>From the Kitplanes article that Thomas J mentioned,  it seems that the turbo turbine needs to be oversized by a factor of 2 to allow for the fact that
 the
 airplane installation has a steady state throttle.   In a car application the throttle is never held open long enough to allow the TIT to build up to a
 hotter
 equalibrium value.  Therfore the "average" TIT in a car is a lot lower than in the steady state airplane application.
 Paul mentioned that another way to make the turbine appear bigger is to drill a hole allowing exhaust gasses to bypass the turbine, thereby increasing
 flow
 rate and lowering turbine inlet pressure and temperature.
 If the wastegate currently seems to have no effect on boost then maybe you could try removing the wastegate flap all together.
 The flap is most likely blocking the flow even when it is open.

 Regards, Erik

That is a pretty good idea. Unfortunately the assembly is welded together
so it might not be possible to restore it. I think modifying
the back cover of the waste gate to allow the flap to open 90 degrees
might be a better approach. At least a first thing to try.

I did a 3D of a steel spacer shown here.

Also a large 3 inch exhaust tube could be welded over a 3 inch hole
in the existing back cover to allow the flap more freedom to open fully.

A bit of porting on the waste gate might help.

Paul Lamar

What if you find that even with the fully open wastegate hole, the TIT and MAP is still too high at WOT?
If you have an old cracked turbo housing, you could try that on the dyno with the flap removed.
Then a least you know if you are in the ball park.

A further option is to try and fit a smaller diameter turbine to the existing stock shaft and housing.
What is Francois using in his turbo?
Erik

Well than if all else fails a hole in the turbine housing next to the waste
gate will work as a last resort.

Francois uses the stock Mazda 1991 2nd gen turbo.

A larger turbine is what you need if you want less boost.
It also dissipates more heat because it is larger.

Paul Lamar

As far as I can tell the goal for the aircraft turbo is to:
1. reduce rpm = less boost  and no overspeed
2. reduce turbine inlet pressure = less boost, less temperaure and longer life turbine

from the Kitplanes article, Turbo Tom says:
"... matching is the process of adjusting the size of the turbine to achieve the desired results from the compressor. The turbine powers the compressor, and
matching can be thought of as similar to changing the pulley ratio on a belt-driven supercharger. To slow the compressor, one would change to a larger turbine
nozzle or a larger turbine wheel. This lowers the operating pressure of the turbine and slows it down. Since the turbine inlet pressure is also engine exhaust
back pressure, this has the additional benefit of reducing the temperature and pressure in the exhaust system.

    To better understand the concept of turbine matching, picture squirting a pinwheel with a garden hose. If you use your finger on the end of the hose to
reduce the nozzle, the water comes out with more velocity and the pinwheel turns faster. What you might not realize, though, is that the pressure in the hose
(back pressure) has also increased. To slow the pinwheel down, you loosen your finger a little (larger nozzle). The pinwheel slows down, the pressure in the
hose is reduced, and the flow through the hose is increased. If you attach a shaft to the pinwheel and make it turn something useful, you have a turbine!

    Because we had already machined the turbine housing to accept the biggest wheel that would fit in it (a P-trim), and we were already using the biggest
exhaust housing available (0.83 A/R), we now tried putting smaller turbine wheels inside the machined-out housing. Making a fixed internal leak or built-in
turbine bypass called a wastegate were our only options to slow the turbo.
We ended up changing the turbine configuration five times to get where we wanted to be, but it works perfectly now. We just kept putting smaller and smaller
turbine wheels into the housing that was machined out for the big wheel, and eventually we got the right amount of boost. If you were to go by the catalog
application guidelines, we used the compressor recommended for a 100-150 cubic inch engine and combined it with a turbine recommended for a 300-350 cubic inch
engine (if we had kept the P-trim wheel in the P-trim housing). The Subaru is 133 cubic inches. "

The method that Turbo Tom used to reduce rpm and back pressure is to have a large turbine housing and nozzle with a smaller turbine wheel.
This meant more exhaust gas flow rate = less gas velocity, less gas pressure, less temperature, less turbo rpm, and less boost.
Erik

Well if money is no object go for it. You are looking at least $2000.
That is about what most people paid for their engine. We have something 
that works pretty good. All we need is a bit of fine tuning.

Paul Lamar
 
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