Category:Carburetors
Contents |
Theory
At the risk of atomizing a little more fuel onto the fire (a little carb humor there...), let's examine the basic function of a carb. Air flows through the venturi, and, thanks to the discovery by G.B. Venturi (1746-1822), is accelerated. This accelerated airflow causes a pressure drop, which in turn causes fuel to be drawn from the fuel bowl through a series of jets and passageways. Small venturies are by nature more responsive to small changes in airflow, with the result that an increase in flow rate will result in a more rapid increase in the rate at which fuel is drawn from the bowl. Unfortunately, a small venturi also poses a limit on the total airflow through the passageway. The traditional carb (Holley 4150/60, 4GC, et al) has a single booster venturi inside the primary bores. The Q-jet actually has multiple nested booster venturies. This, IMHO, is the secret to the greater sensitivity of this carb in part-throttle driving. Naturally, this additional restriction also compromises airflow (some Holley carbs which are clearly designed for W.O.T. only don't even have booster venturies, but use annular fuel discharge from the perimeter of the main venturi - which actually is a clever idea).
Now, the secondary side of the Q-jet is clearly designed for W.O.T. only. Not only are there no booster venturies, there isn't a venturi at all - just a couple of huge, irregularly shaped holes. The secret is the air valve. I'll go out on a limb here (seeing as how I usually _never_ have strong opinions...), and proclaim that while the vacuum secondary Holley carbs clearly have better driveability than double pumpers, the "mass airflow" sensing nature of an air valve (which includes the 4GC, AFB, AVS, and T-Quad carbs also) provides more accurate control of airflow based on engine needs than does an actuator controlled by manifold vacuum. Again, the ultimate goal here is to maintain a sufficiently high airflow (and resulting pressure drop) to properly draw and atomize the fuel without imposing an excessive airflow restriction.
This is true. However, the vacuum secondary Holleys don't derive their vacuum source from strictly manifold vacuum, as in port being plumbed directly to the manifold. The vacuum source is a small port that opens to the constriction in one of the primary venturis. So, as air flow through the venturi accelerates, an ever increasing pressure drop (increasing vacuum signal) will be realized at the point of maximum constriction--another discovery of our friend, Venturi.
So, the vacuum signal applied to the secondary diaphragm housing in a Holley is directly related to the airflow through the primaries. Consequently, both air-valve carbs and vac secondary Holleys respond to airflow. However, the Holley's response to airflow is an indirect response, whereas, the air valve carbs respond directly to airflow. Hence, I would give the air valve carbs an edge in this area. Nevertheless, don't think that vac secondary Holleys are simply responding to manifold vacuum, they aren't. They respond to VENTURI vacuum, which changes with airflow.
Of course, all of this is rendered moot by EFI systems. Electronics (much as I distrust them) replace the need for this delicate airflow balancing act, and fuel atomization is controlled by injector nozzle design, not a booster venturi or air valve. This, more than anything else, is the reason for the incredibly high outputs of today's motors.
All the systems in nearly all downdraft carburetors, whether 1 bbls. 2G's, Q-Jets, Holley, Carters, etc., utilize an "up and over" system that effectively prevents leaks -- for idle, off-idle, main metering, and secondary fuel. I am simplifying quite a bit here, but suffice it to say that all fuel in the carb must first travel through the jets at the bottom of the bowl, then turn UP through a well (either idle, main, or secondary well) to a point slightly higher than the fuel level in the bowl. As it travels through the well it is emulsified with air. Once it reaches the top of it's well, it makes a 180 degree turn and travels back down.
For the idle mixture, it travels back down the idle channel to the idle discharge port. For main system and secondary fuel it doesn't make an abrupt 180 turn but makes a sideways turn of some angle until it gets to the fuel discharge tube, from there it exits the fuel discharge nozzle in the center of the booster venturi and makes it's downward trek through the carburetor bore, itself. Secondary fuel likewise, exits the secondary discharge nozzle. The point is, you could take the jets OUT of the carb and it wouldn't leak.
To see for yourself, the next time you have a Q-Jet apart, gut the main body, take the jets out and everything, and fill the bowl with water. You will have no leaks, unless some plugs are bad or something.
Another thing to consider. The metering rods do not seat in the jet. When it is not running, the rods are up, in their richest position, not seated. When the engine is running at an idle, engine vacuum pulls the rods to their leanest position, which still allows sufficient fuel for operation. Even secondary rods do not seat in the jet.
The secondary orifice in a Q-Jet is a stainless steel disc with a .135 hole. Other brands use a material of the same type as the primary jets. But Q-jet is the only one with secondary metering rods.
[ Thanks to Joe Padavano, Thomas Smith
for this information. ]
Calculating Flow (CFM) / Sizing
It's easy to overestimate the CFM requirements of an engine. You can calculate the cfm your engine flows using the formula everyone on this list has probably seen many times:
engine CFM = (engine CID x max rpm x volumetric efficiency)/3456
Now for a stock 455, I'll assume you won't go above 5500rpm, and stock
heads with stock exhaust manifolds is probably going to keep your
volumetric efficiency no higher than 85% at 5500 rpm. So your engine
needs - only 615 cfm!
Here is where the universal misunderstanding begins. Usually people ignore the formula above, or use a rule-of-thumb like "use 1.5 times the airflow predicted by the formula". The fact is that 4-bbl carburetors are rated for their cfm when there is 1.5" of vaccuum sucking on them. If you use a 600cfm carb on your 455, there will be about 1.5" of pressure drop across it when your engine is at max rpm. Since atmospheric pressure is about 30 inches of pressure, you just took away 1.5" from 30", leaving the engine about 5% down on power compared to a much bigger carb. If your engine makes 300 real-world horses with those exhaust manifolds and stock heads, using the 600 cfm carburetor costs you only about 15 hp, at max rpm. Everywhere else in the rpm band the engine will be more responsive with this small carb, the car will feel better in traffic, get better gas mileage, etc. Unless the car is a drag-race only car, the 5% power drop at max rpm is really not very significant - how many minutes a day does your car spend at 10% throttle? How many at WOT? How many at WOT and 5500 rpm?
So I'd say pick any carb that flows at least 600 cfm (I believe all the true 4-bbl Quadrajets do), tune it to match your car and driving style, and you'll enjoy life with it. On the other hand, bolt on a 1050 cfm Dominator, and you'll probably be miserable with the result.
You probably know that the airflow through a carburetor is proportional to the square-root of the pressure drop across it, so 3" of vacuum will flow (square-root of (3/1.5)) times more than 1.5" of vacuum across the same carb. Since 3/1.5 = 2, and sqrt(2) = 1.4142, that means that 500 cfm at 3" is equivalent to only 353.5 cfm at 1.5" of vacuum!
