Mr Helmholz - simple model

[William T Wilson]

On Thu, 20 May 1999, Gary Derian wrote:

> If one has a sliding clutch, it should be set to hold the engine at max
> torque rpm, not max power rpm.

I disagree, because of the effects of gearing.

A hypothetical engine makes 300 ft.lbs at its torque peak of 4000 RPM.  
At that RPM it makes 228.5 HP.

Let's say it makes 201.3 ft.lbs at its HP peak of 6000 RPM.  At that RPM
it makes 230 HP.  This is an engine that could probably use a new cam. :}

Even so, once gearing is taken into consideration, it is better to use the
HP peak.  You can have a 33% lower gear ratio at the higher RPM, which -
more or less by definition - outweighs the decrease in torque at the HP
peak.  Once we gear this engine down by 1/3 it makes the same 300 ft.lbs
of torque that it did at its torque peak.

But let's say we have an engine that doesn't have its torque fall off so
badly.  Let's say, instead, the engine makes 220 ft.lbs at 6000 RPM,
instead, for an HP rating of 251.3.  This engine, when gearing is taken
into consideration, can generate an effective 328 ft.lbs of torque - a
noticeable improvement.

This is why engines are rated mostly in HP.  Torque is of course the most
important component in acceleration, but what RPM the torque is generated
at is just as important.  HP is a better measurement of torque at RPM,
although of course a full printout of the torque curve is the best :}