dj55b

iTrader: (2)

I think that my other past wasn't going that good so i decided to start another one. What do you guys recommend for piping for a 12A engine? Do you think 3" for a N/A is too much? Will i loose power?

gsl-se addict

iTrader: (3)

I think about 2.25" is what you want for an NA engine. Too large won't help peak power but will be louder and can hurt low end because of lack of the scavenging effect. Turbo is a different story. 3" would be good on a turbo application. You want the exhaust to be small enough to keep the velocity up (scavenging effect) but not so small that it is a restriction up top.

dj55b

iTrader: (2)

really that small? how much off the low end will it hurt though? alot?

BlurComet

Depends on what else your planning on doing to the car. 3" is a bit much for a STOCK port 12A, with the normal 7k redline. However, increasing the air flow (Bigger carb, adding forced induction, built for higher revs, porting) would certainly make the 3" exhaust worth your time (and I do mean time, since there are complications with installing a 3" system if the car sags on old suspension or is lowered). If your looking for just simple mods like I/H/E I would stick with 2.5"

Edit: You might want to also consider the volume increase you will have by adding either a 2.5" or a 3" straght pipe. It will be loud.

dj55b

iTrader: (2)

so maybe i should return the exhaus that i bought and exchange it with the one that has a 2.5 inlet? or could i do this :

Install 2.5 inch piping all the way to the exhaust and from there attach it to the 3" inlet with an adapter ... so that later down the road i can just replace the piping

would that provide the back pressure required?

gsl-se addict

iTrader: (3)

I think that Mazdatrix for that above 2.25" didn't help power. I believe that this was on a stock 2nd gen. As BlurComet said, mods will require larger exhaust. So 2.25 to 2.5 would be best for most situations. It is not a case where bigger is always better.

Edit: That would probably be good. Do you have a header? What about a pre-silencer? Basically, you want good velocity for the low end, but still support the flow required up high. It does become an issue of backpressure in the real world, but it is not really backpressure that you want. As they say "Backpressure is the devil".

dj55b

iTrader: (2)

alright ... but would the idea that i mentioned above work though? Cause the difference between the one with a 3"inlet and 2.5" inlet is 3$ difference

85rotarypower

On a stock engine, 2.25" is plenty big enough. I had a custom 3" exhaust on my 7 with the stock carb and port size and it seemed like there was enough torque in the low end (could start the car off in 4th gear!!), but there was no difference when I went down to 2.25". Now, on my new engine, which is a streetported 6 port 13B, I'm going with 2.25" duals that collect just before the rear muffler. I think it should keep a nice amount of low end torque while still having enough flow for high RPM. Its going to be kind of an adaptation of RB's streetport system.

BTW, you should straighten out your terms a little. It seems that what you refer to as an "exhaust" is just the muffler. The exhaust system is the muffler, header, and all the pipes in between. Also, no car needs backpressure. This is just a common misconception. What your car does need though is exhaust velocity to help "pull" the exhaust gasses out of the engine. This is called scavaging. In a perfect world, an exhaust system would have no backpressure, while maintaining proper velocity of the exhaust to get optimum scavaging. But since this isn't possible, we stick with the optimum of what we can do.

In short, a 3" exhaust pipe will create very little back pressure on the small 12A, but the velocity of the exhaust gasses travelling through the pipe will suffer greatly and the scavaging effect will be lost and in effect you will loose power. With the 2.25" exhaust, you will have more backpressure, but velocity will be greater and help with scavaging and make more power. What we can do to free up the exhaust a little is to avoid restrictive mufflers and catalytic converters, this will help keep backpressure at a minimum for a 2.25" exhaust.

FB II

i ran 2.5" on my first n/a setup. no cats, RB header, straight thru muffler. later on i went to 2.25" and it was noticably weaker up top. if you can stand the noise, go 2.5" all the way.

dj55b

iTrader: (2)

Alright so 2.5 it is then. Thanks everyone for all the info.

dj55b

iTrader: (2)

alright so i got the pipe in yesterday ... had quite some trouble getting it home, i bought a 10' ss pipe with 2.5" diameter ... that looks so big, i hope it doesn't end up being too big. I still have to wait to get the header so that i can bend it. now would you guys suggest making it all one piece from header to exhaust or 2 pieces? Also would you guys recommend weldingi t to the body or should i just have it attached like stock form with the rubber mountings (which one will be the quietess)

jayroc

I also have 2.5". There is no noticable loss at low end that I can tell. Compared to stock, anyways.
I did have to put a "3 stage" dynomax muffler in line and I lost flow at high speeds (70mph+).
How much, I can't say for sure. Was noticable in a race, though but worth the tradeoff for noise.
There is definitely a lot of noise if you plan to run only one muffler.

Surely, you're not talking about welding exhaust pipes to the frame?? Extremely bad idea.
Assuming you got a header, adapt the 2.5" tubing to it and run a single pipe to the rear muffler, attempting to use the stock mount points as necessary. The autostore has hangers and adapters that work well. You could always rig it up like I did and use bailing wire and metal tape. Crude, but effective..and cheap.

REVHED

For a stock port I'd say 2inch piping for the header and 2.5inch after the header would be ideal. For something like a 13B bridge or pp you could go up to a 2.25inch header and 3inch after.

REVHED

Back pressure is never desirable in any way shape or form. It's all about scavenging and velocity.

vipernicus42

iTrader: (4)

When you're talking about rotaries!

I don't know anything about piston engines, but I hear piston engine people talking about needing "backpressure" all the time.... god knows why.

I know the 6-port engines need backpressure to activate the 6-port actuators... but there are ways around that. Wiring them open and using your air pump for the pressure are two ways.

Jon

dj55b

iTrader: (2)

alright then things sound promising ... does anyone know what the output of a road race RB header is ? is it 2 -2" outputs or is it reduced to 1 output?

So as far as bending ... should i be bending the exhaust the exact same way the stock setup was? Also would you guys recommend in cutting it up and doing it 2 pieces (one from header to just a bit before the axle, and the second one from there to the muffler. )

na88lx4d

I have a rb 13b race header and it was cut to fit my 12a and a 2.5 flange was put on the end of it and i have a 2.5 exhaust and a rb primaflow high flow and noise is pleasent and and very streetable. btw my 12a is 1/2 bridged,holley 600, rb intake w/ plenum, ect. with all the goodies and i dont have problems with it the exhaust is perfect for my setup I hope that helps u