Scott Speirer wrote:

Paul,

Thought I'd bounce my turbo matching estimates off the readership for any possible
feedback.  I am assuming a stock 13B turbo with intake and exhaust manifolds that
provide the same airflow behavior/efficiency as the RX-7 car manifolds.  No power
beyond that of the stock version is expected, just normalization of the power up to
15,000 feet density altitude.  (e.i., a critical altitude of 15,000 ft.)  This is
based off of the method in "Turbochargers", by Hugh MacInnes.

For the purpose of airflow calculations as a function of engine rpm, the engine has a
displacement of 2600 cc's.  This converts to 158.6 cu in.  (Paul - thanks for this
info.)

With 80% total system volumetric efficiency (a big assumption) at sea-level STP, the
airflow rate will be as follows without turbocharging:
@5000rpm    190 cfm   or   13.1 lbs/min    (cfm = cu. ft./min.)
@6000rpm    215 cfm   or   14.8 lbs/min
@7000rpm    260 cfm   or   17.9 lbs/min
@8000rpm    295 cfm   or   20.35 lbs/min

Assume the engine performs best at 5psi boost above sea-level standard pressure.
This would be 14.7 + 5 = 19.7 psia, or psi absolute pressure.  (If 5psi isn't the
best boost level, would like to know otherwise.)

At 15,000 ft, STP absolute atmospheric pressure is 8.29 psia.  The turbo will need to
produce a boost pressure of 19.7 - 8.29 = 11.41 psi.  The pressure ratio is (8.29 +
11.41)/8.29 = 2.37.  The turbo compression density ratio for this pressure ratio will
be 1.7 if the turbo is in its 70% efficiency region.  It will be 1.62 for 60% turbo
efficiency.  I'll choose 1.65 as an average.  With these conditions and average
compressor efficiency, the airflow will be the sea-level non-turbocharged flow values
multiplied by the density ratio, resulting in the following values:
@5000rpm    313 cfm   or   21.6 lbs/min
@6000rpm    354 cfm   or   24.4 lbs/min
@7000rpm    429 cfm   or   29.6 lbs/min
@8000rpm    487 cfm   or   33.6 lbs/min

At the critical altitude, the compressor will be able to produce the same boost as at
sea-level, but at a lower turbo speed due to the lower atmospheric temperature.  The
speed reduction factor at 15,000ft will be 1/sqrt(520/460), or 0.94.  Not much, but
it might help.

At density altitudes less than 15,000ft, the waste-gate will be partially open, to
prevent the turbo from overboosting.  I may use a setup like Charles Harbert's to
control the waste-gate.

>From the compressor maps provided at www.turbonetics.com, I looked for a turbo that
would not surge at a 5000 rpm level and still be at least 60% efficient, and also
would be at least 70% efficient at 7000rpm.  I found the T3 "60" Trim and the T3
"Super 60" turbochargers seemed the best.

Any constructive criticisms on my assumptions are welcome.  I'll be contacting
Turbonetics in a while, and let you all know what they think of my estimates.

Scott

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