Carb CFM recommendations

[yellow rose]

I understand what you are saying about needing more air to produce the same power. Problem is, the only way to pull in more air is more displacement. There is nothing I can do to make a 318 engine pull in more than 318 cubic inches of air per combustion cycle. Obviously changes to cams and valves impact the exact airflow in and out of the engine, but the maximum amount of air still cannot exceed displacement on a naturally aspirated engine.

We actually pull in the same volume of air in Colorado as they would in Florida - the air just has less density. That is the reality we have to live with at altitude. So we make intakes as free flowing as possible, but we have to live with the fact that our air has less energy in it. And if we oversize a carburetor, the lower density only compounds the issue of poor signal at the boosters. Thinner air passing through the venturis has less vacuum than denser air. This is even higher than the standard air energy loss. We lose approximately 16.8% vacuum pressure at 5K feet. This means the problem of an oversized carb will only be compounded at our altitude. I think your original advice is pretty solid.

Then how do you explain volumetric efficiencies over 100%? Which is NOT hard to do.


I don't want to offend you, but it sounds like you know enough to have a decent foundation, but you need to keep learning.

You are caught up in numbers and you treat those numbers like they are cold, hard facts.

I can tell you that I've been told (as I have no desire to race at a place like Denver) by Greg Anderson that he never changed a jet from Denver to Sonoma to Woodburn to Seattle. Didn't change a jet.

There is more to it than your simplistic view of things. Starting with a dual plane intake being better for low speed performance. It's not. Never has been. It's another in a long line of bullshit that keeps getting passed around like VD in a high school locker room.

I hope before you buy something that isn't what you really should use, you take some time and start learning about carburation, how they function and what the circuits do. Study Bernoulii and Venturi and see how they apply to carburation.

Or, you can just bolt on what you want and stay your course.

But to suggest you can never have more than 318 inches because that is your displacement is wrong. Competiton Eliminator is a good place to start looking at what small displacement engines are capable of.

Also consider, even if you bolted an 850 on your engine (using an essentially worthless CFM number but that what we are dealing with) you are only using the primary side of the carb most of the time. Think about that. Throttle position makes a difference.

Now, rather that using CFM to select a carb, why not use Venturi diameter? That's a much better way to select a carb.

 

[318willrun]

But to suggest you can never have more than 318 inches because that is your displacement is wrong. Competiton Eliminator is a good place to start looking at what small displacement engines are capable of.

LOL... I didn't even go there, I knew where that would lead... I've had my fun, YR it's your turn

 

[thecatsfan]

Then how do you explain volumetric efficiencies over 100%? Which is NOT hard to do.


I don't want to offend you, but it sounds like you know enough to have a decent foundation, but you need to keep learning.

You are caught up in numbers and you treat those numbers like they are cold, hard facts.

I can tell you that I've been told (as I have no desire to race at a place like Denver) by Greg Anderson that he never changed a jet from Denver to Sonoma to Woodburn to Seattle. Didn't change a jet.

There is more to it than your simplistic view of things. Starting with a dual plane intake being better for low speed performance. It's not. Never has been. It's another in a long line of bullshit that keeps getting passed around like VD in a high school locker room.

I hope before you buy something that isn't what you really should use, you take some time and start learning about carburation, how they function and what the circuits do. Study Bernoulii and Venturi and see how they apply to carburation.

Or, you can just bolt on what you want and stay your course.

But to suggest you can never have more than 318 inches because that is your displacement is wrong. Competiton Eliminator is a good place to start looking at what small displacement engines are capable of.

Also consider, even if you bolted an 850 on your engine (using an essentially worthless CFM number but that what we are dealing with) you are only using the primary side of the carb most of the time. Think about that. Throttle position makes a difference.

Now, rather that using CFM to select a carb, why not use Venturi diameter? That's a much better way to select a carb.

I never claimed any expertise. Yes, there are the rare cases of naturally aspirated engines which exceed 100% volumetric efficiencies. They do so because of internal inertia and kenetic energy creating a vacuum. So yes, I oversimplified my example.

But putting a carb on a street engine that exceeds its 100% volumetric efficiency displacement by more than 40% is WAY beyond the realm of any volume capabilites of any street engine and we both know it.

And by the way, his name is Daniel Bernoulli, not Bernouli, and I memorized the basics of his work in college in the 90s, as well as that of Geovani Venturi. But I doubt you care as this has become a keyboard warrior contest.

Admins, please shut this thread down.

 

[Garrett Ellison]

One thing that can’t be argued about is that 318willrun’s at home head porting is spot on. Anytime that an engine responds that dramatically to a larger than formulated cfm shows that the volumetric efficiency is well above what the formula calculates for. Usually 85% or so. The 750 is probably the actual optimal size, but it could be the “full blast effect” of the extra enrichment allowing air to start moving through the secondaries quicker.