Underdrive Pulley revisited...

[Boxtaboy]

Ok, I've been doing a little research on the pros and cons of installing the underdrive pulley for a little extra hp, and am generally sold on installing one because it seems to give you the most hp bang for the buck. One comment I came across that alarmed me, however, was the following comment in this article: "third, the pulley also is without the ‘harmonic balancer/torsional damper’ meaning if the crank isn’t within certain tolerances (tight factory spec.) it could cause a ‘wobble’ in the crank, leading to premature bearing wear."

Full article here (yes, I know they're referring to Z cars, but I figure it applies universally): http://www.nicoclub.com/archives/300zx-z32-underdrive-pulley.html

So I guess my question is, if installing an underdrive pulley means upsetting the balance of the crank/engine, is there any aftermarket UD pulley out there that addresses this concern? I would like to get an extra 5-8hp and am willing to sacrifice some a/c cooling efficiency, but not at the expense of engine longevity if it means premature engine wear.

[jhandy]

That is the job of the dual mass flywheel.
the original pulley is with out a damper. so replaceing it with a smaller unit, also without a damper is no problem.

dont over think this. Just do it. you will like it.

[Boxtaboy]

Thanks, but I guess I'm confused after reading this thread where even Jake says he's developing a dampened UDP. RSS, it seems, already sells a dampened pulley, and the dual mass flywheel, it seems, dampens the area of the tranny connection, not the engine harmonics, so two separate purposes. Seems there's still a good reason for a dampened pulley. I guess my question is answered however, as RSS puts out a dampened pulley.

986 2.5 underdrive pulley dyno graph

[AndyA6]

On a straight six like a BMW I'd never put an undampened UDP, totally negative effects.
On the other hand I have one on my 1.8T Audi for years and years now, no ill effect. First shorter crank, 2nd here in the US one never drives for extended period of times with high rpm like on the Autobanhn where you'd go pedal to the metal for hours at a time. That would be a bad idea with an UDP!

Maybe not Boxster related but on my last aircooled Porsche ('90 C2) I had a lightweight OEM flywheel and clutch (RS parts). So no more dual mass damping there.

For above reasons and 'cause the crank on our cars is rather short I will go with an UDP. And if the clutch wears out (I doubt it) or the DMF goes kaputt I change from an UPD to a light flywheel/clutch setup.

Besides, I drive the Boxster for maybe 5-6,000mls per year-if that-, for the engine to wear out faster with the UDP it would take me how many years???

[JFP in PA]

With a lightweight (single mass) flywheel, and no other harmonic dampeners, it won't "wear out", but you run a substantial risk of the crank cracking (they are cast) from excessive torsional harmonic stresses (regardless of annual mileage) that are normally absorbed by the dual mass flywheel............seen it more than once. If you want to run a lightweight, single mass flywheel, you should fully harmonically balance your rotating assembly with the LW flywheel indexed to it………

[Boxtaboy]

So is the consensus that it's ok then to run an undampened crank pulley with the stock dual mass flywheel?