Black13B

iTrader: (1)

1986 13B GLX

but this is a rotary in general related question

i know my fare share of piston, but im learning about rotary now

what would be best for an exhaust system? total free flow (straight pipes) or would a muffler (either straight thru design, or what have you) be more beneficial to a rotary engine?

basically what im getting as is how does back pressure effect rotary engines? is a small amount beneficial or...?

Canadian Rotary Man

if u go with something like straight pipes u get a lot of noise and a lot of gain in high RPM Hp. Apparently u loose low end torque though.

The more restrictive exhaust the more low end torque and less high end.

well at least this is what i've heard. I still have my stock exhaust cuz i have to go through emissions testing

Stripgear

iTrader: (2)

Back pressure affects rotary a great deal. Think of them like you would a two stroke piston engine; more overlap, very sensitive to back pressure. Headers and a free flow exhuast alone can add 20% or more.

I don't think any back pressure is benefical from a preformance stand point, although some back pressure helps emissions.

Stripgear

iTrader: (2)

Also rotary exhaust is very loud and very hot. You cant really use regular thickness pipe, and if you use a straight thu system you probably get arrested.

Black13B

iTrader: (1)

ok thanks..

im just considering my options..

i dont really mind if the police dont like it, but i WOULD try to keep it legal, in regards to noise

essentially around here it doesnt matter how loud it is unless you have something representing a muffler's function on the vehicle

would a general striaght through design muffler be able to take the heat from rotary? or is the temperature different not as much of an impact..

diabolical1

iTrader: (3)

well you have to specify the application. backpressure may affect a rotary a bit more in terms of flow and power, but it affects it's streetability just as much as a piston engine. you will need some back pressure to give you the torque in a street-friendly RPM range. a lot of people seem to miss this point when they modify their exhaust. if you have a race-car or a car that you will drive only occasionally on the road, it's fine to run large-diameter pipes and minimal muffling, but if it's your daily driver, you need to keep some backpressure or you will be slipping the clutch for standing starts a lot more, as well as you'll probably have to rev out 1st and 2nd gears more than usual (try that in Manhattan, Seattle or Toronto traffic)

Stripgear

iTrader: (2)

Diabolical,

It is desirable to have a high velocity exhaust gas flow because the velocity and momentum of that flow tends to help purge (or suck) the exhuast from the combustion chamber.
So, when folks put on overly big exhaust components, the exhuast gas goes into a bigger pipe, slowing it's velocity, and lessening the purge effect.

If more exhaust gas is left behind, that would lessen combustion effiency and torque fo a street car.

Is this the same thing you are saying or am I missing something relating to backpressure?

Black13B

iTrader: (1)

okay i understand now..

what stock N/A 13B size pipes are equipped? what setups to people have around here, and how do you find them to be?

diabolical1

iTrader: (3)

well, as far as i "THINK" i know ... most of what you said is true. however, on the point of velocity and momentum, i'm not sure if you're saying what i'm saying. i don't think it's a matter of velocity, so much as it is about momentum since that would involve the amount (mass) of the exhaust gas in addition to it's acceleration (which it's velocity would be a function of).
let me say this up front, i may be totally wrong on this - so please don't take this as me saying it's the way it is.

the reason why bigger pipes are bad for torque (on a N/A application) is simply because they shift the torque further up in the RPM range. it's not that you necessarily make LESS torque. the larger diameter will still promote the scavenging effect, it just takes more RPM before the effect gets to a point where the engine will be using less HP to expel the exhaust and, instead, allow you to put it to the rear wheels. i think it is the momentum of the gas that's changing not the velocity.

we seem to completely agree that bigger is not better when it comes to exhaust tuning, but i'm also saying that total freeflow is not that great for torque either. i believe you'd suffer the same torque shift (upward in the RPM band) if you ran two separate 2.0" pipes that never collect as you might experience running pipes that collect into 2.5 or 3.0 inch crotch that runs out a single inlet muffler.

again, in racing where absolute HP is needed and you're going to be shifting like mad, up and down, it won't matter, but it will make a noticeable difference on the street in terms of noise and low-end. it's true that one can live with it (i ran a 2.5 inch system for about 3 years) but the decreased low-end is very noticeable compared to tuned systems like the RB systems which use 2.0 inch. that was all i was saying.