What are underdrive pulleys

[thexav]

Can someone please explain to me what an underdrive pulley is and what they do?

Why would you want to fit one to your engine?

 

[andy206]

An underdrive pulley is otherwise known as the crank pulley. Id imagine every car has them fitting a billet lightweight item will let the crank etc rotate faster which in turn will help with speeding revs up/down

 

[RobBentley]

Under-drive pulley's are also commonly fitted to superchargers as an aftermarket modification. The smaller pulley will make the supercharger spin faster, and thus create most boost. I've done this numerous times on the 4.2 v8 s/c jag engines.

 

[obi_waynne]

What are the power gains like on those Rob?

 

[RobBentley]

55-60hp with the pully and suitable maps on the engine and gearbox.

 

[obi_waynne]

That's a pretty decent gain.

 

[T9 man]

55-60hp with the pully and suitable maps on the engine and gearbox.

Very impressive.

 

[obi_waynne]

Under-drive pulley's are also commonly fitted to superchargers as an aftermarket modification. The smaller pulley will make the supercharger spin faster, and thus create most boost. I've done this numerous times on the 4.2 v8 s/c jag engines.

Good answer Rob, I see this done quite a bit on Jags, it's strange they respond so well.

The Cooper S also gives a nice power gain with a simple pulley change.

I wonder how far you could go before you'd need a remap to make it work well.

 

[TCJBOLDIE]

I would always recommend checking the AFR's after any mod that increases positive pressure to the motor by turbo or supercharger and remapping if needed.

 

[RobBentley]

You'll be fine usually with small pully mods on an otherwise standard jag 4.2 as there is enough correction available you'll get more fuel as its corrected from the lambda readings.

There is of course additional gains to be had elsewhere so the best results come from a map too. Particularly on the auto jags where good gains can be had from modification of the gearbox software.