Underdrive pulleys?

[TheX1992]

How do underdrive pulleys work? For those who don't know, they are different pulleys that you bolt on to your crank pulley, alternator, and power steering pulley. It's suppose to give you 15-20 more HP. How is this possible? Are they different sizes or what? They all bolt on, so there's no bearings that are replaced. Anyone know?

[Scro]

Possibly a smaller pulley, thus causing more RPMs in the alternator? I'm not really sure.

[duke416ex]

They usually include a larger crank pulley and smaller pulleys for whatever else is included in the set, meaning your alt, waterpump or which ever pulleys you replace will turn more for the same amount of rpm on the crank.

[underpowered]

it is the same priciple as lower gears in the rearend. lower gears are easier to turn than taller gear. this reduces the amount of power it takes to turn all the accessories giving you more power. you can also replace the clutch fan with an electric fan to reduce the power taken to turn it. it is free horsepower since you are not modifying the motor , you are freeing up what is already there.

[MOFO]

I wouldnt use them.

A buddy of mine put them on his mustang... his water pump, alternator and everything else turned slower than they should. His car would run hotter and his charging system did not work as well as it should. It will reduce the speed in which the accessories are spun.

[Guy400]

Originally posted by duke416ex
They usually include a larger crank pulley and smaller pulleys for whatever else is included in the set, meaning your alt, waterpump or which ever pulleys you replace will turn more for the same amount of rpm on the crank. The crank pulley is smaller than a stocker in an underdrive setup.

Horsepower claims differ depending on who you talk to. Some people swear by them and some people swear at them. I had them on my LT1 Z28 and I didn't feel a difference on the Butt Dyno nor any significant gain on the track that couldn't be attributed to better air that day, etc. Not every day at the track has the same track temp, air temp, humidity and so on. After I put the pulleys on my times would still fluctuate around .10 in the quarter either good or bad.

[duke416ex]

^^That hit me later on in the day about the crank pulley. I have heard people have trouble out of them on mustangs too, I think it just depends on the entire set up of the engine. The guy I heard complain about it never could get his car completely right.

[honda350r]

If you want driveablity problems,install underdriven pulleys..

They are not worth the hassle.. Unless you drive your car at redline all day:rolleyes:

[MR.BIG]

Not true in all cases. I had them on my 91 mustang and I had no problems. They increase power by maybe 5hp but that's about it.

[Scott-300ex]

I have them on my 85 Mustangs, and have no problems.


Its a gear thing.

Also, they make it easier to turn, is a big thing with them, so the engine doesn't have to work near as hard to turn your accessories, such as, alternator, a/c, water pump, p/s, and idler pulley if you have one.

Its easier cuz you don't have to turn it as hard as it did with stock pulleys.

[MOFO]

Originally posted by MR.BIG
Not true in all cases. I had them on my 91 mustang and I had no problems. They increase power by maybe 5hp but that's about it.


Yes, it is. The accessories on a car are designed to be turned at a minimum RPM (usually your idle speed). Under drive pulleys reduce that speed...its not an issue while your driving, however at idle, it IS.

If you have your idle turned up higher than factory - you might not see these issues.

[quadrcr161]

for a track only car, im sure they can be great but for a daily driver in stop and go traffic it will kill the alt, and could cause the temp to rise by not flowing the water at the correct rate. for a track car, they are usually turing more rpm's and able to have more air flow and faster air flow throught the radiator.

[Scott-300ex]

Yeah, if your at idle for a long time, it can undercharge your battery.

[AtvMxRider]

Originally posted by Guy400
The crank pulley is smaller than a stocker in an underdrive setup.

Horsepower claims differ depending on who you talk to. Some people swear by them and some people swear at them. I had them on my LT1 Z28 and I didn't feel a difference on the Butt Dyno nor any significant gain on the track that couldn't be attributed to better air that day, etc. Not every day at the track has the same track temp, air temp, humidity and so on. After I put the pulleys on my times would still fluctuate around .10 in the quarter either good or bad.


You just can't drive thats all