Here's a neat one. I used a 600 vac secondary on my 1995 NON-roller cammed 502 chevy engine in the truck while I worked on the 750 vac secondary. VERY RESPONSIVE off the bottom end! Makes a difference with the smaller bores at lower rpms, hence, showing that even the biggest cubic inch engines can run better on the street with a smaller cfm rated carb. HOWEVER, there's a trade off with this. Top end was off considerably with the smaller carb on the big engine (instant restrictor). Sometimes, you can just go TOO small.
This is where I say the Q-jet when properly calibrated, or even Holley's spread-bore, gives you the best of both worlds very economically. (as long as it isn't those high $$ electronic doobies). Of course, once repairs were made to the 750, top end power was restored.
Very simply, the problem with using a "big" carb on a smaller motor somewhat equates to using gas with more octane than your engine requires. Sure, it'll run, but you don't NEED it, and you're gonna waste money. The major concern with the intake system is velocity. If you can keep the velocity up, the more responsive it is. Smaller ports/valves, carbs make this possible. Again, this is simplified. Intake runner size, port size, cam lift/duration and valve size have a LOT to do with it. You can go huge with everything, but lose on the bottom end. The best running engines have MATCHED components working together to make the most power per stroke. Bigger isn't necessarily better for a street engine. I've personally had strong running, properly built Chevelle 327's make me look bad in my stock 69 W-30 400 years ago. (still though, they didn't kill me by a bunch) About the only time bigger is better IMO, is fuel line size.
Ultimately, the way to build power is to move more air and fuel (by weight) in and out of the engine. Since the inlet side is driven by vacuum, huge ports and runners will kill part throttle power due to the inability to get that large volume of air moving. Of course, small runners provide too much of a restriction at high rpm. This is the reason that some newer cars have variable geometry induction systems. A set of small diameter, long runners is used for part throttle driving, with a set of short large diameter runners for W.O.T.
Anybody remember the old Offy Dual Port manifolds? These had two sets of runners, small ones connected to the carb's primary bores and large ones under the secondaries.
I've read an article not to long ago in Hot Rodding if I remember right. They were comparing the 600cfm - 850cfm with a typically built street performance 350 engine. They got the most horse power and torque out of the 750. They pretty much said the formula isn't quite always true but is a good rule of thumb. GM recommends the 750 Holley carb on their 350 ZZ create engines. My ZZ4 just absolutely rips with the 3310 750 Holley. A magazine back in '90 dyno tested a ZZZ create engine with a 750 cfm carb and then changed to the Edelbrock quad setup using 2 600 cfm carbs. The horsepower and torque increased. That's 1200cfm!! It was only rev'ed to 6,000 rpm. It mainly depends on the setup. Big roller cams really like the bigger carbs.
I had a 350 Olds built to the Edelbrock RPM specs in my H/O for 3 years. I ran a 3310 750 vaccum secondary spread bore Holley and it was very responsive and had tons of bottom end, mid range, and upper. The 600 cfm will give you better gas mileage. The engine package is totally streetable. Edelbrock recommends a 750 cfm carb with their Olds RPM package.
[ Thanks to John Carri, Mike Rothe, Joe Padavano, Chad Schwartz
for this information. ]
Firsts
All the talk about Holleys and Q-Jets prompted me to do a little research. What I found is, I think, rather interesting in a trivial pursuit sort of way. Not really significant when it comes to selecting a carb, however. Now, while I maintain my assertion that I like and use Q-Jets, let's just say I'm just doing a little bit of sacred cow tipping!
Things for which the Q-Jet is revered:
Being a four-barrel carburetor Actually, the world's first progressive four-barrel down-draft carb was the Carter WCFB (Will Carter Four Barrel), introduced in 1952. Weighing in at a hefty 18 pounds, they were used on Buick Straight Eights. They are still available on E-Bay, from time to time.
The secondary air valve Used by Carter on the AFB in 1957, fully eight years before the Q-jet. The secondary air valve was taken to it's next level of development in the Carter AVS. However, I don't know if the AVS or the Q-Jet was introduced first.
Primary Metering Rods Again, used in the Carter AFB way back in '57. In fact, the AFB and AVS use a separate power piston for each primary, as opposed to the Q-Jet's utilization of a single power piston.
Triple-Venturi As near as I can tell, the Q-Jet was probably the first with this. However, in the late '60s, years before the introduction of the DualJet, Holley made a two barrel with triple venturis - the 2210. Another version, introduced in the early '70's, the 2211, has a base that allows it to replace the Rochester 2GC.
Secondary Metering Rods Finally, something Rochester has to call all it's own -- sort of. To my knowledge, these are unique to Rochester and have never been copied in a four barrel carb. However, the basic idea behind Q-Jet secondary operation -- a conventional throttle blade, an air valve which varies position in response to air flow, and a tapered fuel metering rod which varies fuel flow in relation to the position of the air valve -- is the idea behind most variable-venturi carbs and dates back to the '50's with the SU carbs (others may also have used the general idea). The air valve mechanism in those carbs was more complex and used pressure differentials to work, but the overall concept of operation is the same. In my opinion, the secondary rods are one of the coolest things in the Q-jet. They can be changed in seconds without any carb teardown, you don't even have to remove the entire air cleaner -- just the lid. They allow for superior tunability of the secondaries -- all the while not affecting the primaries.
[ Thanks to Thomas Smith for this information. ]
Flow Rates
It depends on the size of the venturi. Refer to the info below.
Venturi size CFM 1 3/32 278 1 3/16 352 1 1/4 381 1 5/16 423 1 3/8 435
Keep in mind, 2 bbls and 1 bbls are rated at a 3.0 in/HG pressure drop an 4
bbls are rated at a 1.5 in/Hg pressure drop.
[ Thanks to Thomas Smith for this information. ]
Nomenclature
For a squarebore carb, the primaries are significantly smaller and typically do not share the same centerline as the secondaries in a spreadbore.
In a squarebore, the bores are closer to the same size, though, in some squarebore carbs, like 390 cfm Holleys, the primaries may be noticeably smaller than the secondaries. However, even then the bores share the same centerline.
Many aftermarket intakes, such as mine, and the Edelbrock Performer and RPM, etc, are "dual pattern" intakes (the area where the carb bolts on looks like a butterfly). Either a squarebore OR spreadbore can be physically be mounted on them with no adaptors.
Carter AFB's, AVS's and Holley 4150's, 4160's, and 4010's are squarebores. Carter Thermoquads, Rochester Quadrajets, and Holley 4165's, 4175's, and 4011's are spreadbores.
If the intakes are true squarebore intakes (four individual holes) or true spreadbore intakes (again, four individual holes, though of different size), you cannot really bolt a squarebore carb on a spreadbore intake and vise versa.
