upgrading exaust, need to know about back pressure
 

will my ports still open if i get a dual straight exaust w/o cats, but keep the precat? or will i need to keep all 3 cat to get them to open?, im looking for the best power increse with the system. the pipes are like 2 1/2 i think, w/ mufflers and the exit after the mufflers is 3 1/2 i think.

Actually back pressure is bad... you want exhaust pressure- but that is simple a symantic.

anyway, you would need the main cat, (or a pre-silencer with a aux port hook up) not the precat to keep the ports working.

Mufflers and anything cat back will have little effect.

im getting a 3" cat back with i think a 4" muffler, will i lose a lot of low end?

Non-turbo or turbo, crush-bent or mandrel bent?

non turbo s4, all i know is that its an rsr full catback im getting for $200 from a friend.

You have a S4 car (at least your profile says so). So if you intend to run true duals, the only way you'll get enough pressure to open your ports is to feed a line directly into your header. It must be in the bend and point straight into your exhaust ports. A presilencer with the aux port tube will NOT make enough pressure to open your ports.

Anything bigger then 2" OD primaries on a header and or true dual piping will make LESS power on stock, mild, and large street ports and even 1/2 bridges. You might see a higher peak with slightly larger dia. but it will be very narrow and your car will be overall slower. Bridge ports, P-ports, and heavy nitrous use is a different story.

Any thing bigger then 2.25~2.5" piping on a single and or collected exhaust n/a be it stock, mild, or large street porting and 1/2 bridges will make LESS power. Its a HORRIBLE idea to use turbo 3", 80mm, 3.5", 4" exhaust on a N/A. It'll be LOUD and SLOW. Again, full bridges,p-ports and heavy nitrous is a different story.

This has all been proven time and time again with dyno backing over decades by very very reputable rotary engine builders/racers. Do your own searching before asking "were's your proof?" BS.

And back pressure is absolutely the enemy of exhaust systems of ANY internal combustion 4 cycle engine. The key to power, torque, drivablitly, and peak power is exhaust gas flow velocity vrs. flow, not "backpressure" or "maximum flow".

~Mike...........

OOPS, I just re-read the original post and miss-interpreted it the first go around. I thought you were going to true duals (single pipe coming from each rotor). You'll be running a dual exit cat back. You will have enough pressure to open your ports if you keep a main cat, get a new single high flow cat, or use a presilencer provided either has a tube coming out for it and there inlet/exit diameters do not exceed 2.5”.

~Mike..........

As far as i know, the racing beat presilencer will open the aux ports regardless of the catbacks. I ran it with no catback a little while, and it still opened the aux ports.

i say do it and if they dont open take care of that another way later, ..just what i would do but i like it simple

thx guys, i just bought the setup for 50 bones and just need to find a muffler shop now, thx