Dan :
Very well said !
I agree entirely with your conclusions, but have a few minor points :
> The purely mechanical losses would, I think, not be much more than the losses
> from a simple idler pulley, and would be very small.
Don't forget, the alternator has it's own fan which also takes power.
Unfortunately, there's no practical way to turn it off and on.
> Right out of the starting
> gate, your ignition system is seeing a 14% reduction in power.
True with points-type ignitions. However, most electronic ignitions are
"constant power" and will simply draw more current at lower voltages.
> If you were to be running at the 24hrs of Le Mans, you
> would not last the duration unless you swapped your battery out for a fresh
> one at each pit stop. For normal street use, it just doesn't seem like a good
> idea.
Well, the point is you use a switch that disables the alternator only at
full power. Even at LeMans, you don't keep the throttle wide ALL the
time <g>
> Now, let's talk about electrical vs mechanical fans. First off, we have to
> make some assumptions:
>
> 1) The fan blades (and fan installation details -- shrouding, etc) used on
> both the mechanical and the electrical fans are identical. We have to make
> this assumption so we are comparing apples to apples, rather than apples to
> doughnuts. If the fans are not identical, the differences in fan efficiencies
> would confuse the issue. If this is not true, we could always swap blades (and
> redesign the installation) so that it is true.
A simplifying assumption that is clearly NOT true. The mechanical fan
has to operate over a wide range, and survive being bolted to the
crankshaft. The electric fan can be optimized for a single speed and be
much thinner, etc.
> 2) At very high speed (and corresponding high engine speed), the mechanical
> fan may draw more power than an electrical fan; however, because neither is
> required, the electrical fan will be off, so the mechanical fan will draw
> significantly more power.
Your "very high speed" is actually about 40 mph <g>
> There are too many unknowns to make a definitive conclusion, but in general,
> from a horsepower standpoint, it might be concluded that an electrical fan is
> beneficial if you do a lot of driving at higher speeds, while a mechanical fan
> might be better if you do mostly low speed driving. If you spend a lot of time
> idling, the electric fan may again be the better choice -- not because of
> power or economy concerns, but because of better cooling.
Very few of us PLAN to drive mostly below 40 mph <g>
I think you overlooked the part about lower underhood temperatures when
leaving the starting line er, I mean traffic light.
My conclusions : Disabling the alternator produces no noticeable
increase and isn't worth the hassle. The electric fan produces a just
barely discernable difference but is worthwhile just to avoid
overheating.
Of course, if you are racing where every .01 second counts, then go for
it !
Randall
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