At 10:53 AM 4/10/96 EDT, David Rupert wrote:
>Scions,
>
>Since we are discussing strange measurement systems, I have one that has bugged
>me for years. I recently saw it crop up again in my latest issue of Classic &
>Sportcar, with regard to the early Triumph Glorias and the Southern Cross.
What
>the heck is hp? One of the cars in the article was described as, "A 10.5 hp
>model featuring a 48 bhp engine", or something to that effect. I'm assuming
>that it is NOT some arcane method of measuring horsepower. My first wild guess
>is that it has something to do with weight. Could someone from across the pond
>educate a confused American?
>
>Thanks,
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Dear David,
The best explanation of horsepower I have found is in a tuning book written
by David Vizard, "How to Modify your Mini" 1977 Argus/HP Books (ISBN
0-912656-47-6).
Vizard writes:
"HP is a combination of speed and torque. In the case of an engine,
speed is measured in revolutions per minute, usally shortened to RPM.
Multiply revs by torque and you have power. Not horsepower, just plain
power. You can't turn this figue of power into horsepower until you say just
how strong or powerful 1 HP is.
"Way back, a man called James Watt pioneered locomotive engine
development. he needed to say how powerful his locomotives were. After a
number of tests he found a horse could do 22,000 foot pounds of work per
minute, but he also reasoned that other people might have more powerful
horses. So just to make sure didn't over-rate his steam engines he decided
to say 1 HP was equal to half as much again, or 33,000 foot pounds per
minute, or HP = workdone per minute divided by 33,000.
"In engineering terms, the work done is the force applied in lbs. to
move an object times the distance it is moved in feet. HP is the force
applied x the distance it is moved in one minute divided by 33,000. Or if
you want to use seconds, it is the the force applied x the distance it is
moved in one second divided by 550. You simply divide by 60 to obtain this
second formula. I don't know if Watt ever found a horse half again as strong.
"....How does this relate to your car? This is the bit you need to
understand. Let's go back to torque maesurement: Force (F) at the edge of
the flywheel multiplied by the distance around the flywheel will give the
work done per revolution of the engine. The distance around the edge of the
flywheel is 2 x pi x r, where r is the flywheel radius. The work done per
revolution is 2 x pi x r x F, but r x F equals torque, so the work done per
revolution is 2 x pi x torque. By multiplying the distance the force travels
in one revolution by speed, we have power. In this instance, speed is the
revolutions per minute (RPM). Power is 2 x pi x Torque x RPM. This formula
gives power in ft. lbs. per minute. This in't a nice unit to use so we
divide it by the number of ft. lbs. per minute James Watts' hypothetical
horse was capable of (33,000 ft. lbs. per min.) This gives us the answer in
horsepower. Thus horsepower equals:
2 x pi x N x T
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33,000 "
My own understanding of what Vizard writes is that he is talking about brake
horsepower (BHP). I recall being told that HP (as opposed to BHP) was
defined early this century in regard to intenal combustion engines (and
Watts' measurements) proportional to engine displacement, ie bore, stroke,
etc. The assumption being that all engines of the same capacity would
produce equal HP. It was this misconception that led to the need for BHP to
be used to define the true power of an engine. Perhaps others on the list
will be able to enlighten us all on this.
Owen Michaels-Hardy, Sydney Australia
1980 TR7 Sprint 16V conversion
(and still seeking more BHP for my lovely little wedge)
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