Thorp Air Command - T18.net

Does this sound reasonable to anyone? I have an 0290 bored 30 over with 8.5-1 compression. The cam has some sort of mod---ground for 15 % more duration. Now here is the issue-- it has a MA4-5 carb. The venturi is configured for a 0360. I have been told the carb is too big for the amount of air volume being sucked in by the engine and due to lack of air volume there is lack of fuel being pulled out of the carb creating a lean situation. The builder may have thought since he was building a hot rod engine he needed a large carburator???

Mark Otter
N369W

The goal of a carb is o mix the correct amount of air to fuel ratio, not enough fuel will destroy an engine over time as compared to to much fuel will cause rough running and you will have a "flooding" effect. In your case it will be a crap shoot as to air/fuel ratio. The 0-360 carb has a larger intake bore than a 0-320, probably giving more air/fuel than you can pump through this large compressor we call a motor.

Try this; engine cubic in. x rpm x .09 divided by 3455 equals carb CFM size.....this is a formula he race car folks use. You would need to find the CFM of a MA 5 carb..

The displacement only grew by 1% and the RPM will remain unchanged. The difference in cam timing will only effect where peak torque will occur. The standard carb will in practice be the most practical. Any bigger will most likely cause problems when you least want it like on a go around when a "I need power now" situation happens. I am curious where the 8.5:1 pistons were sourced from, custom?
I also have an O-290. Jahns 9:1 pistons, standard cam advanced 2 degrees and dynamic balancing. I've yet to run the engine but am sure it will run great with the stock carb. Velocity of the air/fuel mixture is critical and too large a venturi will not allow for enough vacuum for to keep the mixture from leaning out.

Bill and DanaL thanks for your input.
Bill thanks for the formula. I will try to find the cfm of the carb.
DanaL--the pistons in my engine are also Jahns pistons.

Thanks
Mark
N369W

I think there might be a typo on the recent formula for carb CFM. Change .09 to .90; the idea is to reduce the final value just a little for "volumetric efficiency", or rather "inefficiency". From what I understand, it's a simple percentage - varying from 80% to 90%, for standard to high performance engines.

With all respect,
Les Krumel.

Yep you are correct, damn those happy fingers.

If I calculated the cfm formula correctly my engine needs 196.4 cfm's. How do I know which carburetor will supply this amount of cfm's.

369w
Mark