Paul,

What would be a safe top rpm for the 1989 stock turbo.  The do not exceed
speed.
Thanks
Rino

We don't know. My guess is around 100,000 +/- 10,000
I am going on the TO4_60-1 compressor map.
As I recall, Dave added the TO4_60-1 compressor shown in the
second jpg to the stock turbo and it fit. This loaded down the
turbine. It seems to be working so far.


Paul Lamar ...No rotor no motor.

Hi Paul,

Turbo overspeed seems like the culprit in turbo failure. The idea of a
turbine tach got discarded some months ago but I didn't see any reason.
Turbine speed could be cockpit monitored-- wow.. another needle to stare
at--or be fed to a processor (Picaxe?) to enable a change to be made
(electrically adjust the wastegate).

In several of Gordon's posts he's pointed to a ratio of turbine inlet
pressure to turbine outlet pressure. Nailing down exactly what the
turbine inlet pressure is may be impossible to do with repeatable (plane
to plane) accuracy. Do you think there is an influence from  factors
including  1) exhaust manifold shape and routing ahead of the turbo, 2)
fuel type varying from 87 octane and E-85 up to 100LL, 3) varying
mixture ratios, 4) compression ratios that vary with each builder, 5)
even ambient air flow within the cowling,  cooling the exhaust manifold
and thus reducing the energy available. The list goes on but the idea is
there may be as many variables as there are individual airplanes and
builders.

Gordon indicates the turbine speed is directly linked to that
differential perssure ratio so why not directly monitor and control that
ratio?  Possibly a pressure tap on both sides of the turbine linked to a
processor linked to the wastegate. Actual turbine size becomes less
important and can simply be a best guess or even what's
available........if you guess too large, the processor has little to
do, you've spent too much money, and carry excess weight, but turbine
shaft output power should  be at its best possible. If you guess too
small, the processor works like blazes constantly adjusting the
wastegate, and turbine shaft output power would never come up to the
potential available. If you guess just right.....Ahhhhh. But in all
cases the turbine will not overspeed and self destruct. Obviously an
educated guess is preferred over a WAG but it only needs to get you in
the ballpark.

For the processor challenged, consider this approach. Picture a normal
automotive  wastegate setup. The W/G lives just upstream of the turbo
and gets its gate positioning input via its vacuum diaphragm fed from a
vacuum line routed from the intake manifold. Normal so far....a drop in
manifold pressure moves the diaphragm which is fixed to a rod that moves
the W/G door toward closed to increase flow and pressure into the
turbine. But as you gain altitude the ambient pressure on the turbine
outlet is lessening so the differential pressure which Gordon has
pointed out the importance of controlling is getting a double whammy.
But what I propose is to go way downstream in the exhaust piping where
the gasses are  cooler and weld in a spud and support structure to allow
a butterfly valve to be installed inside the pipe ( a device similar to
the old exhaust "dumps" of street racers of bygone days). The spud
accepts a pressure diaphragm that has one side sealed from
ambient, the other side has a control rod attached to the butterfly. The
diaphragm senses pressure in the pipe and adjusts the butterfly to
maintain a fixed amount of backpressure in the exhaust system thus
providing a constant and consistant pressure for the turbine outlet to
work against and prevent overspeeding. The back pressure seen by the
engine should be unchanged within the limits of the turbine actually
doing something, after the turbine reaches its predetermined
differential pressure limit then the engine would begin to see an
increase in its back pressure, but at that point the system is max'ed
out anyway. Sounds confusing but think of it as a downstream wastegate
keeping watch on the upstream wastegate with no electrical power
required.                      Dave M

Yes and throw in the phase of the moon. :)
Brought to you from the campground of Sun n Fun by free wireless and
a 12 volt to 110 volt inverter.