hi all;
One of the reasons to use a underdrive pulley (which would be larger
than
the standard gen pulley or smaller than standard crank pulley) would be to
slow the generator down. In a competition situation, a higher rev limit
would be used, and the engine will be running at a much higher adverage rpm
than would ever likely be used on the street. the generator( and
alternators too) have an optimum rpm beyond which they either don't
generate any more power and/or they blow up from overspeed. Generators are
much more suseptable to overspeed damage than alts. If you match the rpm
range you'll likely be using in a rally (say 4000-6800 with a 7200 absolute
max) then you size the pulley so that it makes optimum power as low down in
the rev range as possiable without overreving at max eng speed. Then, you
start making the pulley even larger if you can get away with it because you
want the extra free HP and , more difficult to quantify but even more
important , the extra acceleration and reliability that comes from a slower
gen. In all likelyhood, either twin generators , alternators, or just a
larger unit would have been fitted, all with a pulley sized to keep them in
thier optimum rev range. Generators have a nasty habit of scattering thier
armatures at elevated rpm, and they take a LOT of power to speed up , so as
large of pulley as possiable would be used.Alts are almost immune to
overspeed, but there's no point in running them beyond the optimum rev
range, and they also take power to speed up. Over the course of the rally,
the car will be run through it's rev range perhaps a few thousand times.
The time saved by having less rotational inertia on the generator would be
worth enough time to justify substancial thought and effort to optimise the
system. You can bet the boys at Porsche and other front runners did. You
can also bet that the boys at the TR skunkworks weren't given enough money
to sort out this kind of stuff, so were running on a "best guess" based on
thier talks with the lucas men.
Don Boyd
> Phil said, "We also used
> > an undersize pulley so it didn't run as fast. During competition you
rarely
> > use all the lights simultaneously."
>
> This to me, implied that they did something to make it run slower...hence
I
> thought they put a smaller pulley on the crank...
>
> And I couldn't picture a smaller pulley on our generators.... (it's
already
> small and unless you changed to a smaller belt, I doubt the belt could
handle
> it)...
>
> I've always been confused by this... It would seem to me... that the
amount of
> drag on the motor caused by a generator running normally or slower would
be a
> somewhat minimal improvement... and I'd rather have confidence that my
battery
> is going to remain charged... (while idling in the pits, at the start,
etc..)
> than to have a tiny climb in available horsepower...
> > Message text written by INTERNET:triumphs-owner@autox.team.net
> > >Why build up the generator and then make a smaller pulley? Seems
like
> > you're
> > defeating some of your efforts?
> >
> > I would understand keeping the generator stock and putting a smaller
pulley
> > on...
> > if you're just vintage racing with, at most, headlamps, etc... but
it
> > doesn't
> > make sense to me for a full rally car driving through harsh conditions
at
> > night...
> >
> > - --Justin
> > <
> >
> > Justin, a smaller pulley will give MORE rpms and thereby give more
> > power output. AFAIK....
> >
> > -Tony
>
>
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