I was think about this today while driving my pos capri, I know that v-8's give more power when they have little or know back pressure, but many 4 bangers need back pressure to run properly. I read some past threads on here about it and the from what I've read the less back pressure the better with the rotories. Is this true or is there a limit to how big your exhuast or how short you can make it.
I have some plans for a "street legal" staright pipe. In an article or post I read some where a guy used a Y pipe at the header one side of the Y goes to the exhaust system the other has a removable baffle that is closed under normal, every day operation. That baffel is remotely activated by an electric selenoid, which opens that side of the pipe giving a free but very laud exhaust. This gain could be quite great in our cars for the ocational drag from light to light or if your every day driver is also your track car.

 

you are thinking of a "cut-out". i have run this thru my mind as well. it can be a hassle tho, your car needs to be tuned for both. well depending on what your setup is. when i was n/a i had used both straight 2.25" piping thru two mufflers and then 2.5" piping thru one muffler. made a good difference. i liked the 2.5" for top end.

 

when i started up my engine with no echaust (only headers) she'd wind up.. then just conk out..

IIRC its because once you take ur foot off the gas, then the rotor has no real pressure on it so it keeps spinning, and coz u closed the carb (laymans) then its getting less fuel.. so it cant keep going and shuits down...

thats my interpritation anyway

 

This has been talked about countless times on this forum.

Anyway, no engine be it piston or rotary wants or needs backpressure. BUT, and this is where some people seem to get confused, that doesn't mean you can make the exhaust pipe as big as you want. The pipe needs to be kept down to a certain diameter (depending on the engine) to maintain good exhaust gas velocity.

 

Every car will make more peak power with less back pressure. The reason it screws up smaller engines so much is that just throwing a giant exhaust on slows exhaust velocity too much at low RPMs and the engines can't make much power down there. The same thing happens on V8s, you just can't feel it as much.

So you really need to have the correct diameter. Basically, don't try to flow more out then your intake / carb / turbo / whatever can flow in, and don't try to flow more in or out then your engine can use.

 

My car ran fine with only headers. I had to drive it all the way to the muffler shop LOL. it was LOUD!

 

yes.. loud.. i remember that...

 

not to mention scavenging, which is only possible with proper exhaust tuning.