In the case of putting the squarebore on the spreadbore intake, the primary bores of a spreadbore, though small, are spread further out than the primaries of a squarebore. The squarebore primaries of the carb would not line up well with the holes in the manifold. Also, most spreadbore primaries are smaller than most squarebore primaries, so even if they did line up, the manifold would present a sudden, constriction.
In the case of putting the spreadbore on the squarebore intake, the secondaries of a squarebore intake are smaller than those of a spreadbore carb, so in that case, you'd have significant flow problems. Again, you'd have the problem of the primaries not lining up right. HOWEVER, Edelbrock does make an adaptor for installing a spreadbore on an OEM squarebore manifold, and vice versa.
Most 3310 Holleys are variants of the 4160 family of Holleys and are 750 CFM. It has a metering block in the primary side, which USES JETS and a metering plate in the secondary side which does not use jets. They are vacuum secondary carbs so you can use your secondary diaphragm kit.
[ Thanks to Thomas Smith for this information. ]
Rochester, Holley, Carter, etc Carbs
What is the difference between Rochester, Carter, Holley, and Edelbrock, Barry Grant, et alii.
The first three brands have been available on factory production cars. Edelbrock and Barry Grant, among others, are aftermarket only, but are based on, and thus functionally similar to certain carbs from Rochester, Carter, and Holley.
Holley carbs were found on some production cars through the ages, but certainly not to the extent of the proliferation of Rochester or Carter carbs. please let us not forget the eminently forgettable Holley/Weber progressive (yes, progressive, as in one primary and one secondary) 2bbl carb found on such notable cars as the Vega, Pinto, and a slew of offshore 4 bangers... There were a whole bunch of these "OEM Holley" carbs produced. In truth, Holley carbs became much more popular after they became available only as aftermarket carburetors.
In point of fact, the real rival to Rochester over the years was really Carter. The Thermoquad was Carter's answer to the Q-jet. It is also a spreadbore and has the added advantage (or disadvantage) of having a phenolic fuel bowl. This was intended to prevent heat transfer to the fuel with the higher underhood temps of emissions-controlled engines, but the plastic had a tendency to warp and split.
Sadly, Carters are quite obscure nowadays due to the proliferation of Q-Jets and Hollies. They are still available, but fetch a premium, as do new Hollies or Edelbrock carbs.
Edelbrock has two basic carb models, one based on the Carter AFB (square bore) [right guys???], and the other based on a Carter spreadbore that was much more similar to the Q-jet. The difference between these late-model Carters and Quadrajets was that the Carter had two Power Pistons, one for each primary venturii. This made the Carter eminently tunable: imagine boring only cylinders 1,3,5, and 7 0.030" over (one bank of cyls with bigger displacement), and leaving the other 4 alone, but still being able to tune the carb perfectly for both banks!
Barry Grant carbs are based on the typical Holley squarebore design, but are much more tuneable than Holleys are (and you pay for the advantage!). This because you can change the size of the venturiis by adding or removing sleeves to/from the venturii.
There are other carb brands available, one of which is Predator. The Predator (and the Kendig before it) use 2D venturies - basically a square passageway which narrows in one dimension only. Obviously this makes the variation in the venturi much easier to implement, as you only need to move a linear surface, not a circular one.
Which One????
This is a subjective question. The Holley guys will scream Holley. The Q-jets will cringe and support their local carb.
Whichever route you go, be prepared to expend time and energy learning how it works. There is nothing worse than tweaking a mechanism about which little is known.
My Holley doesn't bog. It can be susceptible to bogging if either the accelerator pump linkage has too much play or if too light a spring is installed in the secondary diaphragm housing (in the case of vacuum secondary carbs).
I don't know for sure what it was "designed" for, but I do know the still very popular 4150 series was introduced in 1957 for production vehicles, and that it and the 4160 model were used on production vehicles for some time after that. Also, I have had my Holley on my car in anything but WOT conditions. Take start-and-stop Washington D.C. traffic, for instance. Gotta love the downtown area around 15th and Constitution when the tourists are in full force! I even have good idle with it.
Now, one thing about Holley that separates it from Q-Jet is that Holley is a company that makes many different carburetors. The Q-jet is, itself, a carburetor. So, to blanketly say a Holley is optimized for WOT would be much more accurate if you're referencing the huge Dominator. However, that would not necessarily be true of a 390 cfm or 450 cfm Holley 4160 with vacuum secondaries. These would provide good economy and part throttle driveability even on small V8's. With enough tuning, I've gotten my 600 cfm squarebore to be acceptably efficient.
For a pure drag car, I dunno. I don't know from experience, but I've read some accounts that claim the Q-jet can be made to provide stellar performance even in drag racing. See, I can say nice things about Q-Jets, too :-)
I will sorta agree, that for any car that sees street driving in real traffic, the added sophisitcation of the Q-jet is unmatched. I will say that even though I've gotten acceptable efficiency from my Holley, it is not as forgiving of lead-footedness as the Q-Jet. However, here again, the carb under consideration is but one model: the 4160, part 1850-3. Another Holley, say a 4175 spreadbore with vac secondaries, (which has primaries almost as small as a Q-Jet) may provide much better economy. Some of the later Holleys even allow full electronic hookup to a car's ECM to maintain OEM emissions requirements.
It is true that Holley carbs are "high performance", in the sense that they are very good for drag racing, and they are almost always the carb of choice on dedicated drag-race cars. At wide open throttle, Holley's work very well, and this is what they are designed to do best. They are also modular, which makes them easy to rebuild, and relatively inexpensive.
However, if you drive your car on the street most of the time, you probably drive at full throttle for no more than a few seconds per day, if that. Under these conditions, there is relatively little airflow through the carburetor; for a carburetor to work well on the street, it has to have sensitive boosters that respond to small amounts of airflow. This is where the Holley carbs are at their worst. They have the least sensitive booster venturis of any carburetor I know of, which makes them poor low-speed performers; the usual result is poor gas mileage and increased exhaust pollutants. In the US where gas is relatively cheap this may be liveable with, in other countries where gas costs as much as US $5 per gallon, a Holley is probably not a great choice for a mainly street-driven car.
Both Carter carburetors and Quadrajets are designed from the start to be very responsive to part-throttle, low-airflow conditions. They use double or triple booster venturis, vacuum-responsive metering rods, and air-valve secondaries, all of which make the carb adapt better to the engine under the varied loads and speeds of street driving. Carter carbs are elegantly simple designs and easy to understand, Quadrajets are quite complex and it takes a little more effort and knowledge to tune one of them correctly, but once you get it done you will get better gas mileage, less exhaust pollution, better part throttle response, and just as good full-throttle performance. In my opinion, for a mainly street-driven car a Carter or Qjet is considerably better suited than a Holley. Between the Carter and Qjet it comes down to personal choice - each has advantages, especially if price is a factor and the Qjet is already on your car! The Qjet is a spreadbore carb (small primaries, big secondaries) which helps it perform well at both part-throttle and full-throttle.
