there's really only 2 sizes, one about 725-730 and the larger 800. THe 800s are pretty rare.

GM used adjustable stuff on the carbs to limit air flow when used on smaller engines. mostly these affect the opening of the secondary air valve, they used a tab on the end of the shaft to keep it from opening all the way.

The 800 Q's work best on BBs, there can be issues with off idle stumble when used on small blocks.

 

there's really only 2 sizes, one about 725-730 and the larger 800. THe 800s are pretty rare.

GM used adjustable stuff on the carbs to limit air flow when used on smaller engines. mostly these affect the opening of the secondary air valve, they used a tab on the end of the shaft to keep it from opening all the way.

The 800 Q's work best on BBs, there can be issues with off idle stumble when used on small blocks.

X2:hurray:

 

Jeff, the easiest way to spot the 800's is by looking down the primary bores, they aren't tapered like most are. Also, I think they have 13 bolts on top? OEM apps for a bbc RV will have one.

 

The carbs themselves are all the same, with a very very few exceptions that came on motor homes and some 455s and such in the early 70s. I don't think I've ever seen one of those in all the years and the hundreds of Q-Jets I've had or worked with.

They are all about 750 CFM, except for those rare few that are a little over 800.

Some that come on smaller motors like 305s have a tab on their air valve that limits its travel, thereby "lowering" their CFM "rating". However, grind or bend that tab, and then they're the same "rating" as all the rest.

"CFM rating" is simply the wrong question to ask about a Q-Jet.

 

The carbs themselves are all the same, with a very very few exceptions that came on motor homes and some 455s and such in the early 70s. I don't think I've ever seen one of those in all the years and the hundreds of Q-Jets I've had or worked with.

They are all about 750 CFM, except for those rare few that are a little over 800.

Some that come on smaller motors like 305s have a tab on their air valve that limits its travel, thereby "lowering" their CFM "rating". However, grind or bend that tab, and then they're the same "rating" as all the rest.

"CFM rating" is simply the wrong question to ask about a Q-Jet.

I guess I just didn't know how to pose the question. Did GM use different jet sizes and/or metering rods to better tailor the carb to the intended engine size/horse power (ie 350/300hp vs 396/350hp)?

 

Yes ,calibration was different depending on the application.
http://www.lbfun.com/warehouse/tech_info/fuel & intake/Q-Jet Tuning Paper.pdf
Scroll down for the different OEM configurations/calibrations.

 

For practical purposes, there are TWO CFM ratings for Q-Jets.

There's the "normal" 750 and the "normal" 800; the difference is shown in the photo below. The difference is in the primary venturi; the secondaries are the same.

There's also a one-year or perhaps two-year only Pontiac version of the "750" that has only one booster venturi. VERY, VERY rare. There were lots of problems even with the factory-made units. The do-it-yourself version--made by cutting the second booster out of an ordinary Q-Jet--is a recipe for disaster.





Q-Jets have so many calibration differences (jets, metering rods, air bleeds, air bypasses, fuel passages, etc) that it's about impossible to count them all.

 

For practical purposes, there are TWO CFM ratings for Q-Jets.

There's the "normal" 750 and the "normal" 800; the difference is shown in the photo below. The difference is in the primary venturi; the secondaries are the same.

All the late model one I've seen look to be 800's & look like the pic on the left. By late model I mean 78 on up.

 

Right: while the "CFM rating" remains the same for all but a miniscule handful of them, there is an ENORMOUS variation in all other possible details. Things y ou can't even imagine as existing, are different from one to another.

There are the "original" design ones, with a bowl vent to the outside world activated by the accel pump lever. There are ones without that. There are ones with an "aneroid" (altitude adjusting) feature. There are ones with computer controls, in the form of a variable-duty-cycle solenoid that time-proportions the pri rods in and out of the jets to tune the mixture. Obviously those won't work right without the computer.

There are not only different sizes of rods, but also different styles; single taper, double taper, and so on. There are probably at least 100 different part #s of primary rods, and probably 40-50 of sec rods, that I know of.

There are 3 common styles of choke; divorced, integral, and electric.

There are several varieties of fuel line entry. In case you think that doesn't matter, try putting an Olds one (long straight out the front) on a SBC (needs a short one pointed to the pass side), and observe how the Olds one hits the water outlet on the SBC intake.

Most of them use a vacuum dashpot to control the sec AV opening rate. However, for the first few years, they used a piston in a little chamber full of fuel.

Then there are the endless variations in idle air bleeds, main air bleeds, power pistons springs, needle valves, a whole long list of internal calibration orifices, pump discharge orifices, linkage, and so on.

There are differences in calibration based on engine size, car weight, gearing, and transmission (manual vs auto).

Basically, there were probably around 75-100 different versions used EVERY YEAR from 65 to 83, declining after that. They break down roughly into 4 families; pre-smog, early smog, late 70s-early 80s, and computer-controlled.

For all that, they're still ALL 750 CFM, except for those extremely rare few as detailed above.

For a truly PREMIUM running one, getting the right core is critical. Just going to the buzzrd and yoinking something off of some old truck isn't going to get you the same thing as finding one off of a 69 350 that had come with double-hump heads, or a 66 396, or whatever.

