A note on carburetor flow capacities

[rich weyand]

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

 

[MadOgre]

Good stuff Rich!

I know people think that if you port and polish and run headers that the displacement will flow more but that is incorrect. The port and polish is simply to maximize the possible flow which is ultimately maxed out by the volume each cylinder can absorb. Cylinder volume is ultimately what determines the maximum attainable flow. This is all influenced greatly by the cam shaft too.

The math you posted shows the maximum attainable flow with a perfect port job and cam

 

[Quadrajet Power]

Good information. I used to say the two easiest things to do are over carb or over cam an engine.

One of the great features in the design of the quadrajet, was it's ability to only supply the flow that the engine demands. And the mixture is metered through the rpm and load range. All original quadrajets are capable of 750cfm, some are 800cfm, with a few rare exceptions over 800. But the quadrajet supplies only what the engine demands when they are set up properly.

 

[firebane]

Good information. I used to say the two easiest things to do are over carb or over cam an engine.

One of the great features in the design of the quadrajet, was it's ability to only supply the flow that the engine demands. And the mixture is metered through the rpm and load range. All original quadrajets are capable of 750cfm, some are 800cfm, with a few rare exceptions over 800. But the quadrajet supplies only what the engine demands when they are set up properly.

So here is a question. I was given 4 quadrajets when I bought my truck and each one of them is a 800cfm. How can I ensure its "metered" properly to not over carb my truck?

 

[MikeB]

Good info, Rich. I've read several posts over the years by guys who wouldn't use anything less than a 750 carb on 350 engine, because they they can feel the difference compared to a 600 or 650..

Well if it's a VC carb, the secondaries are never getting close to fully open, so I'm not sure what they're feeling. And the larger primary bores (compared to a 600) actually slow down air velocity, which may make it harder to get a consistent idle, and will definitely hurt throttle response.

On another note, Holley says if you can feel vacuum secondaries kick in, what you're actually feeling is a slight bog. Kinda kills some old wives tales, huh?

 

[Quadrajet Power]

So here is a question. I was given 4 quadrajets when I bought my truck and each one of them is a 800cfm. How can I ensure its "metered" properly to not over carb my truck?

You have primary and secondary adjustments. Main flow comes in on secondary side. On the baseplate, I always set the secondary blades for 90 degree at full throttle, which would deliver the full potential cfm if needed. The factory would limit this opening and reduce flow on engines not needing the cfm, which was much easier than changing the castings I would imagine.

Secondary flow is adjusted with a tension spring on the air valves. These air valves open from engine demand, and as they open, they lift the secondary rods to allow fuel flow in the secondary circuit. The tension spring is adjustable, so you can tune it to open as needed. I always replace the tension spring and the plastic cam on the secondary air valve rod. The vacuum pull off is in place to prevent too quick of an opening.

Many people rev their engine when truck is sitting and say the secondaries aren't opening on the quadrajet. The upper air valves aren't opening, but the secondary blades are opening. Those uppers only open as engine demand requires, under load typically to deliver the required fuel/air.

So, long answer, but if your secondary rods, hanger stamp, air valve tension and choke pull off are all in tune with the vehicle, you should have smooth transition to the secondary circuit with no bog, and the upper air valves should open to the extent the engine will pull them based on demand, which lifts the secondary rods and meters the secondary circuit fuel supply.