For my money the best street carb of all is one that is no longer available - - the last and best Carter design, the ThermoQuad, which replaced the Carter AVS, which in turn replaced the Carter AFB that Edelbrock and Carter still make to this day. Like the Qjet, the ThermoQuad was a spreadbore design, and unlike the Qjet, it is a very simple and yet very effective design.
Unfortunately, mainly because of nostalgia and the association with 60's muscle cars, people buy older, less efficient carburetor designs in preference to newer ones. Holley found that out with their new-design 4010 and 4011 carbs which no one would buy in enough quantity to be profitable, and the only Carter that attained cult status is the ancient AFB design that Carter had already improved upon even before the muscle car era ended - it was already obsolete in the 1960's!
Some cynic once said something like "a carburetor is an incredibly ingenious and complex device designed to provide exactly the wrong air/fuel ratio under all conditions of load and speed". That's definitely not true, but sadly there's an element of truth in it, which is why every new car uses EFI.
Here's a little idea of mine to think about. A carburetor is supposed to sense airflow and meter fuel in response, which is how it "adjusts" to varying loads and engine rpms. An ideal carburetor would be so sensitive to air-velocity that it not need an accelerator pump at all (all the accelerator pump does is compensate for the carbs inability to respond to quick changes in airflow). It turns out there is at least one type of carburetor that *is* so responsive to air-flow that it does not, in fact, need an accel. pump - that is the Weber carb and its derivatives (Japanese copies , etc).
Carrying this line of reasoning a little further, a carb that gets by with a smaller accelerator pump (a smaller squirt of gasoline is adequate to cover up the carbs deficiencies) is a better street carb than one that requires a bigger accel. pump shot to cover up its flaws, i.e is more responsive to airflow and more likely to give you good mileage and good throttle response. A little investigation shows that ThermoQuads, Qjets, AFB's, AVS's, etc, all have smaller accel pump volume than Holleys. Worst of all are double-pumper, mechanical secondary Holleys, which, while great race carbs, don't really do a very good job of sensing the engines requirements at all - they need not one but two massive accelerator pumps to cover up their poor response to airflow.
While I don't doubt that someone who know what they are doing can make any of the above carbs work on their car, I think it is clear that by design the ThermoQuad, Qjet, AVS, etc, are better suited to an engine that works over a wide range of rpm and loading, i.e., any engine that runs mainly on the street.
While I have never used a Predator carb, I am fascinated by it because it is the only carb I know about that apparently needs almost no calibration to run on any engine. It is supposed to be truly responsive to airflow through the engine, so much so that just about the only tuning needed is to swap one part (a fuel cam) that matches the fuel curve to the engines likes. No jets, needles, air bleeds, secondary vacuum diaphragms, air valve spring tension, booster venturi's, power valves, choke pull-offs, etc, etc. to adjust, because the carb actually responds to what the engine wants, rather than trying to guess it by using a series of complex mechanical do-dads to approximate the desired fuel curve. Unfortunately Predators are very expensive, may cause hood clearance problems with many cars, and don't have many intake manifolds designed to work with them (they have a different fuel discharge pattern from other more common carbs, and the wrong intake manifold will turn that into poor air/fuel distribution to the different cylinders).
Of course other factors such as availability, cost, and reliability go into any carb decision too. But if someone laid all these carbs out in front of me, all brand new, shiny, and functioning correctly, and asked me to pick one for my daily-driver car, the Holley - any Holley - is the last one I would pick. I would pick the ThermoQuad first, the Qjet next, the AVS after that, then the AFB, then vacuum secondary Holley's, and finally mechanical secondary Holley's.
Anyone preparing to vent their displeasure with my choices, remember, this is just my opinion! There's room in the world for a lot of different ones!
I've tried the Q-jet and Holley Double Pumper on my 71's 455. I'm happier with the 750 vacuum operated secondaries. I've slightly modified mine, but it works like a CHAMP. Either should do fine, as long as it's set up correctly for the application.
Difference: double pumper MAKES the engine take fuel, cutting down mileage, the vac secondaries ALLOW the engine to take on more fuel as needed. The DP gets away with the mechanical secondaries opening RIGHT NOW by adding a second pump shot to cover the big air hole it created. Which the engine MAY or MAY NOT be ready to deal with. It will respond, but with a lot more wasted fuel. With the vacuum secondary, the engine "tells" the secondaries when to open by the vacuum signal. Loose metaphoric comparison: try to make a baby eat without making a mess. Won't happen.
Weight of vehicle makes a big difference on vac secondary carbs. Generally, trial and error is the only REAL method when deciding on diaphragm springs. You want to set it up to where you can't even feel the secondaries kick in. A bog or slight hesitation could mean that you need a stiffer secondary spring. Go as light as spring as you can, starting with the stock spring, then go one step lighter at a time until a bog appears. Then move back up to the next stiffer spring. That should do it. This is similar to adjusting the Q-jet secondary air valve "wrap", only more time consuming.
Motor size doesn't make as big a difference as Volumetric Efficiency. If your engine can't move a lot of air, (restrictive exhaust, small ports, etc.) then a smaller CFM carb will do. I think a 70% efficient (generous here) 455 requires something along the lines of around 720 CFM at 5000 rpm. So a 750 VAC secondary seems good. I've seen too many people overdo it with huge CFM carbs. Not good. Even if you used a 600, your street driveability is greatly enhanced due to the somewhat smaller bores, meaning more intake velocity.
Generally speaking, treat carb CFM as you would octane. Don't recommend using significantly MORE than you're engine requires.
Well, Matt is right about the effect a spreadbore has on gas mileage when running on the primaries - and yes the sound is nice when the massive secondaries kick in. However, there are several carbs that Holley has produced, such as the one I will be running on my 468 BBC, which have the same design. It's list number is 6211. It is an 800cfm spreadbore double-pumper. This type of carburetor would give you the advantages that the Q-Jets do. Some people say that even so, the Q-Jets are easier to tune for economy, but if you want a good "race" engine go with Holley. Perhaps. However, like you said, John, there are many more books related to tuning Holley carburetors than there are to Rochester carbs. I have 2 excellent books on Holley carbs which I would highly recommend myself. Both are published by S-A Design. The two books are:
SA08 - Super Tuning and Modifying HOLLEY CARBURETORS SA27 - HOLLEY Rebuilding & Modifying (A Guide To Holley Modular Carburetors)
Both books have a chart in them with all the Holley list numbers and the specs of the carb. Although all the pictures are in black and white, I never really had to strain to figure out what the pictures were - very detailed descriptions, very well written books.
