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Old 01-16-2009, 09:57 AM   #1
craigs
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Default Down Pipe Size vs. Power Loss or Gain

I am in the process of changing turbo location on a customers car.
Working closely with both the engine builder and the car owner, the discussion has been brought up regarding down pipe size.
In moving his turbo from the driverside to front and center (Forward Facing) I have ran into a situation where the optimal 5" downpipe will no longer clear the motor plate and still allow the bumper cover to clear the compressor housing.
So my next choice would be to neck down to 4", as I can get a tighter CLR bend and have plenty of room to clear the motor plate.
My concern was that the 4" would cause some restriction with either back pressure or power loss.
So being the inquizative person I am, I started making some phone calls. First to Precision turbo. Although they stressed the use of the 5", I didn't get a clear answer as to why or why not the 4" would work.
After speaking with a few more companies I get pretty much the same response, yet no clear answer as to why necking down the tubing size will or will not affect performance.
Does anyone have a clear answer on this subject? Has anyone actually performed any comparisson tests that can prove or disprove this theory?
Is there a mathmatical formula for calculating optimal tube size?
I know there are a ton of variables, as each engine is different, but there has to be some facts associated with determining the correct size.

Any help will be greatly appreciated.

Thanks in advance,

CraigS
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Old 01-16-2009, 10:31 AM   #2
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Here is my personal theory:

5" downpipe = 19.63 sq inches of area which would be the same as dual 3.5" pipes.

Now if I was working with a 400" motor with a 101 turbo making 2000 hp, then I sure would want a 2000 hp N/A motor with at least dual 3.5" exhaust (obviously bigger bit we are limited to a 5" outlet on the turbo).

Using a 4" downpipe in the same calculations results in 12.56 sq inches and the same area as dual 2.82" pipes.


Now obviously this is my backwoods redneck way of calculating things. Hopefully someone with more expertise than my one night stay at the Holiday Inn Express will chime in

Chris
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Old 01-16-2009, 11:07 AM   #3
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Gimme the cid of the engine, the redline you plan to use, and what sort of boost pressure you plan on. I can run the numbers and let you know the velocity and backpressure on 4" and 5" pipe.
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Old 01-16-2009, 11:17 AM   #4
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Can you fit 4.5"? Thats what we put on the s10 build, same type deal, turbo location vs engine location...
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Old 01-16-2009, 11:23 AM   #5
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Quote:
Originally Posted by DaveH View Post
Gimme the cid of the engine, the redline you plan to use, and what sort of boost pressure you plan on. I can run the numbers and let you know the velocity and backpressure on 4" and 5" pipe.
Cid = 352
Redline= 9000
Boost = 25lbs

his is with a Precision 98/85
he will also be switching out with a true 94 to run Orsca EZ street and will probably see around 30 lbs with that unit.

Thanks for your help. Do you mind sharing the formula that you are using to determine the velocity and backpressure?
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Old 01-16-2009, 11:32 AM   #6
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Quote:
Originally Posted by Frank02L View Post
Can you fit 4.5"? Thats what we put on the s10 build, same type deal, turbo location vs engine location...
Actually I have concidered that. Does anyone make a flange that will go from 5.00" to 4.50" or did you flare the 4.50" tubing?
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Old 01-16-2009, 11:43 AM   #7
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I was wondering the same thing .
I have 4.5 V-bands on my exhaust out and I am wanting to go to 4" aluminum .
I am having a hard time finding 4.5 tube in stainless or aluminum that is not crazy money . I can build it in 4'' aluminum for 60% cheaper and easier to fit.

Looks to me like this guy here runs 4" and he goes pretty fast.

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Old 01-16-2009, 11:53 AM   #8
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Quote:
Originally Posted by craigs View Post
Actually I have concidered that. Does anyone make a flange that will go from 5.00" to 4.50" or did you flare the 4.50" tubing?
I think we machined our own flange, IIRC.
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Old 01-16-2009, 12:45 PM   #9
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how about sectioning the 5" elbow to tighten up the CLR?
taking a few "pie cuts" out of the short side might get you the room to use the 5" without reducing it
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Old 01-16-2009, 01:42 PM   #10
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Quote:
Originally Posted by go green View Post
I was wondering the same thing .
I have 4.5 V-bands on my exhaust out and I am wanting to go to 4" aluminum .
I am having a hard time finding 4.5 tube in stainless or aluminum that is not crazy money . I can build it in 4'' aluminum for 60% cheaper and easier to fit.

Looks to me like this guy here runs 4" and he goes pretty fast.
Keep in mind that on twins you only have half the engine's exhaust to worry about.

Chris
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Old 01-16-2009, 02:02 PM   #11
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I make some wild ass assumptions on the engine ve etc and then plug in the flow into a duct sizing software I have. at redline:

4" pipe - 8.5 psi of backpressure per 100' of pipe

5" pipe - 2.6 psi of backpressure per 100' of pipe.

Quote:
Originally Posted by craigs View Post
Cid = 352
Redline= 9000
Boost = 25lbs

his is with a Precision 98/85
he will also be switching out with a true 94 to run Orsca EZ street and will probably see around 30 lbs with that unit.

Thanks for your help. Do you mind sharing the formula that you are using to determine the velocity and backpressure?
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Old 01-16-2009, 03:10 PM   #12
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Quote:
Originally Posted by DaveH View Post
I make some wild ass assumptions on the engine ve etc and then plug in the flow into a duct sizing software I have. at redline:

4" pipe - 8.5 psi of backpressure per 100' of pipe

5" pipe - 2.6 psi of backpressure per 100' of pipe.
So if you have 5' of pipe, the difference is .425psi backpressure on 4" and .13psi backpressure on the 5"???
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Old 01-16-2009, 03:11 PM   #13
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Quote:
Originally Posted by DaveH View Post
I make some wild ass assumptions on the engine ve etc and then plug in the flow into a duct sizing software I have. at redline:

4" pipe - 8.5 psi of backpressure per 100' of pipe

5" pipe - 2.6 psi of backpressure per 100' of pipe.
You also need to remember that the backpressure seen at the engine is equal to [(pipe backpressure + ambient)* expansion ratio of turbine] - ambient pressure

So if you have an expansion ratio of 2.5, your engine backpressure will for the 4" will be (8.5+14.7)*2.5 - 14.7 = 43.3 psi

For you 5" it will be (2.6 + 14.7)*2.5 - 14.7 = 28.55 psi

So, the biggest exhaust you can get in their, the better. A lower engine backpressure will decrease your BSFC and increase your VE, resulting in more power.
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Old 01-16-2009, 03:12 PM   #14
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Quote:
Originally Posted by Frank02L View Post
So if you have 5' of pipe, the difference is .425psi backpressure on 4" and .13psi backpressure on the 5"???
no the per 100' is the Length of the pipe.
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Old 01-16-2009, 10:47 PM   #15
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On my car, I have 5" pipe out of the turbo and continues on about 3.5 feet. Now I am planning on reducing at the end of the 5" down to 4", and get a 4" in/out oval straight through muffler and a turn down. It is just too loud with the 5" to drive on the street. How much am I hurting by doing this?
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