Exhaust Flow: Volume verses Velocity

j9fd3s · 01-02-2013, 02:48 PM

first, i don't claim to know EVERYTHING, about the subject, but i have read all the SAE papers, and been around a while.

there is a compromise in the exhaust port between port size and timing. the other consideration is reversion, and the shape of the exhaust sleeve.

the reversion thing is peculiar, if you look at a stock FD exhaust port and manifold, the port is smaller than the sleeve which is smaller than the manifold opening, and this creates an anti-reversion effect. gas flow OUT is unhindered, but gas flow IN is. at minimum you can say Mazda paid attention to the port match between the engine and exhaust manifold. this does make a difference!

with port size, bigger is better, but to the sides the apex seal needs a certain amount of support, and in height you're limited by port timing.

with port timing, the opening has an effect on BSFC, the earlier you open the port the shorter the power stroke. if you want examples the FD opens at 71 degrees and the max power full race engines open at 73 degrees, and the Rx8 opens at 50.

with the closing timing, you are increasing overlap. overlap is bad in a rotary. a little overlap might be good in a crossflow piston engine, just because the exhaust flow OUT can pull more intake IN, but the rotary just tends to pull exhaust gas into the intake stroke, which is BAD.

so port closing times? the early P port engines close at something like 65 degrees, in about 1990 they revised this to 55, which helps ADD power, even on a race engine without mufflers, and the FD closes @48. the Rx8 closes at 3 or -3, and this lets them open the intake earlier, and have less overlap than an FD.

so what this means is that the stock port has a little anti reversion, low overlap and adequate size, so it works really well, and can tolerate high backpressures.

you could do worse.

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01-02-2013, 02:48 PM	#4
j9fd3s Rotary Fanatic Join Date: Mar 2008 Posts: 123 Rep Power: 18	first, i don't claim to know EVERYTHING, about the subject, but i have read all the SAE papers, and been around a while. there is a compromise in the exhaust port between port size and timing. the other consideration is reversion, and the shape of the exhaust sleeve. the reversion thing is peculiar, if you look at a stock FD exhaust port and manifold, the port is smaller than the sleeve which is smaller than the manifold opening, and this creates an anti-reversion effect. gas flow OUT is unhindered, but gas flow IN is. at minimum you can say Mazda paid attention to the port match between the engine and exhaust manifold. this does make a difference! with port size, bigger is better, but to the sides the apex seal needs a certain amount of support, and in height you're limited by port timing. with port timing, the opening has an effect on BSFC, the earlier you open the port the shorter the power stroke. if you want examples the FD opens at 71 degrees and the max power full race engines open at 73 degrees, and the Rx8 opens at 50. with the closing timing, you are increasing overlap. overlap is bad in a rotary. a little overlap might be good in a crossflow piston engine, just because the exhaust flow OUT can pull more intake IN, but the rotary just tends to pull exhaust gas into the intake stroke, which is BAD. so port closing times? the early P port engines close at something like 65 degrees, in about 1990 they revised this to 55, which helps ADD power, even on a race engine without mufflers, and the FD closes @48. the Rx8 closes at 3 or -3, and this lets them open the intake earlier, and have less overlap than an FD. so what this means is that the stock port has a little anti reversion, low overlap and adequate size, so it works really well, and can tolerate high backpressures. you could do worse.