[Diy_efi] Turbo compressor selection, do we do it wrong?

[Grant Beaty]

Lets say we are selecting a compressor with two goals in mind:
1) Not to surge if it spools by the target RPM
2) The least amount of exhaust back pressure over the target RPM range
We will assume we have a bigass 1000+ ci FMIC core, that removes almost all
the heat from the intake charge.

Conventional compressor selection involves selecting a wheel that is
effecient over the RPM range you want, and doesn't surge. I'll looked at a
lot of dyno graphs, on a lot of Supras (mk4, 3.0L I6 2JZ-GTE), with a lot of
different turbos, and one thing always seems to stand out: Bigger is better.
Bigger compressors just seem to make more power, even if they aren't really
any more effecient, unless you assume what would seem to be overly
optimistic VE (ie 115% or more).

I always wondered why, and I always wondered how much shaft RPM had to do
with everything. Then I saw this quote from Garrett's GT catalog:
"This efficiency is simply the percentage of turbo shaft power that converts
to actual air compression."

That got me thinking, isn't the difference in velocity between the exhaust
gas and the turbine wheel directly proportional to the torque the exhaust
exerts on the shaft? If you have two compressors of the same effeciency and
the same hot side, but one spins faster than the other, the slower-spinning
compressor would also have a slower-spinning turbine wheel. Wouldn't the
slower-spinning turbine be able to opperate at a lower pressure ratio than
the faster spinning one, and thus cause less power-killing exhaust back
pressure? The larger compressor would take the same amount of power to
drive, but the turbine would be able to get more power from the exhaust
gases. Too bad we don't have any data on turbine effeciency and RPM.

If this is correct, then we would want to select the largest possible turbos
that don't surge and are reasonably effecient over the target RPM range.

Here is the only exhaust back pressure data that I have for the 2JZ:
GT-42 53-trim compressor, Q-trim .81 exhaust, RPS header, ported head and
272 duration cams.
Bigass FMIC, tons of power, semi-restrictive exhaust (when the 3" exhaust on
this car was changed to a 3.5", the car was totally unable to control
boost).
At 6800: Boost @ 1.4 kg/cm2,  Backpressure at 1.1 kg

T66 .70 P-trim, log-style manifold, stock 240ish cams.
Bigass FMIC, has since picked up a few more HP from a better flowing
exhaust, still probably over 1 psi at the turbine outlet.
At 6800: Boost @ 1.15 kg/cm2, Backpressure at 1.2 kg

60-1 .70 P-trim, RPS header, stock 240ish cams.
Good flowing exhaust and intake.
At 6800: Boost @ 1.2 kg/cm2, Backpressure at 1.3 kg

More evedence:
The 1st car mentioned, owned by Steve Hayes, switched from a T64 .70 P-trim
to a 74mm turbo made by precision turbo & engine, with a GT-42 53-trim
compressor and a .81 Q-trim exhaust. He dynoed before and after, with no
changes, same a/f, same boost. Here is the graph:
http://www.moreboost.org/graphics/turbos/SP74_SH1.jpg
The 74 makes more power at 5500 and greater. Of course the exhaust sides are
different, but the .70 p-trim exhaust has been proven to be unrestrictive
with that much power (600 to the wheels). But at 5500 rpm, the 64 should be
in the meat of its effeciency range, while the 74 is just becoming
effecient. Unless we assume a VE of 115% or so, the 64 should be just as or
more effecient.

Thanks for any help,
Grant Beaty


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