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