Carburetor cfm rating -- adding more air to intake manifold

[Ranger Mike]

TL;DR Summary

will adding more air to intake manifold change engine performance

Carburetor CFM

A Holley Carb rated at 500 cfm 2 barrel carb has venturi diameter of 1.3/8". There are 2 barrel carbs with 600 cfm and have 1.45 diameter venturi.

Looking at the area the 1.378 bore has 5.9 sq. Inch area. The 1.45 dia. has 6.6 sq. inch. 5.9 - 6.6 = 0.70 sq. inch difference.

Keeping the 500 cfm carb in place, if I can introduce 0.7 sq inch more area in the intake manifold, will I have the same potential horsepower as a 600 cfm carb provide? Assume I can change jetting to make optimum fuel /air ratio.

 

[Baluncore]

Assuming no turbocharger or fuel injection.

The induction stroke can only draw in one cylinder capacity of air at atmospheric pressure. At the end of the induction stroke, there will be less than one atmosphere in the cylinder because restriction of the inlet will prevent complete filling in the time available.

By increasing the throat of the entire inlet manifold significantly, only marginally more air will enter during the induction stroke. Engine performance will increase slightly, but not nearly in proportion to the increase in carburetor capacity. The carburetor fitted by the manufacturer, would have been optimised for that engine. It was selected for the air inlet path, the valves, and the exhaust restrictions.

You may get improved performance by replacing an inlet manifold with runners, and an exhaust manifold with extractors, only then might it be worth upgrading the carburetor.

 

[Ranger Mike]

In this case you already have aftermarket intake and header exhaust. The bottle neck is the carb itself. You also can use aftermarket camshaft that promotes scavenging. And you must use the 500 cfm carb.

 

[Baluncore]

In this case you already have aftermarket intake and header exhaust. The bottle neck is the carb itself. You also can use aftermarket camshaft that promotes scavenging. And you must use the 500 cfm carb.

Too much air is now flowing through the small diameter carburetor, so it has become the bottleneck.
Your question has now become what ?

 

[Ranger Mike]

Balun
You and I have had communication problems in the past. How can you say too much air is flowing thru the carb? The venturi is still the limiting factor and thus the bottle neck.
Granted, good exhaust headers and a well designed intake may increase port velocity but you can only push so much air thru the venturi.

My original question is if I can introduce 0.7 sq inch more area in the intake manifold, will I have the same potential horsepower as a 600 cfm carb provide? The answer would be yes or no.

Did I ever say I wanted to introduce more air thru the carb? NO. I said intake manifold.
Will I see improvement in performance, though not a good as replacing the 500 cfm carb with a 600 cfm carb, I would suspect yes.

 

[Baluncore]

How can you say too much air is flowing thru the carb?

Because you say ...

The venturi is still the limiting factor and thus the bottle neck.

Surely, if it is the bottleneck, it is the thing that must be changed, to get improved performance.

Did I ever say I wanted to introduce more air thru the carb? NO. I said intake manifold.

Does the air not flow through the carburetor, and then through the intake manifold ?
I thought they were in series, so I would expect the same flow through both.

You and I have had communication problems in the past.

What Ranger Mike writes, often, does not seem to follow, or be consistent to me. Can anyone else explain this reasoning or language miscommunication?

 

[Ranger Mike]

Surely, if it is the bottleneck, it is the thing that must be changed, to get improved performance.
Really???

Some people are good at thinking outside the box, some people are good at keeping things as they are.
This is one of the think outside the box moments. Like hiding two carbs under the flat head Ford intake manifold in 1948.

 

[jack action]

An intake manifold is composed of two parts:

• Plenum
• Runners

The runners' length and area are important to tuning the desired pressure wave. I wrote something about the exhaust manifold design, but the same applies to the intake manifold design, which is even simpler as the temperature variation is smaller.

The way you must look at the plenum is some sort of "sub-atmosphere" where the air is pumped.

Now, imagine a very small plenum, meaning each runner is almost directly connected to the plenum inlet. Other than artificially lengthening your runners (expect peak torque at a lower RPM), this means that the flow velocity through the carburetor is closely linked to the piston velocity during the intake stroke. Slow at first, then peaking, and slowing down again. The problem is that your maximum velocity is what can be restricted by the venturi size.

But what if you could spread your air intake throughout the whole intake cycle? Then you would have a constant average velocity through your carburetor's venturi that is much lower than the maximum velocity of your piston. This is what sharing multiple runners to a single inlet can do: a 1-liter engine with multiple cylinders will draw more air from a restrictor than a 1-liter single-cylinder through the same restrictor. Ideally, using a compressor pulling from your carburetor and pushing into your plenum would accomplish that perfectly, no matter how many cylinders you have.

But having a larger plenum will have a similar effect. If your piston takes a small volume of air in your plenum, the pressure will lower less compared to a smaller volume. This will help to decouple the runner from the carburetor and thus help make the flow more continuous through the carburetor.

This neat video from Holley explains single-plane versus dual-plane intake manifold which is basically using the same space where you either have longer runners with two small plenums vs small runners with one large plenum:

​

Your plenum is not large enough for your taste? You can increase it with a spacer:

But once you decoupled your runners and plenum, you can go wild on the design:

 

[Ranger Mike]

Love the Chrysler long ram manifold on the 413 cid B block, circa 1962