Ancient History

[John Faubion]

> >Mainly by reducing frictional losses at the lower RPM.
> 
> This would be more of a factor with a big block, since it has higher
> frictional losses due to increased bore, stroke, stc.

Well with any engine for that matter. The bigger the bore, longer the
stroke, the number of rings, the number of cylinders, ad nausem, the
greater the frictional losses. But no matter what engine it is the greater
the RPM the greater the frictional losses.

> I wasn't aware of the magic number of 5252 RPM, how did you arrive at it?

> I can't see this applying to ALL internal combustion engines across the
board...

Well actually I didn't. :)  Mr. Watt did around the 17th century. Draft
horses were used to pump water out of mines. The horses pulled a 12 foot
lever to turn a capstan which in turned operated a pump. He figured the
horses could apply 180 pounds of force to the lever. The circumference of
the circle was 75.4 feet (2xPIx12). The horses could make 144 trips around
the circle in an hour or 2.4 per minute. So the horse traveled at 180.96
feet per minute (2.4x75.4). Watt then multiplied this to arrive at 32,580
ft-lb./min (181x180) which he rounded up to 33,000 ft-lb./min or 550
ft-lb./sec (33000/60). This became the standard definition of horsepower.
Now to get back on track for your question. 

Horsepower is a force in pounds applied over a distance in feet in a time
of 1 minute. To convert the engines rotational force into horsepower, we
need to know how far the free end of a 1 foot lever travels in 1 minute.
That is the circumference of circle with a radius of 1 foot times the RPM
of the engine. The circumference would be 2 times PI times 1 foot or 6.183
feet. So our formula would be 6.183 times RPM times torque (in ft-lb.).
This could then be divided by 33,000 to arrive at horsepower. We end up
with a formula that looks like this:

6.183xRPMxTorque
---------------------------- = Horsepower
       33,000

If we then divide the left side by 2 time PI we eliminate the 6.183 on top
and reduce the 33,000 to 5252.11 which we round down to end up with:

RPMxTorque
------------------- = Horsepower
     5252

This is how you can calculate horsepower from torque if you know what RPM
it was taken at. 

> As far as best cruise RPM, depends if it's a Honda 750cc four or a 512ci
> Caddy, doesn't it?  You're right, I shouldn't have included HP in that
list,

To a point yes. Overall it will depend on a multitude of factors, such as
cam timing, chamber design, flow characteristics of intake and exhaust
systems, and etc... Everything has to be taken in the overall system. This
is the biggest problem of most hot rodders. They will add headers, an
intake and a wild cam, and when the car runs slower that it did stock they
complain about all of the parts they bought. If they considered the entire
car as a system and matched the components to run in a compatible range and
then changed the gear ratios to use that particular range, they would be
much happier and the car would be much faster. 

John Faubion
jfaubion at beaches.net