Which Eddy for me?

Frnknsteen said
I have been doing a lot of reading on CFM to cubic inch needs. I ran across a formula that calculated theoretical CFM needed, but that didn't include the Volumetric Efficiency (VE) of the motor. Most good running street cars have a VE in the range of 80% - 85%. Higher performance cars might be 90% efficient. It is virtually impossible for a naturally aspirated engine to have a VE greater than 100%. That has to be factored in to get the true CFM need in the end.

The calculation factored in cubic inches times the preferred max RPM range then divided by a constant of 3456. I don't remember what the constant was driven by, but basically it was a calculation to convert the combination of cubes and RPM down to CFM. you then take that times the estimated VE of the engine. I think I get it, but may be wrong, but I think it is a conversion from cubic inches to cubic feet, then doubled because you have two full strokes for every compression stroke.

Anyways,.... For instance.....

340Cubic inches x 6000RPM / 3456 = 590.28 cfm (theoretical)
Take the theoretical cfm x VE (.85) to get actual required CFM, so,....

590.28 x .80 = 472.2 cfm
590.28 x .85 = 501.7 cfm
590.28 x .90 = 531.25 cfm
590.28 x .95 = 560.76 cfm
590.28 x 1.0 = 590.28 cfm

Like someone said above,... a 600 - 650 cfm carb would likely do what you want it to do. Bigger gets to be a waste. Keep in mind that fuel is drawn in by the velocity of the air rushing through the venturis of the carb. A larger carb has larger venturis, so with a given vacuum draw from the motor, the velocity of the air going through the bigger carb will be less, which will draw less fuel. It IS possible to run too large of a carb on a motor. Performance sucks, mileage sucks, and you will constantly be battling Air/fuel issues.

Bigger ISN'T always better.

That formula is a little misleading. I see an engine as static and dynamic displacement.
A 340 static displacement would be 340, its dynamic displacement would be 1,020,000 cubic inches per minute or 590.28 cfm. Thats what that formula tells you the actual engine volume at rpm. And when used with the right VE its fairly accurate guesstimating how much air your engine can displace per minute .

But the problem using it to pick a carb is the cfm of a carb and that formula are two different levels its not a apples to apples comparison. You wouldn't use that formula to pick a head. 590 cfm divide by 8 equals 74 cfm. You'd be hard to find a intake port to match that at the standard we use.

From a performance point of view Its the not cfm that matters but the vacuum level (restriction) at wot that matters. A full out race car can get away with very little wot vacuum hence very large unrestrictive carbs. A street car is very different cause wot is not our biggest concern.

But really we don't need any formula we already know what works with sb. Basically stock-mild under 350hp 600-650 is your carb can be put on higher output engines but your sacrificing hp for nothing. Over 350 hp 700-800 cfm is your carb which also can go on lower output engine if tuned right but not necessary. And race anything that gives the best time.