how much CFM'S....
 

Does any one know how much total cfm's go through webber's, say size 40?

It depends on the engine rpm.

Remember an engine is an air pump and you can calculate it fairly easy. First, use the displacement of the engine. Divide this by 6 to get individual cylinder displacement volume. convert this to L/min using rpms. There are 28.317 L/ft^3.

Now, realize that if you increase the air volume through a tube that stays a given size (think carb body) the air speed increases. The volume / cross sectional area will give you a velocity. I think John Luetjen has empirically derived that Porsche engines do not make HP beyond an intake speed of 100m/s. So find out what speed your engine is running at. Then see if 40s will be too small for your displacement. You can do the same exercise for head port dimensions.

why is it then that carb. manufactures list cfm's for every madel that they make?
Tas

Jamie; You forgot to mention that the choice of venturi will also have a lot to do with the flow rate.

Yes, I did. I had to start somewhere. The venturi will speed up the air through the carb body. this provides more vacuum to suck fuel out (hey its a carburetor). If the venturi is too small it will choke off the engine by not allowing enough air past. I suspect this may be a tranition from laminar to turbulent low that prevents the fuel from being sucked out thus reducing power. I could be wrong here. My fluid dynamics are weak.

Tas, I expect that V8 manufacturers are what you are referring to and assume that it is because most consumers can't add and multiply. Its all about marketing - bigger is always better - Right?! Think again. It would be common for people to get a carb that is too big for the engine. This might make more high rpm HP but will likely have poor metering signals for low rpm cruising and idle. It is also common for street engines to be delivered with small carbs. This helps low speed drivability and gets better gas mileage while also limiting HP. Going larger can free up some power.

no way around it, the carbs must be matched to the particular engine specs. Don't forget that cams and compression ratio factor in as well. There are many places to get the formulas for finding the correct size carb for your motor. I know Bruce Andersen's book has them. I can't remember them off the top of my head.

John, I also mispelled your last name. It has since been corrected.

Not a problem Jamie. You're not the first, nor (alas) will you be the last!