is backpressure ever an issue on na rotaries? i know piston engines have to worry about it a bit, but for us how freeflowing of an exhaust would be toooo freeflowing....like would it hurt to put a 3" exhaust all the way thru my car????? not gonna do it just curious....thanks

 

Less restriction the better. You can't go too big though because you have to take exhaust gas velocity into consideration. 3" all the way would be too big.

 

if you pick up the latest issue of Sport Compact car there is a good article in there about back pressure and pipe sizes. If your to lazy or to cheap ill type it up later.

 

i dont think backpressure is the right word, its just hard for exhaust to get up to high velocity in a phat tube. I am running a 3" straight thru on n/a and it is too big, you notice at the low end, it takes a while to get the velocity up in the exhaust, then from 4000-8000rpm it is sweet as lol, I have heard twin 2" pipes would be ideal on n/a 13b ported... a resonator on each pipe, then have them join just before axle (properly timed) then over axle into a good freeflowing muffler.

 

Ok Im gonna type it up neways OMG i need another Red Bull "MY NAME IS MICHEL J. CABOOSE AND ...I..HATE...BABIES" if you dont know what thats from just die. OMG im hyper. lol. oh right the article


Exact words here.

Here's the phenomenon in a nutshell: Exhaust dosn't go out your exhaust pipe in a steady flow. At the tailpipe, after several cubic feet of pipes and mufflers, it's reasonably consistant, but in the exhaust port its a different story. Every two revolutions, the exhast valve opens , some air is pushed out, then the valve closes and the air has to stop. During the closing of the exhaust there is an opportuninty.

If you imaging the cylinder full of air that just got pumped into the exhaust as a soilid, it will be a big tube or column, of air. this air column has some intertia, so it isnt terribly thrilled about having to stop when the valve closes. As the vavle starts closin, the air column tries to keep going. This inertia can be used to suck the last of the exhaust out of the cylinder. And because the intake valve is starting to open as the exhaust vavle closes, this sucking also helps start the flow of fresh combustibles into the cylinder.

Interia is 1/2MV2(squared) so increasing the V (velocity) of the air column is the best way to increase inertia. Make the exhaust pipe smaller and jamming the same amount of air throught it will require one of two things. Either that the air will have to get compressed to fit into the smaller pipe (thats backpressure) or it will have to go faster all the way through in time.

In reality, both things happen. With relitivly low exhaust flow at low rpm, the velocity helps scavenge the cylinder. At higher RPM, though, it takes to much energy to make the exhaust go fast enough, and backpressure starts to build.

Generations of hot rodders have seen the small pipe/big tourqe phenomenon and wrongly assumed it was the back pressure that was causiong the low RPM benifit.

the whole air column inertia thing only applies to the header primaries anyway. As you get farther down the exhaust system, the air dosnt really have to stop when the valve closes, so its velocity is irrelivant. bigger at that point, is better at any RPM. There is a point where a bigger pipe wont help anymore, but you cant boost torque by going smaller.

I hope this helps some. Or at leasts clears up anything about back pressure. BTW Dave Coleman is my hero.