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.

 

what size is your downpipe?
does it expand for the rest of the exhaust system?
you really do not want to shave/cut the turbine or housing where the turbine is present there are tolerances that it needs to function properly
I would say try to "gasket" match the entrance and exit of the turbine
if you want to try doing something,but overall if you are ceartin backpressure is
your problem just break down and get a new turbine combination

 

SPANKTU

The dump pipe is 3,1". It is small but we had space issues and could not enlarge it. Now, we made some more space available and the dump pipe can be enlarged. Exhaust system divides into two main pipes after the dump pipe, each are 2,5". There is not catalytic converter, no resonator. Straight piping and an empty muffler. When we installed the rear muffler, a muffler with no internal louvres, no barriers, EGT dropped 150 F.

There is a separate wastegate outlet 1,6 " ID that goes out of the wg and connects later into the main exhaust system. Could this 1,6" pipe cause any back pressure problems?

 

you should try running a nice 3"cutout as close to the turbo as you can
this should help tonnes with your back preasure issues

 

if you're saying your wastegate is only one and a half inches in diameter, then yes I think that's where your restriction is.

 

you can experiment and try a bigger diameter pipe off your wastegate and see if that helps.

Otherwise you just a need a bigger diameter exhaust all together. I bet at 8200 rpms that car is expelling a lot of gases.

 

I have talked to garrett and learnt that the turbocharger I have now is too small and cannot produce 1,000 hp. I will either have to buy another turbocharger run in parallel or get a bigger turbo.

The whole story is because of the small turbine housing.