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Old 10-22-2006, 03:05 AM   #1
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Default Bigger Restriction:Exhaust size or Cat?

So, I have been looking around and searching. Corky says that the cat is generally the largest restriction. I also here many say that 3" is far superior to 2.5". What is your take on this? Right now I have a high-flow Magnaflow cat with 2.5" turbo-back. I have considered going 3" all the way, 3" midpipe back and just putting in a 2.5" straight section where the cat is. So whats your take?
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Old 10-22-2006, 03:13 AM   #2
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I would personally run 3" with a 3" cat or (if you dont get hassled for no cat) just a 3" pipe all the way from turbine outlet to bumper. I dunno the legal situation in OK.

The cat is definitely a bad thing. It traps heat and pressure on the outlet side of the turbine, which hurts spoolup and probably hurts EGTs as well.

You also have to consider what size DP you can run. The T3 turbine outlet is a lot smaller than 3" and most of the 5 bolt housings have 2.5" pipe connections (and that is with WG and turbine combined). I had someone open up the stock 5 bolt integral WG housing and weld up a 3" DP, but I honestly doubt that 2.5" or 3" makes that much of a difference in overall HP. 3" is good up to at least 6-700 hp, 2.5 is probably good up to at least 4-500.
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Old 10-22-2006, 09:40 AM   #3
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I am running a 2.5" all the way with no cat and I will say that the difference is chalk and cheese from the stock size. I am not sure that (for me) a change to 3" would make that much difference, but it really depends on the size of your turbo and how much it is flowing.

DB
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Old 10-22-2006, 12:38 PM   #4
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It all depends... As da Bajan said, you may not see a difference... Even on the dyno with an FM kit switching from a full 2.5" FM exhaust to a 3" dump pipe made absolutely no difference what so ever, but it was louder.... I still ran it on the street for a while and over to play at the Nopi Nationals that year.
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Old 10-22-2006, 01:35 PM   #5
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Read chapter 11 of Corky's book.

3.0" exhaust flows enough for 600BHP, 2.5" is good for 400BHP.

He's mentioned it a few times, however, his new exhaust is 3.0" isn't it.

He also writes that the cat is less than 2psi to the total backpressure in the exhaust.

and easy way to choose an exhaust size is 10% larger diameter than the turbo exhaust outlet
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Old 10-22-2006, 02:08 PM   #6
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Quote:
Originally Posted by braineack View Post
Read chapter 11 of Corky's book.

3.0" exhaust flows enough for 600BHP, 2.5" is good for 400BHP.

He's mentioned it a few times, however, his new exhaust is 3.0" isn't it.

He also writes that the cat is less than 2psi to the total backpressure in the exhaust.

and easy way to choose an exhaust size is 10% larger diameter than the turbo exhaust outlet
Actually the center exit exhaust under Ike's car is a full 3" system... the employees felt like building a 3" system, so they did... don't know if Corky had blessed it prior to them building it.

Mark
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Old 10-22-2006, 03:16 PM   #7
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Quote:
He also writes that the cat is less than 2psi to the total backpressure in the exhaust.
Measured at what rate of flow?

I'm sure my ******* only produces 2 psi of backpressure when I fart, but try to push 500 cfm out of it and I'd be in the hospital. Given the improvements people have found from removing cats (and especially pre-cats) from turbocharged vehicles, my hunch is that they have a fairly large effect on spoolup.

I'm not disagreeing with you or corky, just curious where that 2 psi figure comes from.
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Old 10-22-2006, 03:56 PM   #8
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Quote:
Originally Posted by beerslurpy View Post
I'm sure my ******* only produces 2 psi of backpressure when I fart, but try to push 500 cfm out of it and I'd be in the hospital.

:gay:

In my case the difference was around $60 between 2.5" or 3" mandrel bent exhaust. Not worth comtemplating and the exhaust has a nice deep sound.
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Old 10-22-2006, 04:08 PM   #9
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I don't know if I mentioned but I am using a Tial 38mm external with an atmospheric dump. I imagine that probably helps quite a bit in the exhaust flow situation.
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Old 10-22-2006, 04:14 PM   #10
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it helps with spool but not on the flow.
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Old 10-22-2006, 04:25 PM   #11
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Actually it does help flow. Thats a decent volume of exhaust gas that is no longer in the main exhaust system. Also as far as exhaust diamter goes....in my experience doing anything but a full 3" system has always left alot on the table. A buddies turbo 16v jetta made another 80 whp at the same boost level going from a full 2.5" to a full 3". Another buddies turbo Vr6 Gti made 40 more wtq going from a 3" dp into a catted 2.25" system to a 3" catless system. This was at only 9 psi.
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Old 10-22-2006, 04:34 PM   #12
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only if the wg is open WOT. bigger exhaust is better so long as header primary size doesnt get to crazy big yaeh.
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Old 10-22-2006, 04:37 PM   #13
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It helps with flow unless you have the WG barf back into the downpipe immediately after the turbine outlet. Joining two flows together will increase turbulence which hurts overall flow. If you get something like this just after the turbo, it will decrease the rate at which exhaust is evacuated from the DP. Not a huge difference, but I would say it does make a difference.

I am sure that I would have problems if I ran a small DP with my internal WG. It does dump straight into the DP, but the DP is so huge that I dont think it really kills flow that badly.

Look at what a lot of manufacturers like ferrari do with turbos- they collect all the WG outlets and give them their own exhaust. I thought about doing this, but my setup works OK (and is simple) so redoing it for minimal gains and coolness factor is low on the list.
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Old 10-22-2006, 04:39 PM   #14
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coolness factor is always on the list
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Old 10-22-2006, 04:47 PM   #15
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Open WG dump is cool the first few times you get people to fall off their bikes when you open it up next to them. After that though it sucks cause even if you wanna make a quick squirt on the freeway or something all eyes are on you. I like quiet.
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Old 10-22-2006, 04:48 PM   #16
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me to stealth speed is fun.
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Old 10-22-2006, 04:53 PM   #17
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My Miata was quieter than stock with the full 3". No exhaust noise. Just SSHHHHHHHHHHHHHHHHHHHHHHHHHHHHH, SHHHHHHHHHHHHHHHHHHHHHHH, SHHHHHHHHHHHHHHHHHHHHHHHH
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Old 10-22-2006, 04:57 PM   #18
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shhhshhshhshhhh and pwhuuu gotta love it my intake is pretty noisy more so than the exhaust.
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Old 10-22-2006, 05:02 PM   #19
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With my 2.5" the car is WAY quieter than stock.

DB
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Old 10-22-2006, 05:21 PM   #20
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Well FM did do that dyno comparison with their full exhaust, which is what im using, vs. a 3" dump and there were no changes, as MarkP stated earlier. That was what sold me originally on their 2.5". Of course that was before I knew how to weld and such. My car is also very quiet at the moment. Not quieter than stock, but very close.
BTW, my wastegate really isn't all that loud. It's hard for me to tell it from all the other noise.
-Michael-
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