What is HP anyway?

[jdbubb at ix.netcom.com (Dan Bubb)]

Really there isn't any difference between HP and BHP. BHP is an acronym for
Brake Horse Power meaning the HP was measure against the resistance of a
brake. But, for all practical purposes that's the only way you can measure
it. They're both HP at the flywheel though. All published HP or BHP from an
auto manufacturer is HP at the flywheel.
There is some variation in the size of horse though depending on what
country they come from. I believe Italian horses are the smallest or perhaps
Japanese ;^) There is also some variation in power output depending on the
standard against which they are measured. As an example the SAE gross HP
rating did not require any accessories to be in place and the intake and
exhaust systems could be non-stock. This was replaced in the early 70's with
SAE net HP which requires the engine be tested as installed in the car,
complete intake and exhaust with all accessories in place. This is why you
see differences in HP if the same engine is put into different cars due to,
for example, a different exhaust system.
In all cases HP is a calculated number. Torque is measured directly and HP
is calculated from the formula HP= (Torque X 2PI X RPM)/33,000 or simplified
HP=T X RPM/5252
The original definition of HP was based on the assumption that one horse
could move 550 lb one foot in one second or 33,000 lb one foot in one
minute. Thus the above formula.
Power to weight ratio, generally expressed as lb/HP so actually weight to
power ratio, is simply the car weight divided by the advertised HP or HP at
the flywheel. Obviously, fewer lbs/HP means the car will accelerate faster
although this is less relevant as speed increases where lbs aerodynamic
drag/HP becomes more important.
Dan
as usual telling you way more than you want to know.




----- Original Message -----
From: "Julie Macfarlane" <juliemac57@hotmail.com>
To: <scirocco-l@scirocco.org>
Sent: Monday, August 25, 2003 8:03 PM
Subject: What is HP anyway?


> I know that 1 HP in electrical power=746 watts. But I see power posted as
HP
> and BHP for cars. Whats the diff? Is there any?
>
>
>
> Julie Macfarlane
> Menlo Park Research & Development
> Internet Application Developer
> www.menloparkrandd.com
> www.montgomeryweb.org
> Amsterdam NY
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