A note about carburetor flow capacities

[rich weyand]

The topic comes up often: how big a carb should I put on my XXX engine.

Bigger cfm numbers -- higher flow capacity -- is always better, right?  No.

Carburetors approximate the correct mixture for the engine.  Exactly correct mixture can only be achieved in a closed-loop system, like electronic fuel injection.  A carburetor approximates the correct mixture.  And since a too-lean mixture won't burn, carburetors err on the side of being too rich.  This is why you will always get better gas mileage with some sort of EFI.

Carburetors are also most accurate when they are running at their designed flow.  Most testing and tweaking occurs at the rated flow of the carb in CFM: cubic feet per minute.  That's where the carb is running at its designed stack velocity, and you get the best atomization, the most uniform mixture.

So, how do you calculate the correct flow rate?  Take the cubic inches of the engine, divide by two (because only half the cylinders fire every revolution), and multiply by the redline rpm.  That will give you cubic INCHES per minute.  Now divide by 12 three times (12 cubed) to convert to cubic feet.

This gives you the maximum flow rate for the engine, at a volumetric efficiency of 1.0.  That is, the volume the engine will pump in the ideal case, where the rings are a perfect seal with no blow-by, the vacuum in the intake manifold is 0", the valves open and close at the ideal time to completely load and empty the cylinders, etc.  A volumetric efficiency of 1.0 is never achieved unless the engine is being blown, turboed, or injected with an oxygenator like nitrous.

So what are the numbers?  Here they are:

305 cu in @ 5000 rpm: 441 cfm
350 cu in @ 5000 rpm: 506 cfm
454 cu in @ 5000 rpm: 657 cfm

Then why are there 800 cfm and bigger carburetors, and why would anyone do twin quads or tri-power? 

A 454 at 6000 rpm will pump 788 cfm, at 7000 rpm will pump 920 cfm, and at 8000 rpm will pump 1050 cfm.

For your 350 at 5000 rpm in your truck?  Anything above 500 cfm is fine.  600 is a good choice.  Go bigger, and your performance will DECREASE as you never get to the carb's design stack velocity and its most efficient operating range.

And I'm not claiming to be a carb expert.  This is just some basic stuff I know, to help out people thinking about carb replacements.

[LTZ C20]

Haha not an expert. Great info as always Rich. I enjoyed it just to read and learn.

[enaberif]

The thing is though this really depends on the carb because qudrajets only use what you provide where other carbs won't and you are always stuck with that magical 700-800cfm number.

Its been stated time and time again there is a reason why quadrajets worked so well.

[rich weyand]

Vacuum secondaries, big deala.  The Edelbrocks have a vacuum-operated air door on the secondaries as well.

Doesn't matter.  The carb will work best when it is not oversized to the application.

[enaberif]

So then if that is case why were stock GMs provided with a 750 or 800cfm carb?

[rich weyand]

There were two body styles of Qjet, one nominally 750 and the other a little over 800.  But there were a ton of tuning parameters, air bleeds, and other details that differed depending on the engine size it was supplied on.  It was basically sized internally.  That is, they were tuned to an optimum performance at a lower cfm than the carb body could handle.  That's how they handled that issue.  Run the Qjet from a 350 on a 454 and it will not perform as well above 500 cfm as one that was supplied with the 454.

But if you are buying an over-the-counter carb, including a Qjet, that is oversized for your engine, you will get less optimal performance than you will with the proper sized carb.  So if you are going to buy a replacement Qjet, you should get it from someone who knows how to set it up for the engine you are putting it on, not off the shelf or from the junkyard from a different size engine.

http://www.chevelles.com/forums/showthread.php?t=297789

[bd]

The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor.  It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs.  Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM.  The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.

[BBM3]

The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor.  It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs.  Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM.  The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.

Thanks for your excellent description.
I wish I understood that years ago.

[rich weyand]

The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor.  It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs.  Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM.  The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.

I've never been as impressed by that.  The Edelbrock, for example, has an air door on the secondaries that functions the same way.

What I think made the Qjet so adaptable was all the pressed-in air bleeds and such that were tuned to the individual application.

[ehjorten]

And that a QJ has no gaskets below the fuel level = no leaks!

[Greybeard]

Ultimately, Rich has it nailed. The QJ on most engines will very seldom even get the secondaries open a quarter of the throw in most street scenarios. Pulling a trailer is where they shined. The more the load, the better they worked. But the air flow really has to build (on a stock carb) to get the secondaries to open.

I have seen two distinct QJ's of the same CFM. One has very small primary venturi(s), the other has huge primary venturi(s). Holley spread bores are the same configuration small in front big in back. Talk to old-time circle track racers and they will tell you that a four barrel sucks, a two barrel is the way to go for performance. The faster the air flows the better throttle response, that is why keeping the primary venturi small helps. It will allow higher air flows (better throttle response) at low RPM's where it's used most often. Then as the engine needs the air, the secondaries will open. Tuning this transition in a QJ is something I could never do very good so I used Holley's with mechanical secondaries more often than not. However, my '74 Monte Carlo had a 454 with a QJ that ran like a raped ape (for it's size and weight). It would run consistent 14.995-15.195's all day every day in the quarter mile brackets. Slow by most folks standards but speed was not the goal, consistency was. Besides, the car weighed 4,700lbs and was as aerodynamic as a cinder block.