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?

[Cloudsurfer]

If you have a dual-mass flywheel on your car, changing from an un-damped "normal" size pulley to an un-damped underdrive pulley is fine, as you haven't made any changes to how things work (now, whether these motors should have come from the factory with a harmonic balancer is a whole different discussion).

If you have a single-mass flywheel, then balancing the rotating assembly and running a damped pulley is a requirement if you expect the motor to last.

Porsche did go to a dampened pulley on the M97 3.8, and I have to assume that they had their reasons, as those cars still came from the factory with a dual-mass flywheel.

Road Sport Supply sells a damped underdrive pulley, really designed for the M97 3.8 but as long as you use the longer 997 bolt it works just fine on any of the M96 family.

If I were going to bother going to a UPD, I would use the RSS unit and add some damping factor to the motor.

[JFP in PA]

That is exactly what the factory setup is. There is also a new underdrive dampened front pulley made for the 3.8L cars by RSS that will also fit on the M96, but it is more expensive and requires a longer crank snout bolt to work. I think before I went to an undampened UD pulley, I'd think about using the dampened one as it gives you the the advantages of UD plus additional dampening as well. You need to do a search 6speedonline, which is where I saw a thread about it......

[Boxtaboy]

Ok, thanks guys. Sounds like the RSS dampened pulley is the way to go.

[Cloudsurfer]

I have the RSS pulley on my 3.8, and can confirm that it fits just fine on any M96/M97 motor once you give the TDC fixing boss a haircut and use the longer, 997 bolt.

[backman_z]

Just a thought.

When installing my un-dampened pulley (same one Jake Raby uses), there was not much working room between the fire wall and the engine. I had a hard time getting my socket and wrench in the tight squeeze to loosen and tighten the bolt. I do not know what the dampened pulley looks like compaired to the un-dampened pulley. If the dampened pulley is in any way thicker, you may have more difficulty doing the install.

Boxstaboy, I did notice a power gain. Can't say it was high, but it is there.

PS: If this kills my 2.7 engine, then I get to upgrade!

[JFP in PA]

It fits in there, a bit tight, but it fits...........

[Cloudsurfer]

Actually, come to think of it, swapping the RSS pulley with the motor in the car is a bit of a squeeze. I first installed the pulley with the motor in the engine stand, and then later had to swap the stock pulley back on to tweak the timing a few degrees and found that it wasn't all that convenient.

Problem is that you can't get an open end or box end wrench on it, since the bolt is recessed into the pulley, but a socket and rachet is too thick in profile to fit. What worked for me was machining a standard 24mm socket down on the lathe to lower its profile, and using that with a flex head breaker bar.

Either way, I'd much rather have some damping in the equation than none.

[Gilles]

Originally Posted by Cloudsurfer

Actually, come to think of it, swapping the RSS pulley with the motor in the car is a bit of a squeeze.

Cloudsurfer, do you have the part number for the 3.8 dampened RSS pulley? Somehow I cannot find it on their website.
Thanks!
.

[Cloudsurfer]

Originally Posted by Gilles

Cloudsurfer, do you have the part number for the 3.8 dampened RSS pulley? Somehow I cannot find it on their website.
Thanks!
.

I don't believe they have it listed, I had to call and specifically ask for it.