What is it you REALLY want to know?

 

Which one is the least complacated all around.i got a 350 iput into a 56 chevy im not going to race it but i love the pick up pull when the secondaries kick in.
Thank you for your imput.
Im Roy

 

You can also find the big one on Caddy 472/500 engines found from about 68-74, Eldorado would have had a 500cid.

 

Thanks to all who responsed. This has been a great learning lesson.

 

AFAIK all the 13 screw versions are 800s.

 

Pass cars had electronic q-s up thru 1988 in the Monte SS with L69 305HO and trucks continued to use q-jets a couple yrs past that.

No chevy motors ever got the 800cfm q-jets,just olds/caddy/pontiac larger displacement engines like 455 & up got them with a few smaller 400 poncho's getting the 800 cfm carbs too.

Too bad chevy never gave the ls5 454 with 10.25.1 comp more cam like the solid 143 that came in the ls6 & an 800 cfm q-jet,would have been interesting.

The 454/365hp ls5 could have put out at least 400hp+/520-540lbs trq in that trim (esp with headers) which would have been very streetable for 454 cubes & the 143 solid cam along with an 800cfm q-jet which would have been a great street perf motor.

M20 & 3.31 or 3.55 gear to keep cruise rpm reasoanble by stayig away from 3.73's with short stock size tires and your all set.

Scott

 

The first 800s were Buick 455-exclusive starting in '71 or '72. Even a 500 Caddy or 455 Olds/Pontiac did not have the 800-style main bodies before '72 or '73.

I have heard but not confirmed that ALL the "Mod Quad" '75-and-newer FRONT FUEL INLET main bodies (including those used on small-blocks) were the 800 style, but the '75 and newer SIDE FUEL INLET main bodies could be either 750 (common) or 800 (not so common.)

In short...better whack open the throttle and LOOK AT THE PRIMARIES to see what you've got.

 

Believe it or don't....

The 70 Frod Latrino Super-Cobra-Jet 429 came with a Q-Jet. Which I guess is appropriate since the 429 looks EXACTLY LIKE a BBC inside, in fact you can even take off that stupid Frod non-adjustable "fulcrum" valve train crap and put BBC rockers on it and make it adjustable! You just have to use some oddball push rods; Pontiac IIRC. It's almost like the Ford engineers said, "This BBC is a damn good motor! Let's copy it and put just enough of our stupid Frod crap on it that we won't get sued."

Just another of the Q-Jet oddities I've come across.

If you don't believe me, see p. 3.... http://www.legendaryfordmagazine.com/PDF/issue27_70torino.pdf

 

the 429 looks EXACTLY LIKE a BBC inside, in fact you can even take off that stupid Frod non-adjustable "fulcrum" valve train crap and put BBC rockers on it and make it adjustable! You just have to use some oddball push rods; Pontiac IIRC. It's almost like the Ford engineers said, "This BBC is a damn good motor! Let's copy it and put just enough of our stupid Frod crap on it that we won't get sued."

That's EXACTLY what happened. Chevrolet did all the preliminary development work on the Boss 302, the 351 Cleveland, and (especially) the 429/460.

Ford purchased two canted-valve NASCAR-prototype "Mystery Motors" from Chevrolet in '63. No wonder the 429 is so similar. All the Ford canted-valve engines sprang to life as Ford's developmental offshoot from those two Chevy "Mystery Motors".

http://reviews.ebay.com/Big-Block-Chevy-quot-Generations-quot-A-primer_W0QQugidZ10000000001563647

 

JET industries sells both the 750 and 800 CFM carbs in Jegs and Summit....These are reman carbs with many application options. All will require your own custom tuning to run just right, but seem to be quality carbs at a descent price....


A........

 

There is one more model to add to the list. The 3.8L Turbo Buick used a small cfm (IIRC around 500) Quadrajet. The position of the turbo caused very high vacuum under WOT and that required a change to the circuits to have the power valve to work backwards. Under high vacuum it enriched the mixture instead of leaning it. I put one on a 350 years ago thinking it would use less fuel and it was a disaster.

 

There is one more model to add to the list. The 3.8L Turbo Buick used a small cfm (IIRC around 500) Quadrajet.

Almost certainly a 750 or 800 that had the secondary air valve or throttle opening angle restricted to prevent using the full air-flow capacity.

Yes! Correct!
800's had the bumps, and only came on 455 Pontiacs and Olds.

All the divisions had 800 carbs; but they started with Buick, and the 800 was fairly rare on Chevys.

 

the 800 was fairly rare on Chevys

Right: the only ones I know of are the motor home applications.

 

Almost sure most any thing 80 up had the 800's. 84 305 truck has one, also 82 Suburban had one. I'd imagine their is still some 750's in the mix during those years but I think they're more rare than 800's.

 

Dont forget that Mopar did use Quadrojets in there vans.... I used to work on some fleet vans and surprised the hell out of me back in the 80's....

 

1977 w72 400 800cfm qjet

 

this thread is a decade old. But thank you, i hit the like button for someone who is an angel now. Passed a few years back. Was a HUGE contributor to this site and the Chevelle community.