More non-efi Horsepower stuff ``

[DICK BREWSTER]

>From: Matthew Prater <prater at mtu.edu>

>Hello,

>First time poster.  Mainly a list lurker.  Not enough money to start 
>a efi project right now so I am just trying to absorb info.
>...
>ANYWAYS, about this horsepower equation issue; the REAL definition of 
>power is Power=Torque*angular velocity, ie 
>hp=constant*trq(ft-lb)*rpm.
....
A way to look at the Magic Torque/HP relationship without a bunch of 
conversions is:

An engine producing one lb-ft of torque is exerting a force of one lb 
at a radius of one foot.  So in one revolution, it does 2*pi*1ft*1lb 
6.2832 ft-lb work/revolution

One horse power = 33,000 ft-lb/minute of work.

So: (33,000 ft-lb/minute)/(6.2832 ft-lb/revolution)= 5252.1 
revolutions/minute.


> Walla we get the following:
>1 hp = 1 trq(ft-lb) * 5252 * 1 rpm.  Equation manipulation shows that 
>the hp=trq(ft-lb)*rpm/5252 is true.  That is as far as I can get with 
>the books.
>So, as four years of MTU schooling has taught me, I must move on with 
>the romantic hope of getting a grip on this.

I hope the 33,000/(2pi) relationship helps with your grip.  The 
beautiful simplicity of physics frequently is a casualty of too much 
to learn in too little time.

...

>To those who may know: My official newbie questions...
>Is this brake horsepower?  Methinks it is.  If it is, what is a real
>horsepower?  What is the difference? How stupid is this question?
>In other words, is a brake horsepower different from a normal 
>horsepower in how it's measured, or are they different units?

They are the same and can be used interchangably. Brake horsepower is 
just a leftover name from when dynos were called brakes. And at the 
time, other non-engineering terms such as taxable horse power (which 
has nothing to do with power anymore) were used.

Automotive horsepower ratings in this country are tested according to 
SAE J(something) and are net power, with the engine almost the same as 
installed in the vehicle.  The Germans use DIN ratings that are very 
close to SAE net.  The US used to use SAE gross horsepower which is an 
engine with no accessories such as air cleaner, muffler, alternator. 
water pump  etc.  Then the marketing department added more power.  
Most of this crap stopped in 1972 and power ratings are pretty honest 
now.

>For trivia's sake: who decided that 550ft-lb/sec roughly approximated 
>what a horse is capable of?  What foul machine produced this?  Did 
>some crazy guy give Mr. Ed a big torque wrench and have him spin a 
>few cycles with it on some sort of active resistance machine?




James Watt measured the ability of draught horses to do work over a 
significant period of time (one shift, I think).  He used horses that 
were lifting loads out of mines so it was easy to calculate.  1hp is 
the continuous duty power rating of an average draft horse.  A human 
can put out over 1 hp, but not for long.

>Thanks.
>Matt


Dick