Underdrive pulleys

[roosterjuicer]

Anyone know much about them?

What negative effects will they have? do you drain the battery if you run the AC on full blast if you have an under drive pulley?

[charged]

To be honest they are a bit of overkill and not really needed. In my track car I rev my thing to 9400 and still run stock ratio pulleys. Most guys run them not for ant mechanical/performance benefit because there anodised and look pretty

I would spend my money elsewhere

[henk4]

what is supposed to be the (theoretical) benefit of using these pulleys (driving what?). If I understand the concept of underdrive correctly, something is supposed to run quicker than with a standard pulley, runner quicker will require more energy, so where does that come from and how can that be translated into "gains"?

[fpv_gtho]

I think the idea is less parasitic drag on the engine, therefore more power going to the wheels.

[CapnBoost]

Less inertia (similar to the flywheel, but on the other side of the engine), and then the underdriven aspect runs your accessories at a lower rpm. Some waterpumps are terribly designed and will cavitate at higher rpms, as well as parasitic losses from running the pumps, alternator, a/c compressor, etc.

You probably won't notice the difference unless the car is stock or near stock. Most people do it for the eye candy factor.

[Sid447]

Under-drive pulleys,

are a smaller diameter and lighter crankshaft pulley. The primary job of a crank pulley or damper is to absorb engine vibrations coming through the crankshaft. Manufacturers spend a fair amount of time deciding the best size and weight of this pulley to suit all applications best. Crankshaft vibration can and does cause fatigue and accelerated wear on internal parts (the rotating assembly).
Some engines of the same family have a slightly different size of pulley, but this is generally because they are in a different installation and the pulley diameter has been decided on and changed to run the accessories (water-pump, alternator, power steering etc., at the best speeds for that particular installation).

Fitting any smaller pulley will need a smaller accessories drive belt as the stock one won't fit any longer, it will be too big.

If you decide to fit a UDP to the car, what it does is it slows down the rpm of the water pump & the alternator to free up some hp, It's not a great deal of power and certainly nothing you can feel when driving.
But the consequences of using a UDP for a daily driver, in a hot country can have dire consequences.
Because the water pump speed is lower than normal there's a probability the coolant isn't circulating around the engine and through the radiator sufficiently fast enough to keep ECT under control. Consequently the engine will start running hotter which means there is a greater danger of detonation.

We also we need to run the air-con and other accessories which rely on the alternator usually running at it's nominal speed of around 10,000rpm. Change this speed and it can't provide enough charge for a full electrical system and be able to charge the battery also.

There are much safer ways to get a handful of hp without endangering the engine.
These under-drive accessories are normally for drag-racers looking for any improvement in the quarter mile time and the cars used are not in frequent daily use.

Here's a good technical article by Super Chevy magazine.

1] They test a Chevrolet V8 here, fitted with a 127% over-driven pulley (i.e. run 27% faster than a stock water pump so would absorb even more hp than a stock pump).

2] Then with a 14% Under-Driven Pulley.

3] Finally with an Electric Water Pump.

Below 3,000rpm there was minimal loss (not measurable).

Above 3,000rpm the maximum gain was 6 (yes, six) horsepower at 6,000rpm, when compared to the 127% over-driven pump. Which means the gain would have been less had it been compared to a stock pump driven at 100% or normal rated rpm.

The Electric Pump had almost the same effect as the UDP. They both gave an average gain of an incredible 3hp compared to the over-driven pump.

Read below:-

http://www.superchevy.com/technical/eng ... index.html


Quote from the mag:
"So, clearly, the dilemma is: cooling vs. horsepower. And the path you choose will be defined by the use of your car. Street or drag? If you can afford a second car for your everyday commute and your hot rod only sees use on the weekends, then running an electric pump might be the way to go. But don't plan any long trips down the highway, because that's where you'll need all those extra gallons of water. Of course, you could always run a belt-driven pump and mechanical fan on the street and then switch to an electric pump and electric fan for the track, but is 10 more horsepower really worth all the headache?"

[Matra et Alpine]

WHO still runs a mechanical fan ??? rofl