Hi Wayne/ Jac Mac....Wayne you numbers are right, but what you need to see is that with your 75mm TB you will be feeding a plentium manifold, and as the valves open at individual times you have in efect 75mm off TB per cylinder. Jac Mac is right, when you are talking IR manifolds you need the TB to be that much bigger or you just can't get the cfm required, I have always used the theory of double the engine capacity to cfm (350 cu in = 700 cfm) remembering that is for a plentium type manifold, So I would think that your IR TBs would need to be capible of around 500/600 cfm. Jac Mac, am I correct in what I'm saying.