The current power level from a non bridgeported 13B is 225 HP..... I looked

at another 13B before it was assembled and went on the dyno last
Tuesday.....It has 259 HP. I will be doing the ports on that one over again
this fall....

I have read various postings on engines, modifications, installations, and
their related HP.  This thread of discussion seems like a good time to ask a
question I had planned to post.

How do you measure HP?

I understand that a dyno can can measure the "work" output of the motor by
"direct" connection.  My question is related to postings that say, "after a
modification I reported 200mph at 8.0gph at 218hp."  Obviously the motor was
not taken off and dyno'ed.  Is there a simple formula to calculate the HP as
installed?  Do you use change in RPM in EGT, in airspeed and calculate the HP
required for that change from a known standard?  Do you use fuel flow?  Do you
use sound (yap, it sounds like I got 197.263 hp)?  DO you guess?  Inquiring
minds want to know.

This would be an excellent time for the engineers and gurus to speak up.  {:-)

Technically "work" is force times a distance. HP is force times a distance
in a given period of time. The dyno per se measures torque which in effect
is force. RPM in effect is distance on the rim of the flywheel in a given
period of time. Therefore torque times RPM is proportional to HP.

For example; if you know the drag of your airplane at a given speed
you can multiply that by the speed in feet per second and divide by
the one HP constant of 550. that will give you the net HP out
of the prop. If you assume the prop is 80% efficient and you divide
the above HP number by 0.8 you will get the HP out of the gear box.

You can also use fuel flow. The Mazda engine burns right around .5 pounds
of fuel for every HP generated for every hour the engine is running. Fuel
weighs about six pounds per gallon. If you are burning 10 gallons an hour you
are burning 60 pounds of fuel per hours and generating 120 HP.

There are several other ways of doing it. If you have a dyno curve of
HP verses RPM you can use manifold pressure to determine HP.

Paul Lamar

Probably the best way to assess installed HP increases is with results.
Both speed and propellor rpm are cubic functions - with the same prop
and plane, hopefully as close as possible the same conditions, record the
maximum level speed rpm for both cases, apply the cubic relationship.
in example if you previously flattened out at 6000rpm and now you
get 6200 rpm divide 6200 by 6000 (1.0333) and cube this(1.103),
you would have increased your power by a tad over 10 percent.
Engine mods can easily change specific fuel consumption (usually
for the worse), and also the manifold pressure to power relationship
is also frequently changed. remember though - same plane, same conditions!!
Vance

The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org