Underdrive pulley: the definitive answer.

[Richie200]

Hi guys, I have a modified 996 C2 and am now at a point of where next to turn. The car already has the following:

-Full Hartech Engine
-CAI Kit
-BMC filter
-200Cell cats
-997 SSK
-M030
-PSE

I have just bought a 964RS LWFW and will fit this along with a new clutch when it arrives.
Various sources claim the Underdrive pulley is a great modification. Has anyone here fitted one and if so what was the result?

[IMI A]

The lighter, smaller crankshaft pulley works in 2 ways, one by reducing weight and drag on the crank. Kind of like a lightened flywheel it helps the motor rev up faster. It also enhances the performance and increases power and torque by reducing drag caused by the belt driven accessories by not having to spin them as hard and fast. It's one of the quickest, easiest, and cheapest ways to gain a little power on any vehicle.

[PorscheGT4]

yes could net you 3BHp ;-)

[Richie200]

Many thanks guys, I'd read all the bumph and understand the principles involved, it was more a case of does anyone have any real life feedback to offer. For instance running the PAS system at low pressure may induce heavier steering at low revs, or maybe the alternator no longer produces the correct output (fails to charge the battery), A/C not cold enough...
On a positive note there maybe reduced ancillary noise and a real tangible power increase. There is talk of 10BHP gain; this seems a little optimistic to me and as pointed out above it may be only 3BHP?
Anyone here tried one?

[Ian_UK1]

PorscheGT4 said:

yes could net you 3BHp ;-)

That much? The pulley slows the ancillaries down by about 10% (from memory). A 3 BHP gain would suggest the ancillaries are robbing the engine of 30 BHP under normal use. The alternator under maximum load accounts for about 1.5 BHP of that, so where is the other 28.5 BHP going?

[edc]

I've had a 4.25" and now a 4" on 2 Boxster S now. I've never dyno tested them so can't vouch for any power claim. The current 4" one is now charging at about 13.9V so clearly spinning the alternator slower. I don't notice any difference in the steering or the aircon, now that I use it that much. The engine does feel a bit freer to rev but for me it's not a game changer mod. I felt more when doing exhaust mods in terms of more power and the engine seeming to breath better and rev more freely. The first time I did it the car was due a belt change anyway. The second time it had only had a belt change just 6 months previously.

[Rockster]

Some USA based Porsche engine experts believe fitting a LWFW to these engines is risky in the LWFW doesn't offer the crankshaft dampening feature the factory flywheel offers.

As for an under drive pulley... you'd gain just as much "hp" by using a laxative the night before your track day.

I'm at a loss to understand why someone would want to slow down the speed the water pump turns and reduce the coolant flow through the engine and radiators especially when the engine is going to be pushed extra hard on the track.

[IMI A]

Rockster said:

Some USA based Porsche engine experts believe fitting a LWFW to these engines is risky in the LWFW doesn't offer the crankshaft dampening feature the factory flywheel offers.

As for an under drive pulley... you'd gain just as much "hp" by using a laxative the night before your track day.

I'm at a loss to understand why someone would want to slow down the speed the water pump turns and reduce the coolant flow through the engine and radiators especially when the engine is going to be pushed extra hard on the track.

Weissach follow this route on their GT3 RSR (LWFW, under drive pulley) and you can purchase these parts straight from your OPC. What fools eh?!

[Richie200]

IMI A said:

Rockster said:

Some USA based Porsche engine experts believe fitting a LWFW to these engines is risky in the LWFW doesn't offer the crankshaft dampening feature the factory flywheel offers.

As for an under drive pulley... you'd gain just as much "hp" by using a laxative the night before your track day.

I'm at a loss to understand why someone would want to slow down the speed the water pump turns and reduce the coolant flow through the engine and radiators especially when the engine is going to be pushed extra hard on the track.

Weissach follow this route on their GT3 RSR (LWFW, under drive pulley) and you can purchase these parts straight from your OPC. What fools eh?!

I kind of get where Rockster is coming from with the M96/97 engine issues. The Mezger is as strong as an ox so not the same concerns. My engine has a full Hartech rebuild with beefed up bearings and the closed deck design so I have absolutely no concerns about the LWFW; in-fact if anything, the LWFW gives me less concern than the stock one. Harmonic dampening is all well and good but the 60% reduction in mass hanging off the end of the crankshaft cannot be ignored.
Rockster may have a point regarding water temp and flow issues. I will contact Hartech to get there take on the matter.

[Richie200]

Just spoke to hartech, fantastic advice and really helpful as usual They have no real concerns with the LWFW (so long as it is balanced) but suggest that there could be a trade off in reliability with regards the underdrive pulley. As this engine is known to be a little fragile they suggest it probably isn't the best idea for road application. Obviously for the track, the trade off in performance would be not such an issue. On that basis, I think I will give it a go and see how I get on. Many thanks for all your advice here guys