View Full Version : how do identify the CFM of a quadrajet?

11-17-2004, 04:56 PM
are there markings on the carb i should be looking for or what about a web site where i can run some sort of serial numbeR?
my dad has a few carbs in the garage and i want to sort them out....

thanks for any input in advance guys.


11-17-2004, 07:03 PM
750 necks down in the primaries (entire circumference) 800 just has a little protrusion in each primary bore. Once you see the two it becomes fairly obvious.

11-18-2004, 02:30 AM
do 750 cfm quads even come with chev 305s? cuz that seems like a big carb for a 305.....

11-18-2004, 03:21 AM
do 750 cfm quads even come with chev 305s? cuz that seems like a big carb for a 305.....

All quadrajets were at least 750cfms. This is not too big of a carb because only the primaries opened on anything other than full throttle. Only at full throttle do the secondaries come into play, flowing the 750cfms.

Also, unless you modify the secondary openings a bit, you probably are not flowing 750cfms.

11-18-2004, 07:37 AM
Even though the Qjet has mechanical secondaries which are completely open at WOT, a Qjet cannot overcarb an engine unless the full flow on the primaries is too much. This is because the air valve on the secondary bores will only open as much as the engine needs.

11-18-2004, 07:07 PM
Here you go:


11-18-2004, 11:25 PM
whats better? a 600cfm edelbrock performer, or a 750 cfm quad?

tighty whitey briefs or boxers? :nuts:

thanks for any imput guys!

11-19-2004, 04:30 PM

Here's a simple primer on 4bbl carbs:

All modern 4bbl carbs are designed as either spreadbore or squarebore carbs. A spreadbore carb has secondary bores that are larger than the primaries...they spread out in other words. Good examples of that type of carb are the Rochester Quadrajet and the Carter Thermoquad which is found on Chrysler V8's.Holley also sells some spreadbore carbs which are designed as replacements for either of those carbs.

A squarebore carb has primary and secondary bores that are the same size...together they would fit inside a square. Examples of this type carb are the Edelbrock(nee Carter AVS) and the majority of Holley carbs and all Demens that I'm aware of.

The usefullness of spreadbore carbs comes from the large secondaries and small primaries. During normal driving the small primaries deliver reasonble fuel economy, and during WOT the large secondaries allow a lot of airflow potential.

I'm partial to Quadrajets because they are a spreadbore with AT LEAST 750 CFM capacity, and also they are an airvalve carb. What that means is, the baseplate of the carb has four butterflies....two small ones in the front and two large in the back. When you stuff the gas to the floor, all 4 open up all the way mechanically. However, up on the top of the carb, over the secondary bores, sit a pair of spring-loaded air doors, basically like the butterflies on the bottom. Those airdoors are mechanically attached to a pair of secondary fuel metering rods which sit in orfices in the carb body.

Airflow acts upon those air doors during wide open throttle. As the engine's demand for airflow increases, the air doors open wider and at the same time raise the fuel metering rods, which have a tapered tip, higher up out of their bores, those supplying the engine with more fuel at the same time . Theoretically the engine cannot be "over carbed" with a quadrajet since the engine only gets as much air and fuel as it needs. You can tune one by changing primary jets and needles, and tune the secondaries by changing the secondary rods which takes maybe a minute.

Where it get even better is say for instance you have a 350 with crappy heads and a stock cam. That engine might never need more than 600cfm of airflow. Ok, so you go buy a new $200+ Holley or Edelbrock 600 carb. 2 weeks later the engine throws a rod. It just so happens that you have a moderately built 455 Olds waiting to be dropped in. Only one problem: That nice new 600 cfm carb is gonna now be way too small for that 455, so you can't use it.

If you were running a Qjet, it would be fine jetted and rodded for the 600cfm 350, and then would be perfectly useable on the 455 with a simple retune, not to mention it has that extra 150 CFM waiting to be used if needed. See, if that 350 needed only 600 CFM, the Qjet would let it have enough air and fuel for 600 CFM. If the 455 needs 750 CFM the Qjet delivers.

GM used at least 750 CFM Qjets on even small displacement V8's, such as the 305 and 307. In fact, the Qjet on all the Olds 307's was an 800 CFM casting, which was debated on here and ultimately proven to be correct. What GM did was artifically limit the travel of the air doors so that the carbs only flowed about 575 CFM, since no way in hell would a 307 need 800CFM.

My vote is for a Quadrajet. It's an awesome carb as long as you don't pay attention to people that hate them because they are scared to learn how they work. The Holley is a good carb IMO, just not as "flexible". Edelbrock...they are great at taking up space on shelves and thats about it(IMO).

11-19-2004, 04:59 PM
Good post! :tup:

11-19-2004, 07:29 PM
Good info, 84cutlass.

As a matter of opinion (and only opinion - no flame intended), I believe a QJet can overcarb an engine - especially if it is out of tune. I had an '81 Chevy pick-up with a 305 and a 750 QJet. It was always running rich, no matter how the carb was tuned - even by a master mechanic how knew how to tune one. Not to mention the two accelerator pumps that it went through...

Since I had a bad run with that carb, now I'm a fan of squarebores, and I usually run a Demon (supporting the local economy as much as anything.)

Yes, I'm afraid to learn how to work on a QJet - ok, more lazy than scared. I would rather read 4 pages to tune a carb than 25 or so. And, I can install & tune a Demon in 45 mins. from start to finish - so I guess it's just whatever you a comfort level with. Besides, a Demon is really only a worked-over Holley in most regards. That makes it really easy to re-jet it at the local parts store...

And, I agree - the Edelbrock carbs are pretty much paperweights. About the only thing I like from Edelbrock is my RPM manifold - and it fit like crap when I mounted my A/C on it. :wtf: :mad:

11-22-2004, 12:14 PM
thanks for the hell of the post man. it really cleared things up for me, as a matter of fact i printed it out so that i have a reference for later!!

thanks again guys,