HOWEVER.....if you did want to stick with Rochester's Q-Jet carbs, there are books on rebuilding and tuning them too. I just picked up Haynes TECHBOOK 10230 (2068) - ROCHESTER Caburetor Manual. It covers all the Rochesters, one and two barrels as well, not just the Q-Jet, with sections on tuning, repairing, rebuilding and modifying them. I have not yet read this one through since I just got, but the Haynes manuals I have used in the recent past have been very good quality. Haynes also publishes a manual on Holley carburetors, but I don't have the number of it.
[ Thanks to Karl Aune, Thomas Smith, John Carri, Mike Rothe, Trevor Lee
for this information. ]
Rochester Carb Specifics
ROCHESTER ID: First note that thru '65 nearly all Rochester carbs had ids on tags, and '66-up were stamped on the side of the carb. Probably the most noteable Olds exception to this is the numbers on tags on the '66 Tri-Carbs. I agree something must be mixed up. Of course *anything's* possible! Generally there are exceptions, but the following applies: The next to the last digit must be a 5 to be an Olds carb. 1 is Ch**vy The last digit: odd is manual, even is auto Third digit is year, 2 is '64-'69 Fourth digit is last digit of model year. Fifth digit is carb type Kurt W-MACHINES@prodigy.net Actually, the Q-Jet wasn't released until 1966. I assume you mean a Rochester 4GC. Joe Padavano Regarding the ID on those carbs, I'm puzzled by those numbers. Not just the fact that they don't show up in the books *at all* (and the book lists all Rochester #'s from 1951-1990), but the fact that these numbers don't match the usual patterns. Here's what you'd expect from reading the two carb #'s: >7029683 >7031575 70= These first two digits simply designate a Rochester product The third digit is the series of the carb, followed by the last digit from the year of the date of the application; Thus, the 2 and 3 designate a carb from the '60's (0 and 1 were the Fifties, into the early Sixties; 4 and 5 were the Seventies). The fourth digit (9) in the first carb # makes this to be a Rochester carb for a 1969 something. The fourth digit in the second number, however, is odd, since the "3" series carbs usually designated the early emissions-controlled carbs used in the '65-'67 cars, when things like A.I.R. and C.C.C. required specially-calibrated carbs, and a "1" wouldn't fit in with that. Then again, Rochester often deviated from the standard practice of the fourth digit being the last digit of the year. Now, the last three digits usually specify the application of the carb within a particular year, and usually, the second-to-last digit correlates to the manufacturer (but not always, and not consistently, but most all of the time). The chart that Year One reprints is pretty accurate: 1,2=Chevy 3=Cadillac 4=Buick 5=Olds 6=Pontiac (although sometimes a 7) Now, a 7 or 8 in the second-to-last position usually indicates a non-GM application (like Checker, Chrysler, Ford, marine and the like), so I'd be inclined to think the carbs you list are something like late-60's Rochester carbs not produced for GM applications. One other possibility; service carbs usually employed the numbering schemes from the years they were introduced, not the years they were actually intended to be installed on as replacements. It's possible that the two carbs are late-60's service carbs for earlier applications. Sorry I can't be of more help; can you describe what the carbs look like, and double-check those numbers? Are there any other ID tags or numbers, or part numbers on any of the carb components? The books really don't even have anything close to a range of numbers like the ones you provided. Robert Barry CA CAR: I would look at the Carb number, of course it may not be original but = the Quadrajets had a specific number for California only cars. This = could at least tell you if it was originally sold in CA. steven@purplemonkey.com (Steven Saraceno) YEAR OF CAR FROM CARB YEAR: >What is the number on your carburetor? That will helpo identify engine >year. Not necessarily; the last few years Rochester got lazy, and used the same carb with an identical number, unadjusted for years. I believe the number was 17086553 or something like that. I can check this tonight. Bob Barry QJET: However, the Q-Jet is a little bit of a different animal. Witness, for example, GM's usage of them on 307's, and 305's. The secret is in the small primaries (smaller than the primaries in a 600 CFM Holley) which optimize low and mid range. The secondaries are important, as well, in the Q-Jet's applicability to engine sizes that, according to the CFM charts, are way too small for the Q-Jet's CFM rating. The key here is in the mechanically operated throttle and the air valve that responds to engine demand, and is not directly linked to the secondary throttle. The fuel flow in the secondaries responds to air valve position, not secondary throttle position. These features make the secondaries, in theory, very much like a variable-venturi carburetor. These two things: small primaries and the air valve secondaries, have allowed the Q-Jet's usage on a wide variety of engine sizes. Now, to be sure, on a 350, let alone a 307, the engine demand may never get to a point where the secondary air valve completely opens. But this is the beauty of the Q-Jet: it effectively limits the carb's CFM to whatever the engine needs. All this assumes proper adjustment and calibration, of course. Thomas P. Smith ELECTRONIC QJETS: The electronic Q-Jet receives commands from your car's on-board computer (ECM -- electronic control module) to constantly vary it's fuel metering to maintain as close to ideal an AF mixture as possible. > and is that correct. Most likely. Q-Jets went electronic in California in 1980 and the rest of the states in 1981. > What's the difference? Nonelectronic carbs don't use a computer. They respond soley to airflow and engine vacuum. Electronic versions do this plus respond to computer inputs, which come from continuous analysis of your AF ratio via the oxygen sensor. For this reason, an electronic carb has the potential to be more accurate and economical. As for the rest of your questions, check everything else, i.e., timing (though the computer probably keeps your timing pretty close, you can still check your base timing), spark plugs, wires, distributor cap and rotor, vacuum leaks...the works. Many times symptoms of other problems are blamed on a carburetor. Also ensure the carb is plugged in. There are a couple small wire harnesses on the carb. One comes from the ECM and and the other plugs into the throttle position sensor. They must be plugged in for proper operation. I'd also ensure the choke is operating properly...and is plugged in, too. A carb rebuild and adjustment may fix your problem, but eliminate other variables first. Thomas P. Smith smithtp@ix.netcom.com EGR MODIFIED CARBS: >Exhaust Gas Recirculation. Basically runs exhaust through the intake to >take up airspace that could be used by the air/fuel mix. This extra >space can cause more horsepower, when used correctly, and Detroit wanted >to avoid that possibility. Well, to be honest, they *did* use all the cubic inches of an engine, but only at wide-open throttle. Under that, however, the EGR valve was in effect, and driveability suffered. You can see how they calibrated the carbs for this: pre-EGR carbs had a very gradual taper on the primary and secondary rods, but the EGR carbs tend to have real fat rods, except the last 15% or so, where it tapers radically; mixed with bigger jets, these fatter rods gave leaner operation when the EGR was working, but