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In a message dated 8/20/2001 7:09:17 PM Pacific Daylight Time,
rbelshe@home.com writes:
<< My engine is a IO320 with 9.5:1 pistons, dyno'ed at 196hp, by Lycon. >>
Bob:
Nice of you to take the time to publish those numbers. However, my response
is based upon the belief that the common cause of greater knowledge will be
better served by the dissemination of realistic data.
First, your numbers don't specify a density altitude at which they were
measured, nor do they indicate the methodology by which the speed numbers
were determined. Without the density altitude numbers, the power being
applied to the prop can't be determined, even though you specified 2700 RPM.
Are we to assume that all the speed measurements are TAS? If so, how were
they determined, and were they all determined at the same density altitude?
If not, what was the actual power being appied to the prop for each dataset?
And is it just coincidence that all three of these fixed-pitch props just
happened to max out at 2700 RPM at full throttle at the density altitude you
chose?
The other issue is: I'd like for anyone (including Ken Tunnell at LyCon) to
explain to the world how an IO-320, which on its BEST day, MIGHT have
actually made 160 HP at 2700 RPM (with a special dyno exhaust system), can,
by means of installing 9.5 pistons (regardless of any unmentioned changes to
the intake system, heads, cam, etc.) increase it's power output at 2700 RPM
by 36 BHP. That's a 22.5% increase, and represents a BMEP of 180 psi, which
is pretty close to the max BMEP from my very best angle-valve, injected, 11.0
cr, trick-heads & induction system-IO-540's at 2700 RPM. (Maybe Visalia HP
are different from standard SAE HP).
I can promise you, from having modified lots of this stuff (and dynoed it on
an honest, computer-controlled, calibrated Superflow 901 dyno) that, unless
your heads have been replaced with angle-valve heads and cylinders off an
O-480, even with flow-improved ports and intake system, your 320 won't make
much more than 180 HP (the same power-per-cubic inch as a 200 HP IO-360-A, C
or D engine), and at 9.5 cr, it's pretty unlikely. Further, the the 9.5 cr
suggests strongly that the heads are the same old straight-valve heads as
other 320's. (However, if I monkey with the dyno calibration, I could show
you 500 HP out of your 320 at 2700 RPM, if that's what you wanted to see!)
If you DO have the angle-valve heads, then with some SERIOUS mods (big cr,
heat-barrier and friction coatings, radical accel-ramps on the cam lobes,
highly-modified ports, very-trick intake tubes and and very-trick plenum
mods) you might see your 196 HP at 2700 RPM.
Just for a reality check, assume you can actually achieve 100% volumetric
efficiency on your engine at sea-level standard conditions (highly unlikely).
At 100% VE and 2700 RPM, your 320 pumps 250 CFM. Since
HP = airflow (cfm) x 2.737 x thermal efficiency,
then the thermal efficiency needed to produce 196 HP is 28.6%. Now, in order
to evaluate whether that is probable, consider that the BEST of the Winston
Cup engines achieve a thermal efficiency of about 29.4% at cr's in excess of
12 and at crank speeds in excess of 7000 RPM. I don't believe for a second
that you'll ever get close to that kind of thermal efficiency out of an
air-cooled engine at 2700 RPM.
Sorry to rain on your parade.
But if those airspeed numbers are TAS, then regardless of the advertised
engine HP, you've got a pretty satisfying machine. Hope you can get another
Performance Prop .
Jack Kane
EPI, Inc.
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