i bought two "performance mufflers" that i put on my rx7 mostly for the sound but i'm not getting the right back pressure that i need. anyone know of a performance exhaust system that will get the correct back pressure for less than $500?

 

Im sure you can find a racing beat system or something similar for less than $500. Why not corksport? Only heard good about them. Just my .02

 

There is never necessary back pressure. Back pressure is evil and bad on a rotary engine.

If you are talking about that you have a S3 or S4 non turbo, that uses Exhaust velocity pressure to open the aux ports and they are not opening now; then you screwed up the exhaust flow with your "performance mufflers". This means you probably have less HP now than you did before you put on your "performance mufflers". Too big of a muffler or exhaust system will slow down the exhaust exiting from a rotary engine, and that slow down results in loss of exhaust velocity pressure.

 

^^ and as it slows down it cools.. which slows it even further as the air becomes denser and harder to move.. double whammy on the "performance" exhausts, add the rediculous heat of a rotary cooking those pipes untill crispy burnt and id call the third strike. go out n buy a RB system

 

I thought velocity and pressure were two different things? Velocity increase equals pressure decrease, that kind of stuff.
You do need a certain amount of pressure to open the aux ports so if you've lost it from your exhaust mod you aren't getting all the port timing your engine has. Other than that no engine itself needs backpressure to run better, they need exhaust tuning.

 

The RB back-pressure thing doesn't work with any accuracy. You might get lucky & have it work at all. The newer ones are supposed to be "better". Been there & done that. If you want your ports to work your gonna have to create something. Do a search... this is one of the most discussed subjects for s4 & s5 motors.

Ramses666

 

I thought velocity pressure was a kind of back pressure

So where exactly is the pickup for the auxillary ports? What's it's orientation? (which way is it facing?). How does it provide the pressure to push the auxillary ports open?

I mean, if it's just a tube in the side of the exhaust, then it should be activated by backpressure alone. Or if there's a reduction in diameter so that the velocity is high enough at the pickup tube, this should creature negative pressure at the tube (pressure = backpressure minus negative pressure, and might be less than zero). If it's pointing into the exhaust stream (towards the front of the car), then the pressure in the tube should equal the backpressure plus additional pressure from the exhaust velocity. I always assumed it was just backpressure but lately this talk about velocity is confusing me.

 

thanks for responses. icemark was right about me screwing up the exhaust so now i need recommendations for doing headers, and a catback system. so who knows of the right system to add horsepower and keep the back pressure/velocity (sorry new to working on exhaust and don't know what i'm talking about) correct?

 

Nope. Back pressure reduces flow. In essence in automotive applications (where you have the burn continuing through to the cat converter) back pressure would cause pooling and limit the exhaust gases escaping, or in worse case, even letting the gases be forced into another rotor face or the intake cycle.

Now the velocity of the gases escaping actually does two things. #1 it does create pressure waves enough to force open the aux ports in conjunction with the air pump, and #2 it helps scavenge out the exhaust out of the engine.

On the stock S3 and S4 systems, the pick up is in the middle of the cat, between the two reaction plates of the cat converter as it is used for two different functions (one to dump O2 into the cat at under 3800 and in 5th gear, and two for the exhaust pickup on the aux ports above 3800). On the RB pre silencer the pick up is fairly high up (closer too the engine), and the input to the pipe faces the incoming exhaust charge.

The key location will be determined on the exhaust itself, and how much is still burning and expanding. Too far from the combustion event and the is not sufficient expansion and pressure to force it's way back up the tube to the aux ports.

No... think of back pressure as the exhaust not moving out. It literally is the exhaust not escaping from the pipe as fast as it should or (in worst case) feeding back into the engine.

I think were most people get confused is the ideal of pressure and the ideal of back pressure. They think they are the same.

Maybe using a water hose analogy will help. If you have the hose just turned on, the water is flowing out at say 20 psi. If you close off the end of the hose to just a few drips, the water backs up in the hose (usually swelling it). This would be back pressure... the water wants to go some where but can't.

Now we put a spray end on the hose. Now the water can escape, but at a higher pressure so the hose barely expands. And the water coming out now can shoot across the yard instead of just dumping out of the end of the hose. It is coming out faster than it could just trickling out out or with an open hose end.

So again, we can have the water just dumping out (open exhaust)... or we can restrict the flow so it just trickles out (back pressure in the exhaust), or we can have the water shoot out (velocity pressure in the exhaust). Now I know there will be some bright HVAC or plumping engineer that will say my analogy sucks because I am not taking into consideration that the combustion event continues into the exhaust, but please remember I am just trying to explain the idea in very simple everyday terms.

When you put too big of a exhaust on a car (particularly a non turbo rotary) the exhaust just dumps out instead of being under any sort of pressure, and of course when you put too small or restrictive of an exhaust, you get just a trickle of exhaust out and major back pressure.

What you want is the exhaust to exit as fast as absolutely possible but not puddle in the exhaust. In an ideal world, exhaust exiting would be timed to stack up creating constant pressure, that any void in that exhaust stream would act as a vacuum for the next combustion event (and that is how tuned headers work and why almost all IC engines that are designed to make power have an exhaust system of some sort).

 

Um... wow... glad I went to aviation college, cuz all this velocity increases as pressure decreses stuff is making wish I didn't take my tests drunk. lol. but I sill remember a lil...