richened enormously at WOT. Theoretically, it allows you to have full power at WOT, but in effect be driving a "smaller" engine at all other throttle openings. Unfortunately, driveability with the pre-FI cars suffered, and the response from such a jetting setup was an off/on switch effect; full power at WOT, but *much* less power at lesser throttle openings. The power from my non-EGR 455's is much more linear than the power from my 403 with it's EGR-era carb. Gotta rejet that thing one of these days... >OK, OK, so maybe it didn't happen ~quite~ like that, but it does cool >the combustion process, (can't burn exhaust very well!) which reduces >NOx, allows more advanced timing, and one more item for technicians to >diagnose! No kidding about that advanced timing thing; many EGR cars have initial timing in the 16'-20' range! Bob Barry Since the EGR isn't active at Wide Open Throttle, you won't gain anything at WOT by disconnecting the EGR. At part throttle you may gain some power, but if you want more power at part throttle you just open the throttle more :-) Not having an EGR will require rejetting the carb and adjusting the timing. The EGR helps prevent detonation and allows a bit more timing to be run. That extra spark advance helps make up some of the power loss from the EGR. Tom Lentz, tlentz@ior.com Subject: Re: Plugging my EGR valve? ><< You can either replace your current carb with a pre-EGR carb, or
[snip]
>well now, i am running a '80 quadrajet on my '72 350 with an open element and >no egr. so im thinking maybe i can get some jets out of an older carb? is it >possible to just swap them out? if i can, it will be done! i have had a >little taste of detonation every now and then and its very annoying. now Replacing the jets with ones from an older carb will not be the answer, most likely (though they'll swap out), since the jets in the emissions-era carbs were pretty big to begin with (in fact, and older carb might have smaller jets, making matters worse if they were swapped in). The jetting on a Q-jet involves not only the jets, but the tapered metering rods that ride in and out of that jet. With the EGR carbs, GM tended to go with larger jets, and even *larger* diameter rods, so the effective area of the jet at most points in the metering rod's travel was smaller, but could be very rich when the thin end of the rod was in the jet. What you'd need is a set of rods that will give you richer operation under part-throttle load, so you're looking at a set of rods with a thinner tapered section. Here's the bad news: the pre-EGR carbs ("70xxxxx"-series), with the rods you're looking for, used a different style of primary metering rods than the '80 carb you've got (which is a "170xxxxx" series carb); those are the only tuning items that don't interchange (jets and secondary rods do). Now, there are thinner-taper "170xxxxx" series rods available, but you've got to do your homework first as to what rods and jets you've got in there now, and what carbs might have the rods you're looking for. A couple other options: 1) You could start with a "70xxxxx" series carb as a baseline, as its fuel curve would be calibrated for the pre-EGR setup you've got. Or 2) get the Edelbrock tuning kit that they've got for the '75 and later Q-jets; it's about $100, but would have a range of primary rods with which you could fine-tune your carb. that >i put on headers, i cant tell the difference between the exhaust and pinging. >so im always a little uptight as to whether or not its detonating. that knock >sensor msd makes looks pretty sweet, so i might give it a try too. Pinging costs hp, as well as damaging your engine, that knock sensor would be a nice addition; is it universal, or is there one designed for the specific frequencies detonation would produce inside an Olds V-8? Bob Barry Actually the '75-'80 (car) carbs are different than the pre '75, and post'80 car carb castings. The 800CFM's are very common after 1974, which will hurt around town mileage. The primary rods/hangars are different on the pre '75 carbs. The 1974-earlier carb castings are essentially "flat" across the front of them. '75 later more angled. Don't forget, if the carb was designed for EGR, not running EGR will cause lean condition. Good thing about post '74 carbs are the availability. You could easily get enough primary rods /jets to solve the non-EGR problem.
Dorian Yeager
'75's had REALLY lean jetted Q-jets. Not just Olds but all GM. '75 was the year Rochester changed everything and then later worked out some bugs in the design. Bob Blanchard '73 CARBS: Can't help you on the distributor, but that carb is an Olds carb from 1973; you're missing the final digit on it, though- something should come after the final "5". If it read "7043250" it would be a regular 350 carb; if it read "7043251", it would be a regular 455 carb. If it read "7043252", it would be a Toronado 455 carb. If the final number were higher than "2", then it might be from something special. If there really is no final number on there, then it's either mis-stamped, or a poorly re-stamped replacement carb. If you do find a final digit on there, post it and I can look up the jets and rods for you, and see how far off from your original '71 carb's settings you are. Bob Barry 800 CFM QJET: From: Michael Ecker Subject: 750cfm vs. 800cfm Q-Jets For those of you out hunting for the bigger Q-Jets, here is an easy way to tell. Look down the primary bores with the throttle held wide open. The 750 models have a ridge all the way around the bores, just below the booster venturis. The 800's only have two raised bumps, opposite each other, sticking out from the sides of the bores - also just below the booster venturis. This may be hard to visualize at first, but once you find an 800 you will know it immediately compared to the 750. By the way - I've found 3 of the 800's recently in the local junkyard, all in 1976-1980 Caddy's. One nice bonus is that all three had electric chokes. Two of them came off 425 Caddy engines, and these had much richer secondary rods than the other (which came off a Caddy 368). I checked my 1976 Cadillac 500 (1706_363) carb and my 1980 Turbo 301 carb. They are the 800 CFM variety. I have a generic Ch@vy one, that is 750 cfm. If I find my 7039273, 69 428 HO carb, I will check its size, but I do not believe there were any 800 cfm units until 1971, but I cannot confirm this. If anyone can, please let me know. Easy check: Look down the Venturi, observe a raised area around the outside of the Venturi (this is shaped specific, don't grind it!!), if it goes around and is the same all around, it is a 750, if it is fairly thin, and has a small bump near where the Venturi booster is supported at, you have a 800 cfm. If you measure the area inside that ring, it measures 1 3/32 for a 750 and 1 7/32 for a 800 cfm. The butterflies on the manifold size appear to be identical in size, the change is in the Venturi. So you can't just bore it out. The 80 Turbo Trans AM is probably the last production car from Detroit with a 800 CFM carb, and it is a special Q-Jet. It will work well on any other GM product that can use the fuel inlet "normal", ie, not like a Ch#vy. It goes straight in. The only significant difference is the vacuum under the APT metering rods is EXTERNAL, not internal. No problem! Add a vacuum line to it!! (Else you will KILL mileage!!) This was controlled by the PEVR, Power Enrichment Vacuum Valve. This is not a computer controlled carb!! 81 was 80 was not, but the 81 carb should be a great hi performance carb for any computer controlled car. This Q-Jet also came with the richest secondary metering