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Turbo Backpressure theory questions

 
Old 10-02-2009, 09:08 PM
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Default Turbo Backpressure theory questions

Hey everyone,

couple quick questions I'd love to start a discussion on regarding backpressure. I'll use a real world example as a start point to help me understand the answer.

Scenario A: Turbonetics T-72 S compressor cover, S trim F1 turbine w/.96 AR. Engine is a 231ci V6 FWD making 750fwhp @ 23 psi w/ non stock cam/heads,etc. Backressure before the turbo = 27 - 29psi. PR = 1.3:1
Scenario B: IDENTICAL turbo (literally taken off the V6). Engine is L92 (completely stock) on stock l92 manifolds, stock cats in place (only see 2 psi drop from the cats though) and motor makes about 600ish fwhp @ 8ish psi. Backpressure on this system is a solid 25 psi+. PR = 3:1

What I'm not getting is if the backup in the system is the wheel/AR combo, why is the same turbo wheel/housing holding back pressure at much lower HP in the truck? The *edit* compressor wheel is too small and working too hard to get it there? If that is the case, will a 1.3 AR help it, or make no difference since the wheel is working so hard either way?

Question 2: if you have 5psi backpressure AFTER a turbo (downpipe) does relieving that 5psi also release 5psi pre turbo, or does it release even more (compound effect?)?

TIA!
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Old 10-02-2009, 10:27 PM
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the bigger a/r will help alot but also look at wheel trim and flange size....what about wastegate, it could be wrong...lets assume thats correct

consider at 8 psi the turbo is flowing similar CFM as the v6 running about 15lbs so PSI doesnt mean anything. its cfm the motor is consuming

think the v6 on 15lbs is the same as the v8 at 8 lbs as far as the turbo sees it. the turbo only produces air/cfm to jam into the motor. the backing up of this CFM is the psi you read......

so if you had a perfect set of heads/turbo that could take all the cfm a turbo puts out your turbo would only make 0psi. But your car would make all the horsepower your turbo could produce and your exhaust pressure would also be close to 1:1. (note some turbos operate good with higher pr ratios like the borgwagner units)

now as i see it the exhaust side can not keep up because of the volume of exhaust it produces in the time it takes the compressor side to move enough cfm to produce similar or more horsepower is where the backpressure/hp thingy is skewed. this skewing causes excessive back pressure not only at peak but through out the range (just a small part of the 3:1 your seeing). this could be expressed in compressor surge or other variables and even as a bad turbo choice....as in this case. when a turbos hot side cannot move exhaust effectively and its backed up at compressor wheel due to the A/R, wheel trim, and/or size its effectiveness to evacuate the air at a rate that benefits the larger motor in a manner to produce more HP is FU*!ED (even at a lower rpm)

as for your after turbo back presure that confirms your turbo is tooooooooo smalllll

think about it you have like 30psi at turbo and only 5 after!!!! your housing is too small and your HP suffers because thats all the CFM it can evacute at a given time you take a reading and this very back pressure makes your V8 heads less effective than it would be with the right turbo (it would raise the HP per psi ratio)

so by now you know your turbo is wrong... a bigger motor will always make more horsepower all things being equal........ so pick the right turbo!!!!!!

p.s. im no expert but im right on this one !! try atleast a 76MM t4 maybe even a 80mm t4

Last edited by abbaskhan; 10-02-2009 at 10:50 PM.
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Old 10-03-2009, 08:51 AM
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thanks for the response. I'm still under the impression that backpressure in the exhaust post turbo has no corelation to the size of the turbo, but more to the restrcition of the exhaust system/muffler, etc. I'm just not sure if the 5 psi post turbo will equal 5psi pre turbo or if the effect is compounded since the turbo can now evacuate the air more easily.

Anyone else like to chime in?
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Old 10-03-2009, 11:26 AM
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You are correct that the 5 psi post turbo has nothing to do with the turbo and everything to do with what is behind it. If you had the exhaust side of the turbo open to atmosphere, there would be 0 back pressure. I think that it it would reduce pre turbo back pressure by a little more than 5 psi if you could completely remove the restriction. Why not disconnect the exhaust, check it again and let us know?
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Old 10-03-2009, 08:37 PM
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I'll post the results as soon as I do that test....

anyone else with input?
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Old 10-03-2009, 11:06 PM
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the turbo is just right or a tad big on the v6.On the v8 the turbo is a bit small.
5 psi after the turbo adds to the backpressure before turbo.
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Old 10-04-2009, 11:45 AM
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If you pull off a restrictive exhaust make sure you make some tuning changes in preperation. It will probably pick up quite a bit of hp and usually a few lbs of boost.
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