I know exhausts only make 3-4 whp in our car (my Comptech exhaust only made 3 whp ). What I don't get is why do we upgrade the exhaust and not the cat? What's the point of having a bigger exhaust, even when FI (turbo, SC, NOS), if the flow is limited to 223 cfm by the stock cat? Logically, the catback exhaust would help if the flow of the exhaust is less than the flow of the cat, right? But, that is not it! I would think that even the stock exhaust would flow better than the stock cat, right? Also, many people say that upgrading to a test pipe or a Carsound exhaust would only give me 1 hp at the most and other say 10 hp, which one is it? It does not make sense to me that a catback makes 3-4 whp, without the cat. Also a header makes 4-5 whp with a stock cat. What's the topic of a better header if the flow is restricted at the cat? So my logic says that upgrading the cat is a good thing and should make at least 3-4 whp, right?
I really need someone to enlight me, please! I work hard for my money so I hate to waste on a cat that only would give me 1 hp
Thanks.

 

thanks for your response and I agree/understann what your seaying. but it did not answer my question...

 

I think I made my topic hard to understand. I just read and I don't understand it myself!
I going to take a nap.... I'll revise my question on a fresh brain...
latezzzzzzzzzzzzzzz

 

A high flow cat will make a little, but only on top, and not really much under 6k at all, if any. To get the most out of the exhaust, you need headers and an intake. Then, you typically want to ad a TAD bit more fuel, and also adjust the cam gears by adding a bit more overlap.

 

the amount of gain with a hi-flow cat or test pipe is relative to the amount of modifications performed on said motor... if u have i/h/e... a test pipe may give u a hp or 2... if cams and hi-comp pistons are in place... u need even more flow than with the stock cat... hence u will gain even more... only because it is being even more restrictive... imagine... a built motor pushing 30 psi of boost with a stock cat... take the cat off and ull gain crazy hp... u cant even run a cat at that point

 

if I get cam gears and have overlap, does that increase flow?

 

thought this might help:
You can hurt your low end torque on a NA engine (or on a turbo
engine not under boost, though not as badly) if you make the size of the
primary tubes larger.

The exhaust flowing through your pipes is not a continuous stream, it's
a series of gas pulses. These pulses have mass, they have inertia, and
they have velocity.

Let's examine something else with lots of mass, inertia, and velocity -
a semi trailer truck. Ever been pulled over on the side of a road when a
semi goes by? If it's moving fast enough, the low pressure area behind
it can almost suck you back onto the road.

OK, now pretend you're standing in a primary tube as an exhaust pulse
goes by you. Same thing - the pulse tends to grab you and carry you down
the pipe, but if it doesn't, then low pressure area behind it might suck
you in behind it. The faster the pulse, the bigger this low pressure
area, just like the semi - except that as the pulse seals off the pipe
(think of a semi in a one lane tunnel) it has a _very_ low pressure
area.

Here's where we get tricky. Whenever the pipe changes diameter, you get
a reflection wave where part of the pulse bounces back up the pipe. And
thanks to resonance and some other gas dynamics, you get reflections of
the low pressure waves as well, sort of an "anti-wave" moving back UP
the exhaust.

If an anti-wave peak arrives at the exhaust valve just as it opens, then
the pressure just past the valve is lower than normal, and the reduced
pressure allows more of the exhaust to leave the chamber. This is
"scavenging", and it makes more power. In some cases, it makes a LOT
more power.

Now if we want to try and make sure that one of these anti-waves is
always outside the exhaust port, we have a few changes we can make.
Firstly, we can alter the resonance of the pipe by varying its length.
This works very well, but for a limited RPM range. This is "tuning" the
exhaust.

The other option is to make the anti-waves bigger, by increasing the
velocity of the pulses. Velocity is related to pressure, so the best way
to increase velocity is to reduce the volume (diameter) of the pipes.
The smaller the pipes get, the better the scavenging. (you can do it by
insulating the pipes as well, but there's a practical limit to this)
However, by reducing the pipe diameter, we also reduce total flow
capacity, so at some point further reduction is counter productive. Like
everything else, there's a compromise.
Bigger primaries trade low end torque for high end power. Race engines
need the top end, so they typically have big pipes. Street engines, even
hot ones, need the low end more than the high end, and so get more out
of smaller pipes.

Note that this does NOT apply to turbos under boost, nor does it apply
to the pipe(s) after the header collector. Once you've merged the
primaries, the bigger and freer flowing you get, the better. (The
collector acts as a wave reflector) For most 4 bangers, this translates
into a cat-back system.



[Modified by john mugen, 8:00 PM 8/21/2001]