Mr Helmholz - simple model

[Gary Derian]

I race go-karts which use centrifugal clutches so I have done some thinking
here.  If you change your gears, you have changed your top speed at max rpm.
If you are pulling stumps, use as low a gear as you can but still set the
clutch stall at max torque.

For a drag racer, not traction limited, you need to maximize the "power
under the curve" for the 1/4 mile and must choose a gear that does this
without blowing up the engine before the traps.  Once you are close on a
gear, the clutch setting can be calculated.  Since a sliding clutch
transmits all the torque input (there is no where else for it to go) and
converts power into heat, it stands to reason the more torque input, the
more torque output.  Therefore set the stall to the max torque rpm.  If you
want to change your gearing, you still must set the stall to the max torque
rpm.

For a drag run with a sliding clutch (again, not traction limited), start
out at max torque rpm while the clutch is sliding, as speed builds up, the
clutch will lock up and rpm increase to max power rpm and beyond.  At this
point, you can shift gears if you have them.  One would hit the traps at an
rpm between max power and the rev limit.

This is only for a sliding clutch or fluid coupling.  A snowmobile uses a
CVT so does not absorb power.  For this case, operating the engine at max
power rpm will be faster.  A torque converter multiplies torque and absorbs
some power so it operates somewhere in between.

Gary Derian <gderian at oh.verio.com>

> 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 :}