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

[andrius tiknius]

Helo,
im facing similar problems now, but I dont see em so
big - all depends what range of rpm u go for - higher
rpm(drag) - bigger turbo, lower rpm power(cornering) -
smaller or twins.I believe(without proper research)
that backpresure mostly depends on exhaust
design.Smaller pipes means bigger velocity and gas
inertia, so it restricts backpressure if proper lenght
and does not depend on turbo speed that much as on
turbo camera space and design.
Anyway if you go for over 600 hp, what dyno says, you
have to consult somebody who is realy experienced.I do
not go over 300,and also my projects are mosttly
budged, so my knowledge is mostly empirical.Regards
Andrew
--- 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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