Does some back pressure help the turbo spool up quicker? - Diesel Forum - TheDieselStop.com
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Upgrades and Aftermarket - 99 & up 7.3L Engine Upgrading or adding OEM or aftermarket equipment to your 1999-Up Super Duty or Excursion with 7.3L Power Stroke diesel engine. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

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Old 09-25-2005, 11:52 AM   #1 (permalink)
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Does some back pressure help the turbo spool up quicker?

I read on one brand of exhaust system for a 7.3 that the downpipe went from a 3.5" in to a 4" to keep back pressure so that the turbo would spool up quicker. Is this true?
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Old 09-25-2005, 12:03 PM   #2 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

The lower the pressure in the downpipe the better.

Here is a simple example with "made up" numbers. If you flow 1000 CFM thru your turbine at 30 PSI inlet pressure and 0 PSI downpipe pressure you will need 32 PSI inlet pressure to flow 1000 CFM with 2 PSI downpipe pressure.
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Old 09-25-2005, 03:37 PM   #3 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

The less backpressure the better. Contrary to popular belief, backpressure is detrimental to any engine!

Exhaust waves and velocity are what make an exhaust system work.
Going to too large of a down pipe may slow velocity and cool the air making it more dense (harder to push out of the exhaust).
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Old 09-25-2005, 06:55 PM   #4 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

I just laugh when people say you need backpressure for an engine to run strong. Best running gasser I ever had would light up the tires in a 3/4ton cheby with diff lock, and auto straight off the headers, after installing a dual exhaust and adding backpressure it would never spin the tires after that, but it was quieter. The big 2000hp engines I work on run dual 10" exhaust not much backpressure there, but with dual turbo setups it has all the backpressure it needs without any exhaust.
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Old 09-25-2005, 08:57 PM   #5 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

All depends on what your doing with it. If your pulling and want to keep some low end you want as little back pressure as you need to help spool the turbo quicker.
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Old 09-25-2005, 09:11 PM   #6 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

Wow, the crap is thick in this thread!

Match the ID of your exhaust dp to the ID of your turbo outlet. That will give you the best of everything. For a stock turbo/ BB that would be 3.5". Then have a gradual increase to 4" somewhere down the line w/ a nice open type muffler.
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Old 09-25-2005, 09:15 PM   #7 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

Yep, a low amount of backpressure will help your top, middle, and low end power! Take a look at this article Exhaust Theory
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Old 09-25-2005, 09:16 PM   #8 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

You guys keep kicking this around, I think some of you are talking about preturbo and some post turbo. I just checked mine preturbo. From 0 psi to 33 psi my preturbo and boost gauges rise equally. My boost stops at 33 psi and back (drive) pressure continues to rise to 50 psi. I've checked for leaks. Pressured the system to 20 lbs and found 3 small leaks. Pressures are still the same, it does spool a little faster.
My question is how should they rise in relation to each other? And how much drive pressure is too much.?


PS: Suns I think I need that HX50.
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Old 09-25-2005, 09:32 PM   #9 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

I thought we were talking about post turbo exhaust pressure (in the exhaust). However, if in fact we are talking about pre (drive) turbo exhaust pressure, the same rules apply. Less backpressure is better.

Also, if know both your pre and post turbo backpressures, you can add them together to find your total exhaust pumping pressure (pressure that is trying to slow your engine). For instance, if you are pushing 50 psi pre turbo and 2 psi post then you have a total of 52 psi trying to slow your engine down.

In a perfect world you would have 33 psi boost with 33 psi drive (pre turbo) pressure. But we don't live in perfectville and we run into turbos with lowered efficieny. Therefore, drive pressure will always be more than your boost.

However by using ball bearing turbos, better compressor wheels, better exhaust housings, etc. we can lower this pre turbo drive pressure a little bit.

That being said, 50 PSI does seem a bit much drive pressure. I would think your wastegate would open before then. I don't know though [img]/ubbthreads/images/graemlins/shrug.gif[/img]!
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Old 09-25-2005, 10:52 PM   #10 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

[ QUOTE ]
Wow, the crap is thick in this thread!

[/ QUOTE ]

I'd have to agree there Sun![img]/ubbthreads/images/graemlins/biggrin.gif[/img]

The less pressure after the turbo the quicker it will allow it to spool up period. On a turbo application there is all the back pressure to the engine that it could ever want and then some so there is no worry about haveing to little back pressure to the engine preturbo. HOWEVER on a N/A engine depending on the appliation and the power desired some back pressure is needed. If you want an all out top end screemer then the less back pressure the better. BUT if you want a good tow rig with a broader powerband and more power down low some back pressure is needed so that scavenging of the cylenders and such is more effective at the lower rpm ranges. But none of this really matters here because it doesn't apply to our application.
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Old 09-26-2005, 12:04 AM   #11 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

On a turbo the less back pressure the better from the turbo outlet on out. It's the pressure differential that spins the turbo up, NOT back pressure.
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Old 09-26-2005, 08:25 AM   #12 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

[ QUOTE ]
You guys keep kicking this around, I think some of you are talking about preturbo and some post turbo. I just checked mine preturbo. From 0 psi to 33 psi my preturbo and boost gauges rise equally. My boost stops at 33 psi and back (drive) pressure continues to rise to 50 psi. I've checked for leaks. Pressured the system to 20 lbs and found 3 small leaks. Pressures are still the same, it does spool a little faster.
My question is how should they rise in relation to each other? And how much drive pressure is too much.?


PS: Suns I think I need that HX50.

[/ QUOTE ]
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Old 09-26-2005, 09:27 AM   #13 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

[ QUOTE ]
Wow, the crap is thick in this thread!

Match the ID of your exhaust dp to the ID of your turbo outlet. That will give you the best of everything. For a stock turbo/ BB that would be 3.5".

[/ QUOTE ]

You've never really measured one, have you? If you have you need a new yard stick.
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Old 09-26-2005, 11:21 AM   #14 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

[ QUOTE ]
Quote:
--------------------------------------------------------------------------------

Wow, the crap is thick in this thread!

Match the ID of your exhaust dp to the ID of your turbo outlet. That will give you the best of everything. For a stock turbo/ BB that would be 3.5".


--------------------------------------------------------------------------------



You've never really measured one, have you? If you have you need a new yard stick.

[/ QUOTE ]


For those who are now curious. The answer is 2 3/4" Actually just a bit over. Maybe 2 13/16 or 77 mm
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Old 09-26-2005, 01:58 PM   #15 (permalink)
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Re: Does some back pressure help the turbo spool up quicker?

[ QUOTE ]
[ QUOTE ]
Wow, the crap is thick in this thread!

Match the ID of your exhaust dp to the ID of your turbo outlet. That will give you the best of everything. For a stock turbo/ BB that would be 3.5".

[/ QUOTE ]

You've never really measured one, have you? If you have you need a new yard stick.

[/ QUOTE ]

Hah, definitely got me there! [img]/ubbthreads/images/graemlins/grin.gif[/img]

I've only played w/ slightly larger turbos and witnessed the improvements to be had w/ matching the exhaust housing size to the dp size on my specific turbo. Of course my results fell perfectly in line w/ the textbook description given to me by others in the know.

Remember, I'm not really a mechanic, I just play one on the internet!
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