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Back pressure test results, when is it too much?

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Old 04-16-2012, 05:26 PM   #1
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Default Back pressure test results, when is it too much?

Ok so I made a real nice piece to install in an exhaust bung I welded in right before the flange to the turbo. I hooked it up to a boost gauge and did some street testing.

Comp 80/79 t4 on a 1.32ar
9.5 compression 370
Stock ported 241 heads
Stock ls6 intake and tb
232 236 115lsa 609ish lift cam
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9lbs intake, 20lbs exhaust

14lbs intake, 30lbs exhaust

Tried 18lbs intake but boost gauge only goes to 30. Id say it was at least 35 probably 40lbs back pressure.

Now I'm running a good size ar with a 3.5 dp. I think this is high but then again I bet everyone with a t4 has to be right around the same.

Is this something to worry about?

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Old 04-16-2012, 05:34 PM   #2
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I would have thought 2:1 wasn't too bad ,
I'm sure mine would be way worse 383, t76 f168 exhaust wheel t4 1.32 ar
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Old 04-16-2012, 05:43 PM   #3
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2:1 certainly isnt good. But on a low boost setup provided that pressure ratio offers fast spool, then wouldnt worry too much

However if it is laggy and with 2:1, something is wrong. It certainly isnt the d/p size.

I'd be much happier around 1.5:1 EGBP pre turbine vs Intake pressure
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Old 04-16-2012, 06:12 PM   #4
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This is also running an open cut out about 3 feet after dp.
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Old 04-16-2012, 07:27 PM   #5
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Stevie, what do you consider low boost?
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Old 04-16-2012, 08:55 PM   #6
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It's nothing to 'worry' about, and those numbers are pretty normal for a single T4 on a 370.

Also consider that for every PSI lost to the cold side is another pound of backpressure. (You can reduce exhaust backpressure by increasing cold side efficiency as the engine doesn't need to drive the turbine as hard).

It goes without saying that you could pick up a ton of power/rpm with twins or a good T6 based single.
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Old 04-16-2012, 09:25 PM   #7
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Quote:
Originally Posted by INTMD8 View Post
It's nothing to 'worry' about, and those numbers are pretty normal for a single T4 on a 370.

Also consider that for every PSI lost to the cold side is another pound of backpressure. (You can reduce exhaust backpressure by increasing cold side efficiency as the engine doesn't need to drive the turbine as hard).

It goes without saying that you could pick up a ton of power/rpm with twins or a good T6 based single.
Also wouldn't a T4 with a larger turbine wheel also help? I know guys make good power with the 75mm & 80mm borg warners because they use the 87mm turbine wheel.
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Old 04-16-2012, 10:24 PM   #8
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Quote:
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Also wouldn't a T4 with a larger turbine wheel also help? I know guys make good power with the 75mm & 80mm borg warners because they use the 87mm turbine wheel.
Absolutely a higher flowing turbine would help.
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Old 04-17-2012, 12:45 AM   #9
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Any idea on actual power numbers? Awhile back I talked with comp on that turbo, they claimed it'd do over 1000rwhp on a 347 ls motor(thru an auto), but I just don't see it happening.
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Old 04-17-2012, 05:52 AM   #10
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777 whp at 17psi with this setup.
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Old 04-17-2012, 06:40 AM   #11
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Jim, I'm going to look into getting a better quality IC and correct the way the cold side is routed to it. I have a harsh 90 on both sides going to the IC.
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Old 04-17-2012, 02:55 PM   #12
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Quote:
Originally Posted by SPRAYED 01 View Post
Stevie, what do you consider low boost?
Anything below 15psi is pretty low really, and the 8psi he mentioned certainly is.
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Old 04-17-2012, 03:25 PM   #13
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In the reading that I have done:
(Intake:Back Pressure)

1:1 is the Ideal state

1:1.5-2.0 is what you would normally expect

1:>2.5 would be unacceptable
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Old 04-17-2012, 04:50 PM   #14
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I would try and aim for 1.5 or less. But again it is very dependant on spool and power delivery.

I'd sacrifice some for a fast spooling setup at the expense of a little power. Or if you wanted ultimate power at higher rpm's you'd sacrifice some spool to achieve the lower pressure with a better flowing turbine.
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Old 04-17-2012, 06:49 PM   #15
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JohnV, what kind of power does your car make?
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Old 04-17-2012, 10:17 PM   #16
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700 rwhp @ 16 psi, M6 & spools very quick....
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Old 04-18-2012, 06:54 AM   #17
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In my opinion you should've measured back pressure downstream (at the downpipe), to get a better overall view of how effecient the turbo/exhaust system is as a whole. High pressures upstream is not as detrimental to performance as it is downstream. In fact higher pressures in the header helps turbo performance more than it hurts. Look at what a quick spool valve does and how much higher it raises perssure, yet improves spool-up.
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Old 04-18-2012, 10:46 AM   #18
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Hmm, not sure if that's right. I always thought you measure before turbine because it would show a restriction to the turbo.
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Old 04-18-2012, 05:52 PM   #19
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Quote:
Originally Posted by SPRAYED 01 View Post
Hmm, not sure if that's right. I always thought you measure before turbine because it would show a restriction to the turbo.
You are quite correct, you want to determine how efficient the turbine is.
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Old 04-18-2012, 06:34 PM   #20
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i know a guy in va with a 365ci that has a 80mm compressor and a 87mm turbine t4 1.25 ar and at 11 lbs boost he had 14psi bp iirc
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Old 04-18-2012, 06:34 PM
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