Thanks Don.... that took care of my confusion over pulley size... makes sense.
...still interested to hear what they did do in the rally cars... We can all
guess... but it would be great to hear what they did... did they install
specific
(larger) generators ... did they modify the standard one? etc..
--Justin
DON BOYD, JOAN IKOMA wrote:
> 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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