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Old 10-14-2007, 01:28 PM   #1
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Default Is 3" all the way a good thing!!

Hello, I can't figure out if i should go with 3" downpipe all the way or 2.5".
What the difference besides more air flow and looks.
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Old 10-14-2007, 01:42 PM   #2
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cost, exhaust volume, and fitting are the negatives of 3 inch vs 2.5.

Frank
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Old 10-14-2007, 01:58 PM   #3
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Quote:
Originally Posted by fmowry View Post
exhaust volume
You mean exhaust noise?

I'm the same boat.
at start I was tempted to do all 3" for future upgrades... but now I think that 2,5" is more than enough for what I want:gay:
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Old 10-14-2007, 02:31 PM   #4
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didn't we JUST have a thread about this?
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Old 10-14-2007, 02:56 PM   #5
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ohh sorry i didn't check to see if there was one
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Old 10-14-2007, 03:38 PM   #6
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3" isn't really any louder with all things being equal. You can have a near silent but well flowing 3" system.
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Old 10-14-2007, 11:34 PM   #7
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corky says that your downpipe MUST start at the size of your turbo outlet flange, and should only increase from there at no more than 15 degrees at a time. a sudden change in size, and therefore pressure, will result in a turbulent flow, and therefore more drag, making it less efficent than a smoothly changed diameter pipe.

after that, 3" would be better(no exhaust would be best) but a 2.5" system that is smoother with less restrictions would be better than a 3" system with shitty welds and lots of bends.
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Old 10-14-2007, 11:47 PM   #8
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I don't think it matters. Hondas make 700+whp with 3" turbo back exhausts. I doubt "turbulent" flow is a concern in their minds when it comes to the exhaust.
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Old 10-15-2007, 12:39 AM   #9
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Is there any dyno proof of corky's theory? I don't doubt the theory, but I do doubt that the turbulence has any noticeable negative effect on flow in a real-world application.
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Old 10-15-2007, 12:48 AM   #10
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There is no dyno proof as far as I know. As far as I'm concerned there is no such thing as non-turbulent flow exiting a turbine.
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Old 10-15-2007, 01:47 AM   #11
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Quote:
Originally Posted by Tesseracter View Post
corky says that your downpipe MUST start at the size of your turbo outlet flange, and should only increase from there at no more than 15 degrees at a time. a sudden change in size, and therefore pressure, will result in a turbulent flow, and therefore more drag, making it less efficent than a smoothly changed diameter pipe.

after that, 3" would be better(no exhaust would be best) but a 2.5" system that is smoother with less restrictions would be better than a 3" system with shitty welds and lots of bends.
I have a 3" vband welded to the turbine housing of my t3 60/63 and I can make 15psi just past 3500rpm on my honda 1.6l.

I'm all for 3" as soon as you can get it. To me its the best compromise of size/cost/fitment/sound etc...
On that note... I won't run any turbo car with less than 3" at this point.

Did I mention that the 3" off the turbo and all the way back... is on my beater?
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Old 10-15-2007, 01:50 AM   #12
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Toyota Supras start at 4" downpipes, and they make 5" and even 6" downpipes. You want a 3" downpipe.

BEGI's 2.75" DP starts at 2.25" or so and steps up slowly like Corky talks about to the end size. You can have them make it into a 3" DP at the end for another $30 or so. Order up a 3" Enthuza setup, and the car will be nice and quiet and flow very nicely.
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Old 10-15-2007, 04:09 PM   #13
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Avoid turbulence (easier said than done).
Avoid steps and abrupt transistions in size.
Mandrel-bent pipes.
Swage the pipes so pipe inlets fit "over" the previous pipe's outlet.
Avoid sharp turns or protruberances.
Use quality catalytic convertor and muffler.
Reduce backpressure as much as possible after the turbo.
Separate the wastegate flow from the main flow.
Don't forget you have to fit all of that under the car and into the engine compartment. The larger everything is the more likely the exhaust will bang on the frame under cornering or when the engine torques. Mazda Competition's racing motor mounts helped my car significantly.

My system that Corky custom-built for me?
Downpipe entry: 4 inches square area
Downpipe to cat: 4.9 inches square
Mid-pipe: 7.1 inches square
Muffler inlet: 8 inches square
Exhaust outlet: 9.6 inches square

And...he guaranteed me at least one horsepower increase over stock. Pretty good deal in my book....

Barry
PS. I'm still waiting to hear confirmation from Corky about the final details of the Bonneville Streamliner on which he modified the intercooling system. The fellow who ran that Streamliner and another one (the guy makes a good living) at Bonneville set a new class record and, first time at Bonneville, became the 8th fastest human on wheels. Corky does good work.

Now...time for me to get back to work.
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Old 10-15-2007, 04:33 PM   #14
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yep sounds like my new setup:

turbo is 2.25" outlet
DP 2.5" steps to 2.75"
test pipe 2.75" steps to 3.0"
catback 3.0" to the tip.
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Old 10-15-2007, 04:35 PM   #15
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Mines gonna be 3" turbo back.
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Old 10-15-2007, 04:36 PM   #16
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Quote:
Originally Posted by BEGITechRep View Post
And...he guaranteed me at least one horsepower increase over stock. Pretty good deal in my book....
Just wondering...How much did that one guaranteed horsepower cost? for more than one guaranteed horsepower
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Old 10-15-2007, 04:38 PM   #17
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Quote:
Originally Posted by neogenesis2004 View Post
Mines gonna be 3" turbo back.


you mean: 4" to 3" turbo back. :gay:
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Old 10-15-2007, 05:09 PM   #18
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thats what i said :P
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Old 10-15-2007, 05:30 PM   #19
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Quote:
Originally Posted by Savington View Post
Toyota Supras start at 4" downpipes, and they make 5" and even 6" downpipes. You want a 3" downpipe.

BEGI's 2.75" DP starts at 2.25" or so and steps up slowly like Corky talks about to the end size. You can have them make it into a 3" DP at the end for another $30 or so. Order up a 3" Enthuza setup, and the car will be nice and quiet and flow very nicely.
I used to make a turbo kit for the MKIV Supra, 3.1 inch ID on the DP, if I remember correctly. Which was an exact match for the turbine outlets we used. No difference on a car making 913 to the wheels when we switched to a 4 inch downpipe.
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Old 10-15-2007, 07:13 PM   #20
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Thanks BEGITechRep and others for stepping in and helping me out. Maximum boost and the BEGI website is fresh in my mind, but since I dont have a turbo yet, I feel like I dont have any real rep points here. when someone starts spouting their HP PSI details at me, ive got nothing to go on, unless they have tried scientific studies with various sized pipes.

i also am aiming for a similar goal as corky's engineering goal, which is the most HP per PSI, not total PSI. once I get my system up and running, then I can start bouncing numbers back when people say "ZOMG, ive got 30 more HP than you running 15 more PSI!!!1"
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