RE: OPS .21
WFFF, first get a baseline rpm with open exhaust. It is also helpful to have head shims [or the ability to cut your own head chambers], to get the best compression worked out. Then I get a reading with just the header on. Now it's time to get a reading with the full length system. As you take your reading, lengthen the system slightly and watch the tach, hopefully the rpms drop, if the rpms go up, then the system is too short [or small]. Chances are very good a new, properly engineered system is too long. The painstaking steps come next, hack off a 1/4 inch from the header and take another reading, you should see either improvement, or no worsening. Don.t use a tubing cutter because it leaves an internal flare after the cut is made, use a cut off wheel or hacksaw. Keep going in 1/4 inch increments until the rpm improvements end, you will know when the engine hits resonance, it is an explosive jump in rpm. If you don't get that jump, either the prop is too big or the engine has a problem. Be ready to open up the needle to allow for the extra fuel demand, this is a juggling match on a throttled engine, much better off running a fixed venturi with a fuel shut off.