Dynoed and wondering what a Cat would do

[hoveringuy]

Judging by the dynos I did with my motor you won't lose anything significant, maybe 1/2 hp. I think 3" is too large for that motor.

[lolcantturn]

From my experience, gaining a cat didn't do ANYTHING on the butt dyno. Also on my friend's S52 with a full 3", he lost shit tons of low end torque, it was more than noticable.

[VinniE30]

You want exhaust gas velocity coming out of the exhaust ports to be as fast as possible.
At lower RPMs, gases will move faster through a smaller diameter pipe.
At higher RPMs, past a certain point the smaller pipe will be restricting flow, and create back pressure, so it will be slowing down gases from flowing out the exhaust ports as well as they could be. A larger pipe will relieve that back pressure.
Substitute RPMs above for engine output as well.

So there is a point of diminishing returns for a larger diameter exhaust parts with any engine, even at high rpm, unless it was designed to run with no exhaust.
Finding the ideal exhaust design depends on the output of the engine and what engine speed you want to optimize for.

I would get the high flow cat but not the 3 inch exhaust.

[WADE30]

So I am getting mixed opinions here.

Maybe I should just get the hi-flo cat and re-dyno and we can observe both dyno charts. The before (3 inches all the way no cat) and after ( 3 inches to hi flo cat).

[WADE30]

Oh and the reason why it is 3 inches all the way is because I eventually want to go turbo, so I was just planning ahead when I did the swap.

[hoveringuy]

Oh and the reason why it is 3 inches all the way is because I eventually want to go turbo, so I was just planning ahead when I did the swap.

That makes sense. Post your dyno data when you get it!

[BoostinEuros]

How is this relevant? How many threads did you have to shit in to get 2579 posts in and year and a half?

[nando]

So what you're saying is, whether I have a straight 3 inch pipe or a catalytic converter in place, the end result is the same?

This was all for curiosity's sake, very interesting.

sort of.. there are still resonance effects in the rest of the exhaust. theoretically, if your headers and merges were very finely tuned, and you threw on an exhaust after the final merge, it would alter the tuning. Adding a cat acts like a sort of filter to "hide" the rest of the exhaust from the tuned section. I don't know that a "normal" header setup would make as much of a difference, though.

On the other hand, I don't want my car to stink, and a good cat really doesn't add a lot of restriction. it also quiets things down a bit.

backpressure definitely does not increase torque - velocity does. it just happens that if you increase the size of your primaries and secondaries too much, you lose velocity. it still has nothing to do with the exhaust after the headers merge together... except that people do all kinds of wierd stuff that their headers aren't really working like they should either. I doubt the backpressure = torque myth will die anytime soon though..

[WADE30]

Wow. Incredible Knowledge there. Thanks for clarifying. I have a V-band setup so luckily I can remove the middle straight section and put in my cat for emissions, etc.