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Old 12-06-2012, 07:48 PM   #1 (permalink)
 
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Low end torque loss with cat backs?

So I was curious if going from the 2 1/4" stock pipe size to the 2 1/2" of most aftermarket cat backs causes the torque to shift further up the power band?

I ask because I am currently trying to map out an exhaust for my 2004 GTO with an M6. I am thinking the most HP I am looking for would be say 400 at the wheels. With full exhaust(headers on back), heads, mild cam, Svede OTRCAI, and a tune when all is said and done.

My exhaust plan is to do SHORTY headers from JBA along with catted mids(inspection up here in RI) and then the cat back...I would like more tone than stock provides BUT did not want to lose low end torque by going from 2 1/4 to 2 1/2. I noticed that Borla is the only system, it seems, that uses 2 1/4 and NOT 2 1/2" pipe.

Any thoughts guys?

'Moe
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Old 12-10-2012, 10:03 PM   #2 (permalink)
 
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51 views and no one has anything to say?

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Old 12-10-2012, 10:11 PM   #3 (permalink)
 
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Catbacks don't really do anything for HP. Long tube headers will though. They will also make your stock exhaust system, which sounds great on the 04s, sound even better and louder.

The stock piping size is plenty large for your HP goals.
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Old 12-10-2012, 10:31 PM   #4 (permalink)
 
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Thanks for the information man. The problem with the internet is there is a massive amount of information to cycle through and much of it conflicts with other like information.

I was thinking a cheap way to get a bit more tone with my shorty headers would be to just replace the stock mufflers with a set of Borla pro XS mufflers and maybe add an X pipe. I am told by Borla that their Pro XS mufflers are somewhere between their touring system and their S type system in terms of volume. Street side auto has them on sale for like $70 each for a 2.25" pro xs muffler.

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Old 12-10-2012, 10:49 PM   #5 (permalink)
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Moe,

I would suggest the shorties, high flow cats, remove the resonators and add an H-pipe for a muscle car sound. The H-pipe is said to have better low end torque. The X-pipe will have an exotic sound and may produce a couple hp more than the H-pipe. If you want a little more volume then replace the mufflers.

JMHO

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51 views and no one has anything to say?

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Old 12-10-2012, 11:06 PM   #6 (permalink)
 
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Thanks for the input JMHO.

I am not looking to turn this into a track super star. I just want to play with it and keep it as my DD and gain a few horse in the process.

I think I can convince the boss(read as wife) to let me swap the mufflers out and grab a set of shorties. I am tempted to pick up an OBX cat back but frankly I feel like if the concensus is the stock 2.25 tube is ample to support my 50hp goal then why spend the money to change it? I think a simple muffler swap and the headers will yeild some nice tone without going crazy and going to Borla or a muffler from MRT there should be minimal drone (if any at all) at the dreaded 2,000 RPM range. Drone is my biggest concern sound wise. I had a FM 40 Delta flow many years ago on a truck and it sounded great but man was that thing a big red flag to cops and MAN did it D.R.O.N.E. It was like driving a boat around.

My hangup with 2.25 tube is I am not sure if that will give me the warm baritone sound I am after. Not looking for raspy.

Forgive my ignorance here but does the 04 have resonators? It seemed like it has 4 cats. 2 right after the manifold and 2 more further down stream. Also wouldn't taking off the resonators decrease low end torque as well as make it louder?

'Moe
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Old 12-11-2012, 06:09 AM   #7 (permalink)
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About your original question regarding low-end torque: It's hard to make specific predictions because so much depends on the details of where it's at NOW. In general though, yes there are boundaries between, "too small", "big enough", and "too big".
"Too small" makes the engine have to work harder to push the exhaust gases through to overcome the restriction, and that costs torque/power. Most folks understand that intuitively. "Too big" also will hurt torque/power because the engine has to work harder as well, but for reasons not immediately obvious. It has to do with exhaust flow velocity. Once those gases get moving, their inertia tends to make them want to keep moving. High flow velocity is your friend in this case. Get the exhaust passages "too big" for the engine though, and velocity slows down to the point where now the engine again has to work to "push" them through - you lose the benefit of that flow velocity and the inertia that results from it.
That's why it's hard to make specific recommendations: there's no way to know for sure where you're at on the "too small" - "just right" - "too big" spectrum right now.

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Old 12-11-2012, 12:24 PM   #8 (permalink)
 
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Quote:
Originally Posted by BearGFR View Post
About your original question regarding low-end torque: It's hard to make specific predictions because so much depends on the details of where it's at NOW. In general though, yes there are boundaries between, "too small", "big enough", and "too big".
"Too small" makes the engine have to work harder to push the exhaust gases through to overcome the restriction, and that costs torque/power. Most folks understand that intuitively. "Too big" also will hurt torque/power because the engine has to work harder as well, but for reasons not immediately obvious. It has to do with exhaust flow velocity. Once those gases get moving, their inertia tends to make them want to keep moving. High flow velocity is your friend in this case. Get the exhaust passages "too big" for the engine though, and velocity slows down to the point where now the engine again has to work to "push" them through - you lose the benefit of that flow velocity and the inertia that results from it.
That's why it's hard to make specific recommendations: there's no way to know for sure where you're at on the "too small" - "just right" - "too big" spectrum right now.

Bear
He is looking for 400hp per his origional post. It is safe to say with his goals in mind, stock exhaust size is plenty.
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Old 12-11-2012, 12:42 PM   #9 (permalink)
 
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I know from having built very large industrial boilers as a boilermaker that the exhaust starts at one diameter tube and will end at a tube much smaller. Same principle with an HVAC system. The main is much larger than the lines coming off of it. The idea is to keep the air flowing at a certain rate...not to much and not to little. I feel like dual 2.5" is more than enough to support far more HP than I plan to make. My goals are to put down 350 AT the wheels. I just want a DD that has a touch of street cred.

I feel like I might grab the OBX exhaust and just use it for the tube and put different mufflers on from MRT in Michighan. Not sure anyone fabs custom SS exhaust near me.

Honestly I am 100% sold on the SS exhaust. I had plenty of cars with standard aluminized steel tube and they lasted 4-5 years before they needed attention.

'Moe
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Old 12-25-2012, 07:27 PM   #10 (permalink)
 
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correct me if i'm wrong, isn't smaller diameter catback produce more back pressure which would result in more torque downlow?
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