A petrolhead mate of mine has just installed underdrive pulleys to his car. These were new to me and I wondered if any of yout had used them.

Basically the utility (water pump, air con, etc) belt pulley is reduced is size and weight. Therefore the engine has to use less power to turn them and therefore more power to the road. Additional 5 to 8 BHP with good torque gains on his car, even more on a scooby.

Any you guys got em, know of them?

 

I fitted one on my MY00, The original one weighed in at 2500g and the new one was a Perrin and weighed in at a massive 500g so a weight saving of 2kg.

I was sceptical at first but the difference is actually amazing.

If your going to start modding then get one of these first, you won't regret it. However you will now have thrown your engine out of balance so you will need to wind up the tick over to compensate for the weight change or change the flywheel aswell.

Steve

 

I have them on mine with a lightened flywheel at the same time (5.5kg rather than 12kg). Well worth doing mod- much quicker pick up

 

This is a very underrated mod, that can give noticeable gains, particularly to throttle response and engine braking.

Anything that reduces the rotational mass on the crank is going to be a good thing.

We have them fitted to all our own cars and Andy Forrest has a set of our pulleys on his 10 sec Subaru.

Rich

 

what mods need be done to ensure pulley weight difference is taken account of?
martin

 

The weight, I can go with, but surely if you decrease the size, whatever it's running will run faster? Will this not lead to increased wear/premature failure of ancillaries?

Alcazar

 

Bit of a problem if running a big ice set up I'd have thought.

 

It doesn't run faster thought does it? It just spools up quicker...

 

They are smaller in size as in less material not in diameter.

 

Ah.........OK, I'd also thought of belt length probs with a smaller diameter

Alcazar

 

Well theres a bit of debate on that, it depends on what you mean by faster; Due to less innertia, the engine changes it's speed (revs) faster. Technically you get an increase in the acceleration's acceleration! (rate of change of acceleration) Practically, the top speed would be increased because you'd be decreasing the potential energyof a great big heavy pulley revving, but in practice this increase could be absolutely negligible.
There are some rumoured issues of problems with hill-starts and carrying large trailers often associated with lighter pulleys, crankshafts and in particular, flywheels. In truth, if you need the flywheel's inertia to do a hill start, you're not driving very well, quite frankly. Deffinitely lightening of reciprocating parts gets you a performance increase without the increase in fuel consumption, like mods to the exhaust, by reducing wasted energy.
-Antonis

 

2 solutions offered. 1 a shorter belt, or 2 a larger dia pulley on something else to compensate. The larger dia pulley is still lighter than standard.

Worth consisdering though, any overheat probs if the water pump is running at a different rate?

 

then theres the harmonic balancer argument.

 

Quite true. The alternator is underdriven.

But it seems contradictory to fit dirty great ICE installs in a car that you are trying to enhance the performance of. Or maybe I'm just getting old.