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
I'd love to see a good dyno run to nail down the BSFC better for these
motors. I think that the number is more like 0.6 lb/Hp-hr, but I have no
really SOLID basis. I think Paul's 0.5 number is possible, but probably
too low. Most spark ignition engines get about 0.42-0.45 lb/HP-hr and
the rotary is known to be "thirsty". If it can get 0.5, then it's great, but
in the absence of hard dyno data, I'll bet on 0.6, and we can see who's
right. I'd be willing to bet up to $1, but no more ;-)
The difference is significant. If you are burning 8 gal/hr, then you are
burning 48 lb/hr, and at 0.5 lb/HP-hr you are producing 96 HP. If the
BSFC is 0.6, then the HP is 80. With a pretty efficient spark ignition
engine you will have about 0.42 BSFC (brake specific fuel consumption)
so 8 gal/hr works out to about 114 hp. This about 75% of 150 hp, and when
you fly a 150 HP C172 around at 75% power, the fuel burn is 8-8.5 g/hr.
Higher compression ratio increases efficiency, so diesels get better BSFC,
like maybe 0.375 lb/HP-hr. Diesel fuel is heavier, about 7 lb/gallon. One
aircraft engine, the liquid cooled Continental Voyager series can get a
BSFC at the 0.375 level. This is phenomenal and is due to a very high
compression ratio and a special combustion chamber design. This engine
powered the Voyager "around the world on one tank of gas". Unfortunately,
it has not been commercially successful.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
Bill, Tracy, on his trip up from Florida, with head winds towards the
end of the trip, averaged about 160 MPH on 6.6 gallons an hour as I recall.
How does this compare with a Lyc powered RV4?
Paul Lamar
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