Quote:
Originally Posted by john marcella
I here a lot of people talking only about CFM. There is so much more to a head than the flow #s. Correct cross section and design. Then CFM.
And by the way I/Ex flow ratios are crap. Ex area vs HP and rpm and desirable Valve timing. Most of the good stuff on the exhaust side will happen when cyl pressures are so high the the air speed will be at the local speed of sound witch will react to area ,not flow #s. And as long as you can decrease the air speed to less than the local speed of sound by the time the piston is at BDC, there should not be much pumping losses. JMO
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Perhaps the way I've worded it is crap.
Yes All the exhaust "wants" to leave when the ex valve 1st cracks open. It cant due to restriction. A large % of exhaust AND pressure IS relieved by BDC even on a stock motor. But even on a race motor we are not likely hitting 100% pressure relief at end of bdc dwell. What doesnt get relieved adds to pressure on the upstroke due to exhaust restriction. This creates a pumping loss and also impacts intake column movement at overlap, overlap requirement, cylinder purge etc..
An "ideal" "goal" would be all exhaust pressure relieved by end of bdc dwell and no new pressure created on the upstroke. That would leave only swept volume to deal with.
Flow "balance" DOES play a role here. But not in the way most think. Also flow capability at low lift on the opening "side" of the exhaust is important. The more exhaust and pressure that can be relieved by end of bdc dwell the better. You can only open the exhaust valve earlier to a point untill it negatively impacts the power event. You can only open the ex valve so fast before you have issues doing that.
And you cant flow into the cylinder until the pressure in the intake port is greater than in the cylinder. Exhaust pressure influences that.
That leaves reducing restriction to flow as the next means to get to that "goal" of swept volume only on the upstroke of the exhaust. On the closing side of the exhaust event better flow can assist the purge that occurs during overlap. Until cylinder pressure is lower than intake pressure the intake charge cant move in. Good exhaust flow influences the purge at overlap and aids VE
Please, no offense to anyone,.. stating "I've acheived "x" with a low exhaust percentage" is ok for bragging rights. However it doent answer the question "what could I have acheived if I had better exhaust flow?". IMO you should look at percentages... but in a far different way. Perhaps from differing ends!