rods ever, DX, near needle sized at the tips, and a very rapid rate to get there. It has the absolute richest 70% to 100% secondary operation. No other is richer from 70%+ secondary operation. This is needed for the Turbo boost at 10 PSI and no computer to control it. ESC is part of the distributor, but is stand alone. (hint, make that 11:1 compression live on pump gas??? Works for me!!) AND, they also have pull over enrichment for the primaries, something to deliver even more fuel when under high load/high rpm situations. This is a special feature found only on large engine carbs. Ok, the drawback? It has 51 primary metering rods, sorta lean, so, you change it to be richer based on whatever car you have. I was designed for a single plane intake on a 301. No mine is not for sale!! But I can dig up the carb numbers if anyone needs them, just email me directly. Thomas Martin I beg to differ on this one. I have removed tons of Q-jets from late model cars that were 800 cfm. The bigger carb was more common in the later models than the earlier ones. My CCC carb of an 85 Olds 307 is 800 cfm. All I did was drill the vacuum hole to send the signal to the power piston and get a proper float bowl and air horn and it now makes my 350 run great without a computer. I am now waiting for my 403 short block to arrive and I will make this same carb work fine on it also. Q-jets are the best carb around if you understand them. But you must know them to make them work as you wish. Mrmagic442@aol.com Sorry but Joe is correct (as usual! :)) The only 800 cfm Q-jet ever produced was put on 455 Buicks from '71 to '76 period. The increase in 50 cfm comes from a larger primary venturi. The secondary side flows the same as 750 cfm units. Tony Waldner According to Doug Roe's book on Q-jets, it's the primray venturies that are bigger on the 800 cfm models, 1 7/32" dia vs. 1 3/32" dia for the 750 cfm. Dude retrorockets@snet.net Seem to be most common on Olds 75-8(?) Even CCC ones off of the 307's can be (mine was) no use to a non CCC person, but interesting. Stamped numbers, unless you have Karls big book, are probably worthless for you, as every year the number changed for the most part. WAY too many combinations of Q-jet numbers to look through to find the same number. The ones I have came off of the following vehicles: '77 Cutlass w/350, '78 Vista Cruiser with 403 (800's rumored to be on most every 403) '75 Buick Apollo w/350, and '86 Cutlass w/307 (CCC though) If you can stand dealing with the EGR/smog nature of the mid 70's carbs, look in those years. Most Chev's don't seem to have them, plus due to the different fuel inlet, not very useful. To spot one, look down the primary venturi, and look for the "bump" in the lip down an inch or so. If there is a solid "lip" then it's 750. Single bump, 800. Get Doug Roe's Q-jet book. Perfect picture on one of the pages comparing the top down view. Dorian Yeager A "Tech Tips" page I got from Dick Miller Racing says pre-EGR 800 cfm q-jets are "relatively rare," and found on 1971-1973 Buicks & 1973 Pontiacs. I have a carb from a 73 Buick, & it is NOT 800 cfm. Hitson, Roger HITSOR@mail.dmh.state.mo.us TRI-CARB: ..I thought the tri-carb set-up was for the 394's, not the newer style blocks (455's, etc.). Is this an aftermarket manifold, or an original tri-carb set-up (thought you've said before that you had an original tri-power, but could be wrong)...
This is the relatively rare L-69 tri-carb setup offered only on the 1966
442s. Something like 2000 of these units were made, with 54 of them
serving as the basis for the W-30 option that year (when fitted with the
O.A.I. setup). It is based on a cast-iron manifold with progressive,
fully mechanical throttle linkage (didn't the J-2 operate the outboard
carbs with vacuum diaphragms?). Three individual small chrome open
element air cleaners were used, but were replaced by the large chrome
O.A.I. shroud on the W-30. The other unique feature of this manifold is
the block-off plates for the carb heat riser. A small removable plate on
either side of the carb flange can be unbolted and repositioned to block
or open the center carb heat riser passage without removing the manifold
or resorting to epoxy or aluminum filler.
Joe Padavano
Drew Senko wrote: ..Also, I am told, GM made some sort of rule after 66 that you couldn't install tri-power on a factory car (except C*rvette). If this is true, this would explain why the option was for 1966 only, and wasn't available in 1967.
You can treat this all as heresay, if you are worried about receiving incorrect info. It is correct to the best of my knowledge...
You are absolutely correct, Drew. This is also why the GTO was
limited to a single Q-jet after the 1966 model year. By the way, the
66 W-30s cleaned up in C/Stock in NHRA that year, so that tri-carb
O.A.I. combination really made some HP. I meant to add this to my
other post and forgot. Thanks for finishing the story.
Joe Padavano
evans1@serv01.net-link.net Subject: Tricarb dilemma (Was Re: Rochester 2-Jet I.D.) Actually, I have two #7029683 and one center 7031575 residing on a '66 tricarb setup that I purchased in 1984 *not as cheaply as Chris Witt's, but close*, and the numbers puzzled me as well. And, yes, I've checked, rechecked, and rerechecked the numbers, and those are correct. No, the Tri-Carb isn't original to the car. I just found it odd that these carbs aren't even original to this intake, and I am trying to get a little background on them, is all. Mostly in case there may be some specification oddities that I'd just as soon not be surprised with if I have to do any work to them in the future. The center carb appears to be a typical run-of-the-mill 2-Jet, but the end carbs have no idle circuits and, doggone it, certainly look like they came on there. Mike Evans Mike- the Rochesters for the Tri-Carb have the triangular "tin" tags held on to the carb by one of the cover screws. The tag has the carb number on it. Kurt & Rose wmachine@en.com >Actually, I have two #7029683 and one center 7031575 residing on a '66 >tricarb setup that I purchased in 1984 *not as cheaply as Chris Witt's, but >close*, and the numbers puzzled me as well. And, yes, I've checked, >rechecked, and rerechecked the numbers, and those are correct. > >The center carb appears to be a typical run-of-the-mill 2-Jet, but the end >carbs have no idle circuits and, doggone it, certainly look like they came >on there. This setup *in fact, the entire car* has been in storage for the >last ten years, and I only routinely checked the numbers as I was cleaning >them. Frankly, I couldn't care less about the originality or the value of >this "custom" setup, as I have no intention of ever selling it, but now I'm >intrigued as to the origin/birthdays of these carbs. The fact that the end-carbs have no idle-circuits certainly argues for them being factory tri-carb units; do they have mechanical or vacuum linkage on the end carbs? >I also was leaning toward the service replacement reasoning, though I have >no way to check that.....any further thoughts? One problem with the "service replacement" idea is that Rochester utilized the numbering scheme of the year that the service replacement was produced, not the year of the application it was produced for, so, for example, service carbs for early-70's cars that were produced in the late-70's bear the "170xxxxx" numbering scheme, so even if they were service replacement carbs, they would have been produced in the late 60's. Actually, the #7029683 carbs are somewhat less mysterious than the one center #7031575 carb, as they seem to be 1969 service replacement carbs for some