exactly!!!...i suppose though most 12a'ers are not turbo'd so i suppose less is alittle better...but for the turbo'd guys...for those with good sized turbos...more is usually better

 

To hell with all that nonsense, just rip the hood and fenders off! Less fabrication work!

 

Yeah, I am running 2.5" across the axle...3" is possible, but you will probably need some good mandrel bent 3".

 

I'd go with 2" thickwall for NA or 3" stainless for turbo. Nothing else matters.

 

Anything over 2.5" is just going to be loud with an N/A and you wont really make anymore power.

With a turbo,a 2.5" system will actually support a fair amount of power.My old S4 13BT setup ran with a simple crush bent 2.5" system that worked just fine with around 250HP.When I upped to a ported S5 13BT with a bigger turbo,I opted for a mandrel bent, 3" stainless system.It fits fine over the axle,although I can see one little spot where the Watts link has kissed the pipe,although its light contact,no denting.

 

Wow, what muffler is on your car, Steve? That is a cool flange setup, btw.

 

i have had two rx7s with 2.5. id say to go with no more than 2" on a NA car as jeff has suggested. i also would normally suggest 2.5" with a turbo for a DD, but 3" would be fine in some cases also. it all depends on the set up.

 

I have 2.5 on my NA setup. Love it. Sure its loud, but thats why I love it.

http://www.youtube.com/watch?v=V1cPD1TyY1U

 

i had 2.5 on my NA also. i loved it!! BUT i think the car would have performed better as a DD with a 2". the low end would of been better. i dont care about sound

 

Steve, do you swap out your RB muffler for a straight pipe when you go racing?

 

The flange at the inlet is for easy swap over to my RB TII presilencer.That unit is a smaller, straight through muffler that is a full 3" from end to end.I had intended to use it for racing or whenever I "felt" like a little more noise.
But,with substantially more noise and no apparant gain in performance,I rarely ever use it.I like's em' quiet,although the turbo sounds pretty cool with the louder muffler.My direct comparison between my full RB muffler and essencially "no muffler" at all, furthur convinces me in the belief that RB mufflers are not restrictive.

 

That's interesting because the inlet pipe on the streetport RB muffler is only 2 1/4" and 2" on the '79-80 and '83-'85 versions. I figured since you had to squeeze your 3" pipe down to whatever the inlet size of your particular RB muffler, it would be restrictive enough to notice. Then I remembered that it is actually ok to shrink the exhaust tubing diameter a little bit towards the rear of the vehicle to account for the cooling effect that length of pipe will produce and the resultant contraction of the exhaust gasses. This is especially true on rotaries as the temperature differential inside the pipe vs outside is far greater due to the higher exhaust temps than in piston engines. To put it in other words, because a rotary has hotter exhaust gasses than piston engines, the temperature of the exhaust gasses leaving the exhaust ports on a rotary powered car is a lot hotter than when they exit the tail pipe. So if that is true, rotaries actually have something to gain if you reduce the exhaust pipe diameter somewhere toward the back. Steve, perhaps you accomplished this with your RB muffler because it maintains some velocity as it cools, and maybe that is why you didn't notice an improvement with the straight through 3" presilencer even though logically you'd think you should have.

Oh and if that's not enough 'proof' for anyone, the stock Mazda Cosmo and RX-4 exhaust systems reduced from 2" to 1 7/8" right before the rearend and continued into the muffler. From what I can remember of how my Cosmo drove before I pulled the stupid automatic and decided to rebuild the engine (so it's not running yet), the higher the revs, the more power it had; especially up hills. This is even with the thermal reactor and heat exchanger, which are a couple of corks in the system. Power just kept increasing as the revs went up. It was really surprising because the car was so dog-slow at low RPM. You'd think that a restrictive exhaust would hamper performance the higher it revved because it's simple physics. Heh think again. So much for the restrictive stock exhaust myth eh?

Velocity is most important.

 

does anyone have solid info on N/A short primary exhaust that collects right after header into a 2.5" pipe all the way back, vs the traditional 2.0" long primary exhaust that collects right before axle?

In terms of:
Width of Powerband
Peak HP
RPM Peak HP is at
Amount of noise supression
Tone quality

 

does anyone have solid info on N/A short primary exhaust that collects right after header into a 2.5" pipe all the way back, vs the traditional 2.0" long primary exhaust that collects right before axle?

In terms of:
Width of Powerband
Peak HP
RPM Peak HP is at
Amount of noise supression
Tone quality

 

I have not noticed anything about ground clearance here. My V8 car has 2 1/2 inch collectors and a Y pipe into a 3 inch, which goes all the way to the back. My exhaust guy is really expert at this, has been building headers and exhaust systems for drag cars and road racers for many years. He got the 3 inch over the axle with no clearance problems. Of course it is all mandrel bends - you would not expect to see anything else in a good quality exhaust system.

However, the forward portion does hang lower than anything else on the car. Mazda just did not leave room for anything bigger than 2 inch. So if you have the car lowered, that is certainly a consideration. If you have enough money, it is possible to buy ovalized tubing, which could help with the ground clearance.

And as Jeff said, the cooling effect means reduced diameter further aft is less critical. I would think this is especially true with a rotary because the exhaust temps are high, so the cooling is more rapid, assuming it is naturally aspirated. That would not be as much of an effect with mine because the header and Y pipe are ceramic coated inside and out, and don't conduct much heat.

And as someone said, with a 3 inch exhaust you will get plenty of noise. Someone once commented that my exhaust sounded like there was a hole in it, and I said yes, it has a 3 inch hole!