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.

Hans Conser wrote:

Scott, a couple of thoughts.  When you say that you want the same power as the
stock engine, do you mean peak power?  If you do, then you will probably want
higher boost levels, as most aircraft aplications seem to run 5-6000 RPM, so to get
good power that low you are going to need more than 5 lbs of boost.  I would shoot
for 8-11 lbs myself.

Also, you are likely to gain a lot of efficency from a more open exhaust.  3rd gen
RX-7s gain 50+ horsepower with an open exhaust alone.  Remember that the cats (3 of
them) are very restrictive.

I'll try and check your numbers later.

Hans

 
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