tri-power application (though the "6" in the fifth digit there is mysterious, as the 2-jets invariably have an "0" or "1" there, and it's only with the later emissions carbs that the higher numbers enter in). The 7031575 is mysterious because there is simply nothing like a "7031xxx" carb in the books; it wouldn't be a 1971 service replacement carb, as that would bear a "7041xxx" number, and the only "703xxxx" carbs are the '66 and '67 emissions carbs, which all read "7036xxx" or "7037xxx". Are you sure you can't make a scratch so it reads "703_7_575"? It still wouldn't match anything in the book, but at least it would seem to be a carb from within our universe... ;) No, these are real mystery carbs you've got there; they don't even fit in with the oddball variations that you find on some Rochester carbs. Maybe you can claim they're "experimental" carbs??? :) Bob Barry From: "Brian Lorway" Subject: Re: '66 tripower carbs (L-69 and W30) front unit = 7026055 (L-69 and W30) center unit = 7026056 (L-69 and W30) rear unit = 7026057 Info. was found in book "4-4-2 by the numbers" from Supercars Unlimited. Problem here is that there are something like 5 different venturii diameters for 2-Jet carburetors. Pontiac could have used a different venturii carb, in which case the carb could be jetted differently. _________ In some cases, they did. Early Poncho tripowers used the small venturi carb in the center position, with the larger ones on the ends. I believe the 66 Pontiac tripower does use the same size carbs in all three locations; in fact, I seem to recall that they are effectively identical to the 66 Olds carbs, though with different tag numbers. These carb numbers are unique to the Olds tripower application. More importantly, the numbers are on a separate tag held with one of the air horn screws. The tags get torn off and can be easily switched. A better approach would be to get the jetting specs and modify a set of large venturi 2GCs to work. Yes, you'll need to remove the choke mechanism from the end carbs. Alternatively, as I noted, the Pontiac carbs are functionally equivalent and much more prevalent at swap meets. Joe Padavano I'm not much up on Pontiac stuff to say for sure, but I believe they use a different throttle linkage than the 4-4-2. Although the Pontiac carbs and linkage could probably be adapted if you are not concerned with originality. The 66 4-4-2 Tri-Carb used #68 jets in the front and rear carbs. And #63's in the center carb. These are the 60 degree, long taper style of 2 bbl jets. You'll also need to do something about the idle circuits on the front and rear carbs. Greg Rollin ANEROID CAVITY: While "aneroid cavity" does sound suspiciously like a naughty body part on an alien female, the words rang a bell from my childhood, when I was reading a long obsolete physics textbook that one of my older siblings had once studied. The book talked about an "aneroid barometer", which was a barometer which used a small metallic enclosure evacuated to high vacuum to measure atmospheric pressure - like squishing a sardine can with your fingers, the vacuum enclosure flexes slightly as atmospheric pressure on the outside changes, and a linkage detects that motion and displays it on a dial. The evacuated metallic ocntainer was called an aneroid cavity or aneroid chamber. So what's the connection to carburetors? I know that many smog-era carburetors attempted to provide altitude compensation. As those of you who have driven a carbureted car through Denver or drag-raced at high altitude tracks know, the carb jetting needs to change with altitude to compensate for the thinner air at high altitudes. The reason is that x molecules of gasoline need y molecules of oxygen to burn properly, meaning a given mass of gasoline needs a matching mass of air to burn properly. But that given mass of air has less volume at low altitudes than up in Denver, and unfortunately carburetors respond to the volume of air flow through them rather than to the mass of air flow through them. The "aneroid cavity" in the quadrajet, by Dorian's description, moved a metering rod in and out of a jet in the air horn; the purpose of this was undoubtedly to try to modify the jetting as required to track changes in the altitude at which the car was being driven. Probably so the car could pass smog checks in Denver as well as Los Angeles. John Carri aneriod cavities are not found on CCC carbs, as the computer takes over this function with the M/C solenoid. Jeff Newman newsance@netzero.net
GM apparently sold Rochester Product Division (or just the carburetor producing portion) to Magnetti Marelli. Karl mentioned this previously.
Apparently, MM bought the capital tooling equipment (Karl reads this as heavvy stuff like CNC lathes....) from GM and in the guise of Weber U.S.A. is producing Q-Jet parts for GM's Service Product Order department. Also the finer tooling seems to still belong to GM.
Further; Q-Jet part production will apparently cease in 1999.
So Technically, one could say that new Q-jet parts are still being made, but are being used for carbs being serviced.
[ Thanks to Karl Aune
for this information. ]
Factory / Stock Information
Identification / Decoding
As for a quick decode, conside Q-jet model number 17056259. Q-jets all begin with either "70" or "170", indicating a Rochester product (the "170" carbs are the newer ones, usually post 1974, though trucks used the "70" carbs until the 1980's; big difference is that the primary rods don't interchange between these two series carbs).
The next two digits indicate the year. Actually, the "6" is the last digit of the year, and the "5" is just different from the "2" that you would find in that place on a 1966 Quadrajet (I believe the 1986 would probably have a "7" in that place)
That leaves the last three digits, which usually distinguish the particular application of the carb for that year. The second to last digit usually indicates the manufacturer, with "5" being Olds (but just like that i before e, except after c thing, there are exceptions, especially in the mid-'60's). For example, most 350 4bbl Olds carbs ended with "250", and the Toronado engines ended with "252" for a long time. The standard big-blocks had the "251" carb, and the 442 and W-31/W-30 carbs would end in "254" to "259". No, this example is not a 1976 W-30 carb, though.
A good decoding explanation can be found at
http://www.oldengine.org/unfaq/leadfoot/qjet1.htm.
After 1974, to meet the more stringent emissions requirements, makers specified specific carbs for engines in particular models. So while every 1971 350 4bbl carried the carb # 7041250, in 1976 there were different numbers for the Cutlass carb, the Delta carb and the Omega carb.
As to whether it's a good choice for a performance buildup, it's not a *bad* choice, though I know I have built up a better selection of the "70" series primary rods than of the "170" series rods. Whatever you use it for, you'll likely have to rejet it and use different rods; with the "170" series carbs, GM seemed to favor larger jets, but thicker primary rods, which netted less fuel flow. Jets and secondary rods will interchange between series.
A few Carter Spreadbores ended up on some Cutlasses in the early 70's. Rumor has it that Rochester could not produce the amount of Q-jets needed so they subcontracted Carter to "pick up the slack" . You can id one of these pretty easily. On the side of the carb it will say Quadrajet by Carter instead of the usual Rochester Quadrajet.
[ Thanks to Jeff Easton, Bob Barry, Chris Smetana, Steve for this information ]
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