masrapido944

Hi All, have seen a set of under drive pulleys advertised and am wondering do they really work and what benefits anyone using them here??, pros cons..., thank you for your help!!

Danno

No they don't work. That's because the load on your alternator comes from the power required. With little power being used, it doesn't pose much of a load. However, if you turn on all your lights and blast your stereo, then it's going to put a load on the pulley. Doesn't matter what size your crank pulley is, the alternator load/drag will adjust to be the same with the same power needs.

Tim

Actually under-drive pulleys do work, to a point. Forget about all the loads from the accesories that the pulleys drive and consider the pulley itself. Its really just a small flywheel, one way to make flywheels easier to rotate is to reduce the diameter of the rotating mass. Moment of inertia, (or MOI) for a disc is I=(1/2)mr^2, so the radius is a significant player in this. Are they worth it? I guess if you want to get every last hp out of your engine they are, but they really dont make a huge difference, they just let the engine use its hp to spin the "real" flywheel, rather than use some of the hp to spin up heavy pulleys

Rog

I'm with T on this.

They definitely play a role, but, as he said, a small one. Danno's example of the alternator _is_ the area where the underdrive makes the least difference, since it's resistance to turning is a stronger function of load than it is the constant inertia. Note though, at high loads where the alternator is saturated (maxed out) then the smaller pulley will yield a small benefit. This is a pretty darn small part of your driving time though.

Naturally, the faster you spin your motor, the bigger the improvement, given the velocity-squared term in inertia, but it's small. There's a bigger benefit for powersteering, and the waterpump (not that the water pump example is readily applicable to these cars) AC benefits too (like the previous two, it's a fluid pump)

The question is, do you want to deal with lower outputs on alternator or AC. Depends on how much stop and go driving you do, how warm it is in your climate, and how heavily you load your elec system (stereos, high output lighting, heated seat, short drives etc etc etc.

All of them are small, but they can add up.
Think of the times you'v driven a smaller car, flipped on the AC, and then felt acceleration drop. That can be dramatic. Still, how often do you have your AC vs how often do you need peak output. If I want that extra bit, I just shut the AC off.

to each his own.

KASH '95 993

I also agree that a underdrive pull does free up some power, how much? very little i believe. However, the key to its benifit is not because it's acting as a flywheel (moment of inertia), but drive ratio (kinda like your gear box)....i.e. if the drive pully (your crank) has the same diameter as you accessory pully and the accessory requires 2HP to operate it would take only 1HP if you reduce the diameter of your accessory pully by a half, because the engine will have a chance to rotate twice to do the same work.

Snafu

[quote]Originally posted by Rog:
[QB]I'm with T on this.

QB]<hr></blockquote>

Danno

Ok, so there's two areas of performance that under-drive pulleys affect. Rather than looking black & white yes/no qualitative statements, perhaps we can come up with some quantitative measureable results.

1. mass reduction: went out and weight my front crank-pulley. Weight = 3.5lbs with bolt. Here are some common weight-savings areas on the drivetrain:

• crankshaft -10lbs
• rod & piston combo -1lb each
• flywheel -15lbs
• pressure-plate -5bls

So lets add this all up: 10+(1x4)+15+5= 34lbs. Given that the power gains from drivetrain weight-savings is constantly being debated. A savings of 34-pound weight-reduction in the central rotating parts of an engine being debated? And no horsepower gains shown on most dynos even. So I doubt saving a measly 1-lbs with an underdrive pulley would be as significant as a 34-lbs savings on other directly connected larger-diameter parts. Don't forget the two balance-shafts that weigh about 6-lbs apiece and they're spun at double engine speeds.


2. rotational drag: Let's say you have a 120-amp alternator. And you manage to fully load it up with a lightbar of 9" 250-watt rally lights. And you crank up those bass-thumpers in the trunk until the alternator is just screaming for mercy at its 120-amp capapcity. Let's see 13v x 120a = 1560watts = 2.09HP. So at full drag, the alternator sucks up a full 2.09HP to put out 1560-watts of electricity (assuming perfect power-conversion here). Let's assume that the Porsche alternator is only 50% efficient, we're sucking up 4.18hp in the alternator at full load.

Then we put in under-drive pulleys that reduce rotational velocities and mechanical-leverage by a more realistic 20-30%. We've just saved 0.84-1.25hp on a maxxed-out alternator. If you're not driving your alternator at full capacity and powering 1560-watts of lights and accessories, your savings will be commensurately less.



So... while we can qualitatively say that underdrive pulleys do save you some wasted power, the actually quantity of savings is minimal. You're better off spending your money shaving off much, much more weight from other components like the crankshaft, flywheel, pistons & rods and pressure-plate.