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

[Mr Motor]

One thing you have to realize as well that with a bigger exhaust
housing you are going to have less backpressure for one and for
two the newere style GT Garret turbos have much more efficient
turbine wheels than ever before...so you really can't compare
the brand new GT-Q wheel with the P-trim wheels IMO.

For a good test you'd have to use the same exhaust housings and
the same turbine wheels with changes in compressor wheels and
seeing the differences in backpressure.




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---- On Mon, 26 May 2003, Grant Beaty (gbeaty at ufl.edu) wrote:

> 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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