This shows the arrangement of pulleys on a 3.8L V6 Mustang engine.
The air conditioner clutch pulley, water pump pulley, alternator pulley, and power steering pulley are
all DRIVEN pulleys. The crankshaft damper is the only DRIVER pulley.
The speed and diameters of the pulleys are related as follows:
Using this formula you can calculate the speed that the stock pulleys turn on the 3.8L engine shown above.
Underdrive pulleys reduce the speed of accessories relative to the crankshaft speed. The claim is that this can significantly reduce parasitic drag on the engine, increase horsepower, torque and gas mileage, and by turning the accessories slower, extend their life. Most of this gain probably comes from slowing down the water pump. The A/C pulley is a clutch, and does not produce any significant drag while the A/C is turned off. The alternator creates drag when charging, but that drag is fairly constant regardless of crankshaft RPM. I don't know how much power you gain (if any) by slowing down the power steering pump. The water pump and the heavy rotating mass of the crankshaft damper are the main things you are fighting.
Underdrive pulley installations usually include a crankshaft pulley that is smaller than the stock pulley. This smaller pulley slows down the serpentine belt and turns all of the accessories slower. You can also change the speed of an individual accessory by installing a larger or smaller pulley on that accessory. A larger pulley will slow the accessory down, and a smaller pulley will speed it up. For example, the ASP underdrive pully set for street driven 96-00 4.6L Mustang GT's includes a 25% underdrive crankshaft pulley, a 26% underdrive water pump pulley, and a 10% underdrive alternator pulley.
If you replace the 7" crankshaft pulley on the 3.8L engine with a 5.25" crankshaft pulley, the accessories will be underdriven by about 25%. That is the norm for street applications. Using the formula above, here are the new pulley speed calculations.
If when you replace the 7" crankshaft pulley with a 5.25"" crankshaft pulley you also replace the 2.5" alternator pulley with a 2.25" pulley, the alternator speed will not drop as much as the other accessories. The alternator will only be underdriven about 17% while the rest of the accessories will be underdriven by about 25%. Maintaining close to stock alternator speed can be useful if, for example, you are running a high power audio system that places a lot of demands on your car's battery.
As you can see from the above discussion, you can juggle pulley sizes to achieve the amount of accessory speed that you need. Generally, on street cars the alternator can be safely run at about 2 times the speed of the crankshaft. For drag racing, it can be run at about 1.75 times crankshaft speed, which still provides for charging on the return trip and in staging. On a circle track car, which keeps its revs up almost all the time, the alternator can be run at about the same speed as the crankshaft.
Some Commonly Expressed Concerns about Underdrive Pulleys
1. Lowering the alternator speed will cause the battery to run down. That may be true for some people. If your driving style includes idling the car for long periods, or if you have a high end audio system that requires constant power from the alternator, underdrive pulleys may not be for you. The hot idle speed for a 2000 Mustang 3.8L V6 engine is 700-800 RPM. ASP claims that with its 25% underdrive set the alternator will provide a full charge when the engine is turning over at 850 RPM. I don't expect any problems in this area.
2. Lowering the water pump speed will cause the car to overheat in traffic. If your cooling system is in marginal condition, that could become a problem. An underdrive pulley will remove some of the margin of safety that Ford builds into its systems to account for clogged radiators, slipping belts, etc. If you keep your cooling system in good shape, it should not be a problem. I live in Southern Arizona where it gets HOT in the summer. I haven't had any overheating problems.
3. Lowering the power steering pump speed and the A/C compressor speed will result in decreased power steering and A/C efficiency. The people I've talked to who have installed 25% underdrive pulleys don't seem to notice any difference in A/C or power steering performance.
4. Installing a smaller crankshaft pulley can cause the crankshaft to fail. The people who say this always seem to be talking about the (stupid) practice of removing the harmonic damper and not replacing it when installing a smaller crank pulley. The damper is there for a reason, and removing it probably will result in damage to the crankshaft over time. Manufacturers of underdrive pulleys for the V6 Mustang agree. Their underdrive crank pulleys either piggyback on the stock pully/damper, or they come with a new harmonic damper to be installed along with the new pulley.
5. Underdrive crank pulleys are not compatible with superchargers. True. You don't want to underdrive a belt driven power adder. If you have any plans to install a supercharger at some later date, don't waste your money on underdrive pulleys now.
UPDATE August 2003: It's been ten months and 7,000 miles since I installed this pulley, I've had no problem with overheating, even when sitting in Tucson traffic in 110 degree weather. My battery has not run down. And I haven't noticed any difference in A/C or power steering performance.
UPDATE February 2006: More than three years and 40,000 miles with this pulley and still no problems.
I'm no expert on this topic. If you think I've got any part of this story wrong, let me know.