In Reply to: Underdrive pulley survey (more) posted by Marshall on September 11, 2000 at 17:15:42:
Installed the 02 underdrive pulley on my 95 M3 last week, here's a few initial impressions on the part. Wanted to drive it for a while and see if it threw the AC belt first. It hasn't but some more extensive thrashing may be required for peace of mind.
My car already has a euro HFM/Conforti chip, ECIS intake, so this may influence things.
Performance: Yowza. Noticed it from the first time I got into the throttle a bit in second. Very obvious in all gears, all throttle positions, especially low end as you tip in from say 2000 rpms on up. Runs up to redline a lot faster. Would definitely believe peak HP/torque improvement numbers quoted by manufacturer. Maybe even more at the low end. Planning to dyno the car in the next few weeks and will post numbers somewhere.
Installation: This was something of a headache. Instructions suck. One page of print, 12 lines or so devoted to the whole install. No torque specs or pictures. Basically you must remove both belts, the fan, and water pump pulley. Getting at the crank pulley bolts with the fan shroud in place is pretty difficult, as it can't be pushed out of the way completely unless you drain the radiator/block and remove the hoses from the water pump. On a 95, the bolts sit inside the deep well formed by both the crank pulley and the AC pulley, so you need an extension (or long socket), and the lower lip of the fan shroud is right where you want to put the ratchet. Anyway, finally got both pulleys off, after needing a hammer and a piece of wood to get the crank pulley off of the crank. Light tapping was sufficient but it took a while, very snug fit onto the crank nose.
This whole shimming thing bothers me. The issue is whether or not the new crank pulley with the old AC pulley slid into it puts the grooves of the AC pulley into the same plane as the AC condenser pulley. If not you throw the AC belt (possibly), so you get shims in case you button the car up this happens (groan!). Well, this is kind of stupid if you ask me, you'd have to go through the complete installation again, and oh by the way getting the proper number of shims between both pulleys and then getting two sets of holes lined up for the 6 bolts would be quite difficult. I guess gluing them onto the front of the crank pulley first might work. Or assembling the whole pulley/shim/pulley thing with bolts looose through the holes and hope they don't fall out while you tap the thing on the crank nose with a hammer.
So before installing the new pulley, I set the AC pulley into the old crank pulley and used a micrometer to measure the distance between the last groove on the crank pulley and the first groove on the AC pulley, with the AC pulley centered in the crank pulley. Did the same for the new pulley and the numbers were identical. So no shims, but I am keeping my fingers crossed. I need to go out into the country and thrash this thing a bit to see if it's OK. Not looking forward to redoing this if necessary. New pully is also a very tight fit onto the crank nose, and took quite a while to get lined up to the pin on the crank. Tight is good here (isn't it always?), though, so I guess it's worth it.
After getting the new/old pulleys on, it's a simple matter of reinstalling belts (new shorter one for main acc drive) and water pump pulley, fan, underpanel, etc.
Part: Having bitched about the install, the part itself is literally beautiful. It appears to be CNC'd aluminum, black anodized (?), with a ferrous toothed ring press fitted/pinned onto the outer edge (so the Hall effect crank angle sensor can work). It weighs a lot less than the stock pulley, which was pretty heavy. So you get two effects, one a reduced moment of inertia (like a lighter flywheel), and two, a reduction in parasitic losses from the smaller diameter pulley that runs the accessories.
Other comments: Some folks had thought with the Euro HFM and intake, that I wouldn't see anymore power at the wheels, or that I'd run lean, etc. Well, the engine isn't using anymore fuel, just less HP is now lost to spinning up the crank and running accessories. Also, it seems to me that the more HP your car has prior to install, the more benefit you'll see. Just a guess but this should look essentially like an increase in the correction factor from flywheel HP to RWHP (i.e.; less % drivetrain losses). So more flywheel HP to start, more increase in RWHP. Like I said, just a guess.
Idle rpm stayed the same. Idle seems a little bit lumpier, sort of like a hot cam in an old muscle car. Noticed this right off first time I started it. Have not had any problems yet, either from the belt or electrical abnormalities. Will update if this changes and also when I get dyno numbers.
Very happy with the performance improvement. Keeping fingers crossed on the AC belt/shim thing.
Note: '96 & up cars use a completely different set up.