Some of the alternator pulleys for the 928 are so small now that to go any smaller you might as well run the belt right on the shaft itself.

You can see how they chased this problem (not charging at idle) through the years, and I have seen several 6-rib alternator pulleys on different models of 928. Swap into a smaller one if you are not in the smallest one now.

The problem with going to a smaller drive pulley is that you loose tractive force on the pulley and at the same time you have increase the need for tractive force because you decreased the torque-lever arm and the alternator is harder to turn. A smaller pulley makes both of these worse.

What I usually see on superchargers when smaller pulleys are installed is the customers will start to experience belt slip issues, or they tighten the belt so damn tight they start to have bearing failure.

My suggestion: learn to live with the idea that the alternator does not charge at the stop light with the AC on (it will charge the moment you drive away) or, if its a manual trans car, turn the idle speed up a wee bit. Neither solution costs a penny.

 

Todd has two alternators running simultaneously. He has VERY large fan motors pulling air through the intercooler.

 

Dave (Sharkskin) hit the nail on the head with the references to bearing friction, inertia and windage losses in the alternator. In his example he zeroed those out to make the point that they are the things that might change with the larger pulley. All of those factors reduce with the underdrive pulley. So while Greg Brown is correct on the generating part of the work-done in and out, and Doc Mirror is correct to a point, ultimately the differences come down to speed-related friction losses, and inertia-related issues on acceleration. Under constant electrical load and constant engine RPM conditions, only the friction differences in the bearings and the windage losses differ.

I'm not sure that the inertia difference at the alternator would be appreciable, compared with claimed pumping differences in water and steering pumps with underdrive setups. Pumps use more power when asked to pump more fluid, plus the increase in fluid flow shows up as additional discharge head due to friction in the water system. ASSuming that the thermostat is managing coolant flow to regulate engine temps, slowing the coolant flow works right up to the point when the thermostat opens to pass more coolant mass through the radiator to keep temps the same. Ultimately, the only thing that changes in the system is the reduced discharge head on the pump. This may or may not be an advantage; the cooling system depends on a certain amount of pressure in the heating side of the system to avoid local hot-spot boiling of the coolant. Lower pump speed and lower discharge head with the more-open thermostat make for less even cooling in the block, especially if there is any significant local boiling.

Lots of racing "science" seems to make sense until all of the factors are considered. That doesn't keep people from making things that "make sense" on first blush, even though they really don't contribute much to the net power number. Often, bling alone is a good enough reason. So long as they contribute to the net profit number, the real net power number doesn't make much difference.

 

Non-GTs could just sharktune to run GT idle speed as well...or manually set the idle motor to a high value in the summer, etc.

Many ways to chip away at that problem..if you are able.

 

Carl,

Any dyno numbers for this pulley?

 

I don't know how fine a point you want to put on it, but all of my links were to pulley ratios, and none to the electrical dyne flux field equations. However, now that you've brought it up, in considering the case for a underdriven pulley providing a fixed current load, I'm gonna have to cogitate on that a bit. Because, alternators are not a steady state linear device. That's why they charge better at lower voltage, due to the stator being energized unlike a gen. Food for thought. Maybe nothing, but I'll look into it a bit.

 

The nay-sayers seem stuck on the concept of preservation of load - that it takes the same effort to spin the alternator anywhere in the powerband that you spin it. I already gave my answer to that in post # 42.

But you can you deny the other two benefits provided: the lower moment of polar inertia helps accel and decel the engine faster (like the benefit from an aluminum flywheel, but obviously much smaller) and the
benefit from the increase in torque-lever arm that the new pulley with the larger radius provides, making it mechanically easier for the engine to turn the stator thru the fields?

 

+1

 

+1

 

No one mentioned the improvement in pumping losses on the alternator fan...

 

Only because your wallet would be $80.00 lighter...other than that, no.

 

Maybe the real deal here for a racer would be to have an electric clutch type pully (like the AC compressor pulley clutch) for the alternator - that way you could engage the alternator basically when decellerating or cruising and not when accellerating or idling, A differently organized TPS could probably handle everything (e.g. maybe a switch active at <<WOT throttle position)

So exact control parameters to be worked out but certainly disengage the alternator drive whenever you are at WOT (or idle), and ideally also whenever you are accelerating much more than mildly...

This way you can probably still get adequate charging in typical laps while preserving all engine power for acceleration. May also need to modulate the fans somewhat depending on speed & battery size - but an interesting thought...

Alan

 

Hi Alan, Still need to meet up sometime soon.

Wouldn't the weight of the electric clutch negate the hp savings?

 

Exactly.

the ONLY thing measurable that it will do is spin the alternator at a slower speed, so the brushes dont wear as fast. Period, end of story.

Oh yeah, you do save .3lb, so thats like .5 lbs as if it was in the car not spinning.

Mark

 

wow, i dont know how I missed all the action on this thread!

back to the facts here. I just put one on my car last year, because I had a GT alternator fail and it needed a new winding . so, i got a new rotor, and then while I was in the alternator shop, i saw a larger pulley and thought, hey, spin it slower, less brush wear. I forget the dimension, but it was like a 30% increase. there is so little mass on the alternator. yes, it is spinning 2x the engine speed, via a ribbed belt, but still very little mass. insignificant on a dyno for two reasons. the acceleration rate in 3-4th gear is SLOW, and this factors out almost all inertial effects. reving in idle, might be effected, albiet to a very very small degree.
next, electricallly, the alternator puts a load on the engine based on current requirements, not voltage. f its putting out 40amps at 12,000rpm or 40amps at 60000rpm, the power loss will be near identical. (refer to power= torque x speed/5250). factor in a 50% efficiency factor and you got the scale of what is going on.
you cant fool mother nature on this one. the ONLY thing you are doing is saving brush wear and saving .3 of a lb. by the way, you could get a stock pully to weigh this much if you wanted one.
want proof? do a dyno run with the alternator attached. then, cut the belt and drive it off the battery. you will not see any difference, unless the lower battery voltage changes fuel mixture in the computer.
In theory, the car needs 10amps to run. 10amps at 13 volts is 130 watts , x 2 for efficiency, there is less than 1/4hp. thats TOTAL! we are talking modifiying a part of the alternator, so its a fraction of that 1/4hp.

people bulding race cars dont worry about these kind of factors. however, its a desire, if you want the belt to last longer and alternator brushes to wear less.

lighten the flywheel? not measureable on the dyno. lighten the wheels, brake rotors, etc? also not measureable. on a brake dyno, certainly not!