horsepower and fuel burn (not a Rebel issue per se).

[Peter Cowan/Lexi Cameron]

My initial reaction to Phil's comments was to ignore them and go on but second thought tells me that to do so would ignore our responsibility to do what we can to make this internet medium and this list accurate and informative.

Phil has made two assertions: dyno results are unquestionable and fuel burn is no indication of horsepower being developed.

I am not an engineer and have precious little theoretical training. I have been flying a Lycoming powered homebuilt and now an auto engine homebuilt for 20 years. I also listen with a sceptical ear.

First fuel burn rates or specific fuel consumption numbers. I think Lycoming states theirs is .47. I have been told by more than one engineer that ALL engines are within a few percent of that number. Thus calculating aproximate horespower (being developed) from your burn rate is quite acceptable and reasonably accurate.

Years ago I got quite excited about selling my 180 Lyc and installing a Blanton reduction drive on a V6. His dyno PROVED that engine produced 250hp and with roughly the same weight as my Lyc. I'd even end up with thousands in my pocket. So I went out to Kansas and had a ride in his V6 powered Cessna 175 (a plane very similar in size, weight and wing to mine). What a let down. That extra 75 horsepower produced no noticable performance difference.

Subsequent discussions with "experts" have led me to believe that the real strength of the dyno is to accurately measure differences on a given engine that tuning changes make. Because there are several constants that the dyno operator has to arbitrarly select, it is not truly meaningful to compare the dyno numbers from one operation with those of another.

That is why I did not make any challenge of Phils max horsepower number. I have no data to allow me to do that or to even suggest that it is inaccurate. But his cruise number is PROVEN wrong by his fuel burn rate. Also by common sense. Has anyone ever seen a 160 hp engine that when running at full throttle burns 5 US per hour? My first 100hp continental wouldnt do that.

Phil himself in an earlier note on reduction drives when he was concerned about my negative reaction to the Ross drive said: "statements without back up can be so damaging on the Internet". I hope I have provided sufficient back up to set the record a little straighter.
Peter.

[klehman]

Very good post Peter.
Modern auto engines can get a little less than .47 lb. per hp. hr. but
nowhere near half that as some salesmen and advertising claim.
Thermodynamics nor honest dyno numbers support that claim as you have
said. Higher compression helps. Running leaner than stock automotive efi
helps as the stock efi runs rich of max efficiency to keep the catalytic
converter working. Not needing excess fuel to assist air cooling at high
power helps (Lycomings are well above the .47 number at max power). One
is kidding themselves if they think they are getting better than about
.4 in a real world gasoline engine. (.4 is very very good)
Ken

Peter Cowan/Lexi Cameron wrote:

My initial reaction to Phil's comments was to ignore them and go on
but second thought tells me that to do so would ignore our
responsibility to do what we can to make this internet medium and this
list accurate and informative.

Phil has made two assertions: dyno results are unquestionable and fuel
burn is no indication of horsepower being developed.

I am not an engineer and have precious little theoretical training. I
have been flying a Lycoming powered homebuilt and now an auto engine
homebuilt for 20 years. I also listen with a sceptical ear.

First fuel burn rates or specific fuel consumption numbers. I think
Lycoming states theirs is .47. I have been told by more than one
engineer that ALL engines are within a few percent of that number.
Thus calculating aproximate horespower (being developed) from your
burn rate is quite acceptable and reasonably accurate.

Years ago I got quite excited about selling my 180 Lyc and installing
a Blanton reduction drive on a V6. His dyno PROVED that engine
produced 250hp and with roughly the same weight as my Lyc. I'd even
end up with thousands in my pocket. So I went out to Kansas and had a
ride in his V6 powered Cessna 175 (a plane very similar in size,
weight and wing to mine). What a let down. That extra 75 horsepower
produced no noticable performance difference.

Subsequent discussions with "experts" have led me to believe that the
real strength of the dyno is to accurately measure differences on a
given engine that tuning changes make. Because there are several
constants that the dyno operator has to arbitrarly select, it is not
truly meaningful to compare the dyno numbers from one operation with
those of another.

That is why I did not make any challenge of Phils max horsepower
number. I have no data to allow me to do that or to even suggest that
it is inaccurate. But his cruise number is PROVEN wrong by his fuel
burn rate. Also by common sense. Has anyone ever seen a 160 hp engine
that when running at full throttle burns 5 US per hour? My first 100hp
continental wouldnt do that.

Phil himself in an earlier note on reduction drives when he was
concerned about my negative reaction to the Ross drive said:
"statements without back up can be so damaging on the Internet". I
hope I have provided sufficient back up to set the record a little
straighter.
Peter.

-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
-----------------------------------------------------------------