This a touchy subject especially for engine builders. I would have to take a whole day to explain it in detail, and some engineering discipline would be required for the average person to understand in detail. So I will keep it short and sweet. This is where the timing wheel starts to come in play.Always measure blowdown in degrees of crankshaft rotation from the point where the first exhaust port (highest one) opens to when the first transfer port opens. By definition, that is blowdown.When designing the porting for its intended purpose, consider the percentage of exhaust port area, if any, that's above the rest - such as in staggered triple or quad E ports. Use that number to estimate its affect upon the overall porting layout. With power valve version engines consider valve up and valve down blow down measurements. Target the porting toward the best blowdown number for the engine speed and load it'll encounter the most. In all cases, start on the conservative side.I can't give you a perfect number for blowdown, but I can tell you that with the help of some software and a clearly defined target use for the engine, you can determine exactly what you need to have. Simply stated it'll be based upon the amount of air passing through the engine. That will be based upon your targeted use of the engine.Below is just and example. This again is done with a degree wheel.Have a nice day! Phone is ringing off the Chain. Stumper Intake duration = 160 Exhaust Duration = 167 Blowdown round 22
