Too big and the airspeed is too slow. Fuel droplets have a hard time staying in suspension, booster signal is weak, etc. Too small and the airspeed is too high. In certain areas it can become "Sonic" and/or turbulent and the fuel droplets can literally get pulled OUT of suspension.....and the engine starves for air.

Here is some of the things i can tell you about the heads 200cc up you have to use a victor jr intake or one like it 180cc and smaller heads you can use a dual plain The heads are thicker then most on the market by 1/8 inch.125 thousands so if you use them on a street car you will need to modify your brackets.The valve guides come tight some times and need to be checked push rods on the intake side only most of the time hit the...