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Discussion Starter #1
Finally we have found out what keeps us from boosting 30 psi and what causes high ehaust gas temperatures: EXHAUST GAS BACK PRESSURE.

Engine is inline Euro M3 3,2L. JE Forged pistons with 6,8 cr. Turbocharged and front mount intercooler air to air. Fuel octane was 105.

Turbocharger: Garrett. Exhaust turbine GT40. A/R 1,06. Wheel trim 84. Wheel diameter 2,16/2,36.

We had very safe AFRs such as 11,5:1. But EGT was 1450 F at 140 mph. The pull was made from 62 mph till 140 mph. At 140 mph EGT was 1450 F. We backed off and did not want to see higher than 1450 F. Timing could be advanced and we advanced timing. Starting from 4000 rpm till redline at 8200 rpm, timing was increased gradually and from 5000 rpm till 8200 rpm 10 degrees of advance was dialed in. We were able to decrease EGT 50-70 F but were afraid to add more timing advance at these high boost levels.

WE measured the exhaust back pressure before the exhaust turbine of the turbocharger. At 26 psi of manifold boost we have seen 63 psi of exhaust back pressure.

The ratio of exhaust gas back pressure to intake pressure should be maximum 2:1. Is this correct?

Now my question is: Can I shave off the exhaust turbine of the turbocharger in order to reduce exhaust gas back pressure?
Shaving may be done in three ways:
1- Shave off the exhaust turbine housing's entrance, this is the area where the exhaust gases come into the turbine.
2- Shave off the turbine housing's snail shaped curves.
3- Shave off the area where the turbine housing is bolted into the dump pipe.

Which method would reduce back pressure?

There is one more option which is expensive: as I have a Garrett, the exhaust turbine can be changed by a GT40 housing with 2,45/2,68 wheel diameter or with a GT42 turbine housing which has 1,22 A/R, 78 wheel trim and 3,03/3,43 wheel diameter.


What should I do? I would like to solve this back pressure issue by just shaving off the exhaust turbine.
 

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What type of exhaust sytem are you running (post turbine), freeing this up will reduce your EX/IN pressure differential.

For example stick your pressure probe inthe DP about 8" away from the turbine outlet.

Say if you have 7 psi as measured in your DP at the levels you describe, if you reduce this to 2 psi you will reduce the pressure in your exhaust manifold by around 20 psi !!!

I would measure this first to make sure you exhaust system is not overly restrictive, if this checks out the only best way to reduce your ex manifold pressure is to increase flow rate capacity in the turbine whell/exhaust housing.

You realy want to be running a 4" dia exhaust with little muffling if any to support that BHP range without creating too much back pressure in the post turbine exhaust system, seperate exhaust for the waste gate also helps greatly if you have not done this yet.
 
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The backpressure is a fundamental result of the turbine housing and wheel you are using, with too much mass air flow. You need to go bigger.

What compressor are you using?
 

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The easiest way is remove the exhaust system (just run the DP) and do your test again. If the pressure ratio is too high for your car then you need to increase flow in your turbine section. Best way is a larger exhaust wheel trim.
 

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Discussion Starter #7
The car was naturally aspirated and we had space problems in the turbo conversion.

The dump pipe will be enlarged.

I have talked to Garrett and was told that the charger that I have now is too small for 1,000 hp. I need either a bigger turbocharger or two of the current one.

The current charger's exhaust turbine and housing cause the main restriction. Sure the complete exhaust system must be redesigned and enlarged.

What do you say for getting 2 turbos instead of a single big?

2 chargers mean damn a lot of piping. But less lag. Bigger turbo means more lag, less piping.
 

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2 turbos will not necessarily equal less lag. You will be spinning twice the wheel mass and splitting your exhaust such that each turbo only spins with half the cylinders unless you go sequential and THAT is alot of piping. Put a bigger (single) turbo on it...
 

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Don't DOUBLE POST. This is your 1st warning.

Adam
Mod

EDIT: 2 turbos is not necc less lag at all. Please read my write up on that, I think in the FAQ section.

Adam
 
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Twin turbos will spool faster if the comparison is apples to apples... in other words, don't pair two 10-year-old junk yard turbos up against one brand new GT turbo and call it a fair shootout... if the twins and the single all share similar wheel technology (aerodynamics and materials) then the twins will out-spool the single... it's true that each of the twin turbos will only get half of the exhaust energy but the inertia of the wheel is proportional to the 5th power of its diameter -- a wheel that is only 15% larger in diameter will have twice the inertia of the smaller wheel... so small changes in wheel diameter make huge changes in spool...

second point, a single turbo is more efficient than twins because the tolerances inside the housings are roughly the same, independent of the turbo's overall size... this means that 1/8" of clearance, for example, on a 15" housing amounts to a higher efficiency percentage than 1/8" of clearance on a 6" housing...

bottom line: once you get the things spooled, the single will produce more efficient boost, but the twins will make boost sooner...
 

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My exhaust back pressure was ~16 psi with 16.5 psi of boost at most RPM points. T66 .81 a/r P-trim on a totally stock Supra engine. Exhaust backpressure post-turbine was 1 psi at redline (where the exhaust manifold pressure was 17 psi) and significantly less at lower RPMs. This is with a cast iron log manifold. I tested another car with a very similar setup and got very similar results.

