Exhaust Flow: Volume verses Velocity

[Fendamonky]

So I was thinking...

When it comes to power and response which is better? Exhaust flow velocity, or exhaust flow volume?

Obviously there will be pro's and con's to both, I was hoping that the more knowledgeable and experienced minds here would be willing to share some information.

The reason I got to thinking about it was because I've seen some reputable shops that will leave the exhaust ports stock for "ported" engines. I figure there has to be a reason for that and thought it might boil down to volume verses velocity.

I have my thoughts, and my ideas which feel right in my head, but I know that I'm relatively uneducated in the realm of engineering and flow characteristics.

So please, share your educated opinions!

Cheers,

Levi

[FC Zach]

I had a very experienced builder/ racer tell me he leaves the exhaust ports stock (just cleans them up a little) and then explained to me why he did it this way. He used a water hose as an example. A water hose without an attachment (ported exhaust) flows a lot of water but at a slower rate of speed as compered to a hose with a nozzle (factory Port) which will move less water but at a more compressed and higher speed.

I imagine keeping ports stock will keep the EGTs at a different rate too, , something to consider as well.

Edit: This seems to be mostly a concern for the turbo guys where they want quicker spool up times, NA guys like myself probably wouldn't give a shit, for the most part (the bigger the better lol)

[rxspeed7]

^ kinda a bit off from normal logic, The main reason to leave ports alone is because og the actual port timing not the flow increase.

For a n/a application velocity is almost more important than flow gain(yes you want flow gain but you want the velocity to stay the highest) same goes for N/a intakes. Since you arent compressing the air into the motor you want it to enter and exit as fast as you can to pack more in.

For turbo applications velocity isnt as important cause your forcing air into the motor. You can make up for the slower velocity by adding more boost.With forced induction playing with the port timing can help tremendously on getting more air out and more in since your compressing it.

hope this maes sense. kinda hard to explain by typing.

[j9fd3s]

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.

[rotar bones]

That was a great explanation man.

[diabolical1]

it would be really great to get a decent discussion on this. everytime it seems like I notice trends or rules (on paper) to exhaust porting, I see something (in practice) that goes in direct conflict with it. for what it's worth, I think I've noticed that most seem to favor velocity over volume though.

the one thing that does seem for sure (and j9fd3s illustrated this) is that timing sort of dictates the confines of practicality for what you can actually do.