Under-drive Pulleys
Written by FivePoint0hhh
Under-drive Pulley Installation
Mustang World 4.6L
Under-drive Pulley Installation.
This is a FAQ on Under-drive pulleys. The majority of the information was taken from HERE but I converted it to a question/answer type thing. Note: The crank bolt is a torque-to-yield bolt. Buy a new one if you're going to install pulleys or else it will snap. Also, do not forget the washer on the crank pulley. You might want to have a shop install it for extra insurance. Steeda is now offering reimbursements for those who have broken their crank pulley bolts.
4.6l SOHC Pulley Diagram
Pulley Approx. Diameter
1. Air Conditioner 5”
2. Water Pump 6”
3. Alternator 2.5”
4. Power Steering 6.5”
5. Crankshaft Damper 7”
Q) What is that a picture of?
A) This shows the arrangement of pulleys on a 4.6L V8 Mustang engine. The air conditioner clutch pulley, water pump pulley, alternator pulley, and power steering pulley are all DRIVEN pulleys.
Q) What do under-drive pulleys really do?
A) Under-drive 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.
Under-drive 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 under-drive pulley set for street driven 96-00 4.6L Mustang GT's includes a 25% under-drive crankshaft pulley, a 26% under-drive water pump pulley, and a 10% under-drive alternator pulley.
Q) How fast do my accessories spin stock?
A)
|
Stock 7"
crankshaft pulley turning 1000 RPM |
||
|
Accessory |
Calculation |
Speed |
|
Air conditioner clutch |
1000 RPM x 7"
pulley/5" pulley |
1400 RPM |
|
Water pump |
1000 RPM x 7"
pulley/6" pulley |
1167 RPM |
|
Alternator |
1000 RPM x 7"
pulley/2.5" pulley |
2800 RPM |
|
Power steering |
1000 RPM x 7"
pulley/6.5" pulley |
1077 RPM |
Q) So if I buy a 5” Crank Pulley, then how fast will they turn?
A) The under-drive crank pulley will significantly reduce the Speed of the accessories. The Air conditioner clutch is reduced by 400RPM, the water pump by 334RPM, the alternator by 500RPM and the power steering pump by RPM, as you can see in this diagram...
|
5"
crankshaft pulley turning 1000 RPM |
||
|
Accessory |
Calculation |
Speed |
|
Air conditioner clutch |
1000 RPM x 5"
pulley/5" pulley |
1000 RPM |
|
Water pump |
1000 RPM x 5"
pulley/6" pulley |
833 RPM |
|
Alternator |
1000 RPM x 5"
pulley/2.5" pulley |
2000 RPM |
|
Power steering |
1000 RPM x 5"
pulley/6.5" pulley |
769 RPM |
Q) I run a high-end audio system, is there something I can do to help my alternator reach a closer to stock speed so I don’t end up with a dead battery?
A) Yes, you can either leave the alternator pulley alone, or buy a 2” pulley which will only under-drive about 10% of the Speed. Here is the diagram when using the 2” pulley on the alternator.
|
5"
crankshaft pulley turning 1000 RPM |
||
|
Accessory |
Calculation |
Speed |
|
Air conditioner clutch |
1000 RPM x 5"
pulley/5" pulley |
1000 RPM |
|
Water pump |
1000 RPM x 5"
pulley/6" pulley |
833 RPM |
|
Alternator |
1000 RPM x 5"
pulley/2" pulley |
2500 RPM |
|
Power steering |
1000 RPM x 5"
pulley/6.5" pulley |
769 RPM |
As you can see, by using the 2” pulley on the alternator, you have gained 500rpm back to the alternator. Stock is 2800rpm, so now you are safe.
Q) What did I learn so far?
A) You can juggle pulley sizes to achieve the amount of accessory speed that you need. Generally, on streetcars the alternator can be safely run at about twice 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.
Q) If I lower the alternator speed, can I get a dead battery?
A). 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, under-drive pulleys may not be for you. ASP claims that with its 25% under-drive 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.
Q) If I lower the speed of the water pump, will my car overheat?
A) If your cooling system is in marginal condition, that could become a problem. An under-drive 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.
Q) When I lower the speed of the power steering pump, will my car be harder to steer?
A) The people I've talked to who have installed 25% under-drive pulleys don't seem to notice any difference in power steering performance.
Q) What about my A)C, will it still be as efficient.
A) Yes. Everyone I have talked to about the Air Conditioner after installing U/D Pulleys said that they had noticed no change in cooling.
Q) When I replace my crank pulley with a smaller one, will my crankshaft fail?
A) 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 under-drive pulleys for the V6 and V8 Mustang agree. Their under-drive crank pulleys either piggyback on the stock pulley/damper, or they come with a new harmonic damper to be installed along with the new pulley.
Q)Under-drive crank pulleys are not compatible with superchargers.
A) True. You don't want to under-drive a belt driven power adder. If you have any plans to install a supercharger at some later date, don't waste your money on under-drive pulleys now.
Q) Where can I get Steeda pulleys for my 96-01 GT? Bullitt? How about 01 cobra and Mach1?
NOTE:Midway through the production run for the 2001 model year Ford changed the design of the water pump on the 4.6L engine. The older style pump produced from 1996 to mid 2001 had a longer shaft. In January of 2001 the design of the pump was changed to a shorter shaft. This necessitated a change in the design of the pump pulley. click HERE for Steeda's guide.
A) www.Steeda.com The price is 169.95. For Bullitt, The price is 159.95. For 01 cobra or the mach 1, the price is 169.95.
Q) I would rather have ASP pulleys, where can I get them?
A) http://www.thunderracing.com/index.cfm?fuseaction=subcategory&subcategoryid=1128
Prices vary quite a bit because they offer numerous sizes of pulleys to better suit your needs.
Q) I would rather have March Pulleys, where can I get them?
A) For 94-95 GT’s and Cobras. http://www.marchperformance.com/pg31.html
For 96+ GT’s and Cobras, http://www.marchperformance.com/pg32.html
Code’s Info
Aftermarket Pulleys will NOT produce any charging issues with the battery as long as you don’t have a $3000+ audio system. They will also not harm the cooling system. In fact, they help it by reducing cavitations. The best pulleys are ASP and Steeda as of now (they’re the same thing). However, I have been hearing about more and more problems with the stock oil pump gears due to non-SFI approved aftermarket pulleys. I personally would stay away from the headache. If you do buy them, be sure to replace your stock crank bolt as it is engineered to be torqued only once. There are also no special pulleys as said in this FAQ portion. The author who wrote this adapted it from the 5.0L pulley FAQ so our items aren’t quite as extensive.