Did you test backpressure directly after the turbo? Remember the turbine opperates on a pressure ratio like the compressor.
How does the turbo spool?
What kind of manifold are you using?

Is the compressor in its effeciency range? Obviously the turbo's hot-side causes backpressure, but remember the less effecient a compressor is the more energy it takes to drive it.

The way I look at it is that the boost threashold of a larger (heavier) compressor is the same as a lighter compressor, but the transient response is different.

I did a 3000 rpm spool test in 3rd gear with 3 Supras, all of which had .70 a/r P-trim hot side turbos. 1 Supra had a SP60(59.0/76.2mm inducer/exducer), 1 had a T66 (65.5/91.0mm), and 1 had a SP63 (63.4/84.2mm). All 3 cars spooled the same, neither pulled any noticable difference. Of course there were variables that can effect spool. The 63 and 66 cars had stock fuel systems, MAFs and zero tuning. The 60 car had a VPC/SFC. But my point is that I think the turbine side impacts spool MUCH more than compressor wheel inertia.
 

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Discussion Starter #12
Grant,

I talked to SP and also Induction Motorsports.

I am about to purchase a new turbocharger.

We measure backpressure right before the turbo's exhaust turbine.

My exhaust manifold is hand made. 6 header pipes merge into 2 main pipes with 1,9" ID each. These two 1,9" ID pipes merge into one 2,8" single pipe and this goes into the exhaust turbine.

The headers are on the right hand side and the turbocharger is on the lef hand side. This sounds awkward but we had no place to locate the tc.

The exhaust manifold has many bends and curves. So, these bends may cause restriction and increase exhaust back pressure.

On the other side what would you reccomend me to buy?
PT 74
PT72
T 88
SP 76
SP 74 HO

Thanx
 

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For 1000hp crank or rwhp? 74ho is a good turbo, but prob won't do 1000 wheel hp. Crank, no prob.

Adam
 

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Is the SP76 the same old 76 everyone has always used? If so then it shouldn't flow more than a 72, and less than a 74. And it should surge more. This is all judging by the compressor maps, YMMV.
 

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Discussion Starter #16
The choices are narrowed.

I can either buy a water cooled ball bearing one or a non ball bearing one. The subject about ball bearing has been covered on this forum. OK but why do people pay extra for the water cooling and the ball bearings? I do not care about the life. The cooling of the shaft and the oil and the sleeves preserves oil quality + less heat under the hood + what else? Non ball bearings cost 1/3rd of the ball bearings.
I want to remind one thing: here in Istanbul, TURKEY we race at weekends at night time. We can go as fast as 200 mph. Sometimes you go at 190 mph for more than 20 seconds. Then go down to 150 mph, accelerate back to 180 mph. race and race with SL55s, Modenas.... I know none of them are at my and your power figures but we race and go as much as the road enables us. To summarize, we do not drag race, nor do we pull from 60 to 150 mph once or twice. The whole car including brake system, engine, drive train must hold and be durable. In other words, the turbocharger will be under load for longer time periods on my case. Would this requires a cooler working turbocharger? (I have some mpegs on www.racingflix.com. On those mpegs the car was a miserable Supercharged 415 hp one. You ll see what we mean by autobahn racing. By the way new mpegs will come soon.)
 
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Sounds like a job for water/methanol injection to me. :)

Keeps the motor cool on long periods of WOT.
 

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mert said:
The choices are narrowed.

I can either buy a water cooled ball bearing one or a non ball bearing one. The subject about ball bearing has been covered on this forum. OK but why do people pay extra for the water cooling and the ball bearings? I do not care about the life. The cooling of the shaft and the oil and the sleeves preserves oil quality + less heat under the hood + what else? Non ball bearings cost 1/3rd of the ball bearings.
I want to remind one thing: here in Istanbul, TURKEY we race at weekends at night time. We can go as fast as 200 mph. Sometimes you go at 190 mph for more than 20 seconds. Then go down to 150 mph, accelerate back to 180 mph. race and race with SL55s, Modenas.... I know none of them are at my and your power figures but we race and go as much as the road enables us. To summarize, we do not drag race, nor do we pull from 60 to 150 mph once or twice. The whole car including brake system, engine, drive train must hold and be durable. In other words, the turbocharger will be under load for longer time periods on my case. Would this requires a cooler working turbocharger? (I have some mpegs on www.racingflix.com. On those mpegs the car was a miserable Supercharged 415 hp one. You ll see what we mean by autobahn racing. By the way new mpegs will come soon.)
wow i never knew turkey was that intense on street racing! i say call up a major shop on this board, eg. SP or MVP and they'll hook you up with what you need, you might want to try derek's methanol kit to lower your EGT's.

-Brett
 

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Discussion Starter #19
Neonafk,

The EGT was a by product of high exhaust back pressure. So I have to buy a turbocharger with a bigger exhaust housing and decrease the exhaust gas pressure. At the end EGTs will go lower.
 
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