CatBack/ Exhaust size.

[FirebirdGT]

Originally posted by Adrenaline_Z:
"I am assuming that the larger the engine rating the higher the flowrate - safe bet?"

Here's a link to Pacesetter. I'm waiting for their
tech support to mail me the flow rates. Notice
the engine displacement vs. inlet/outlet size:

http://www.pacesetterexhaust.com/perf_exhaust.htm

"And that the inlet size is not relevant"

I'm thinking you're correct. From a marketing
standpoint, we can't be sure. From a design
standpoint, the inlet size will have no effect
because the catalytic internals will dictate the
rate of flow.

IE: A tighter 'honeycomb' will be more restrictive.

It's up to the mfg. how well they want it to flow
based on materials used, and emissions standards
they need to meet.

http://www.clearimageautomotive.com/.../V6Headers.htm
http://www.car-sound.com/universal/941.htm OR
http://www.car-sound.com/universal/940.htm

These are the pieces I am considering. Already have a B&B 2.5" cat-back.

[mjparme]

Between all the ad hominems I was having trouble following the conversation.

If I read everything correcly you are saying that a bigger pipe actually slows down exhaust flow because of less pressure and thereby increases backpressure?

That is what I gathered but I could have interpreted it wrong [img]smile.gif[/img]

Can the PCM be tuned to take advantage of a 3" exhaust?

[HBHRacing]

yes the larger pipe will slow volocity but it will not create backpressure. restrictions will do that. what you want is low restriction and high velocity.

PCM tuning will not make up for lost volocity due to large ehaust.

[Adrenaline_Z]

Mjparme,

This is probably one of the coolest animations
I have seen to explain velocity in a tube.

Scroll down to the animation. Click and drag
the yellow boxes up and down, left to right.

You'll see how the flow speeds up when the pipe
diameter decreases. You'll also notice the
pressure in the pipe drops which allows for
scavenging and more charge to enter the cylinder
Because they are evacuating sooner.

http://home.earthlink.net/~mmc1919/venturi.html

[Adrenaline_Z]

Robert,

I would contact the mfg. and get the flow rates.

The 941 series seems to be more free flowing by
desciption, but that's just a guess.

What I would do in your case is select the highest
flowing catalytic which meets your emission needs.

Get an inlet size that mates to the y-pipe.

If you need to reduce the diameter, do it AFTER
the catalytic. Install a reducer after that cat.
if you need to match pipe sizes.

There are two reasons for this, can anyone guess
why? It's been touched on already in this thread.

[RGB]

Originally posted by HBHRacing:
yes the larger pipe will slow volocity but it will not create backpressure. restrictions will do that. what you want is low restriction and high velocity.

OK, I get exhaust comes in pulses and you want to keep the pipe dia. small enough to keep velocity up so low pressure after the last pulse helps pull the next pulse out of the cylinder. (Just like having too large of intake runners is bad.) What I'm thinking is that is if you have something that damps the pulses (cat., turbo, muffler that you can't look through) you might as well go with the 3" because you've lost the scavenge effect already.

[Adrenaline_Z]

"yes the larger pipe will slow volocity but it will not create backpressure. restrictions will do that. what you want is low restriction and high velocity. "

Actually Harlan, the pressure will increase causing
a backpressure due to higher atmospheric pressure
in the pipe.

RGB,

You have it down pretty good. The timing of that
pulse exiting will depend on the RPM and length
of the pipe.

Pressure pulse lengths will vary throughout RPM.
The exhaust length will be resonant at one length
and create increases at certain harmonic lengths.

[RGB]

Originally posted by Adrenaline_Z:
Actually Harlan, the pressure will increase causing
a backpressure due to higher atmospheric pressure
in the pipe.

Needs a little refinement. 1) We know smaller and smaller pipes (higher and higher velocity) do not improve exhausts 2) With small or large pipe the atmosphere is not flowing into the exhaust.

[Adrenaline_Z]

"We know smaller and smaller pipes (higher and higher velocity) do not improve exhausts "

True

"2) With small or large pipe the atmosphere is not flowing into the exhaust."

Reflected pulses from hitting the atmosphere may
cause reversion. The waves may create a higher
pressure in the exhaust when out of the tuned
range.

A pipe with a larger volume is able to contain more
atmosphere.

The inertia to overcome that volume is greater
in a bigger diameter and length pipe.

[FirebirdGT]

Originally posted by Adrenaline_Z:
Robert,

I would contact the mfg. and get the flow rates.

The 941 series seems to be more free flowing by
desciption, but that's just a guess.

What I would do in your case is select the highest
flowing catalytic which meets your emission needs.

Get an inlet size that mates to the y-pipe.

If you need to reduce the diameter, do it AFTER
the catalytic. Install a reducer after that cat.
if you need to match pipe sizes.

They don't have that info, tried already. You confrmed what I was planning to do. However, as the 941 is smaller to do the same job - may have a higher pressure drop than the 940? They are both rated to the same engine size.

The 2.5" 940 is the one spec'd by Magnaflow for the F-body.

[Adrenaline_Z]

No clue, I was waiting for specs back from their
tech support.

If you say they don't have that data, then your
guess is as good as mine.

Someone should know this information, sort of weak
that they don't.

[mjparme]

This is probably one of the coolest animations
I have seen to explain velocity in a tube.

Scroll down to the animation. Click and drag
the yellow boxes up and down, left to right.

That is a very helpful and informative animation.

[RGB]

Adrenaline_z

You missed my point. Dropping pressure in the pipe using Bernoulli's principle can not be the guiding principle in exhaust design because then we would all have 1 inch pipes and the air around us (high pressure) would be rushing into the exhaust (low pressure).

[Adrenaline_Z]

Bernoulli is certainly not the ONLY, or may not
be the guiding principle, but it sure is a factor.

There are still considerations such as flow
calculations and capacity of pipes.

One inch is too small for 3.8L @ 6000 RPM
It might be fine for 3000 RPM and lower.

Three inch is too big for anything less than
6000 RPM. It would best for 6000+ applications

Two inch is probably just right for best power
from 2000-6000 RPM. This size creates enough
velocity at the specific range to make it more
efficient.

You all did see the sizing charts from Pacesetter
and Magnaflow? If you'd like more examples, I can
post pictures of the pressure diagrams from my
tuning text to help further.

[gojo83]

Re: CatBack/ Exhaust size.

well I may seem like a dumbass for asking this but is there a super big difference in using a 3 in pipe if it is crush bent vs using a 2.75 or 2.5 also being crush bent. I am going to be running pp heads and intake and ram air as well as a cia and headers. I live up here at a higher altitude and would that make a difference. I am not looking to build a monster but a nice 230 -240 if possible out of that and maybe a cam to get there.