Over-Sized Pulley Upgrade

pulley-comparison.jpg (53766 bytes)

    Recently, the VW aftermarket introduced a new part.  It's an over-sized crank pulley which is 20% larger than a stock sized pulley.  Common sense says that this could be a nice addition for those of us who live in hot climates since it should spin the alt./gen. and fan 20% faster.  Higher revs on the fan equals more cooling air for the motor.  It will also increase your generator output at low RPMs as well so you can say good-bye to dimming headlights at an idle.

I picked mine pulley kit up at a local shop for $29.99.  It included the larger, cast aluminum pulley (made in Taiwan) and the correct sized belt to run with it.  Immediately upon inspection, I noticed that the TDC and BDC marks were silk screened 180 degrees out.  At first, I thought that the reason my kit was so cheap was because of this.  Well it turns out that everyone I know who has purchased one of these kits even from the retailer featured in the VW magazine, has received a pulley that is 180 degrees out.  No big deal, really as long as you know this going in, but it could be really confusing for someone who is unaware of this flaw.  Below are some comparison shots.

pulley-comparison2.jpg (55056 bytes)            pulley-flaw.jpg (42323 bytes)

    You will need to remove the old pulley from your motor.  Now slide on the new, larger pulley and note that the pulley tin will need to be trimmed a bit in order to fit.  There is no way around this so don't forget it!!  Below is a shot of the tin where it needs to be trimmed.

pre-tin-trimming.jpg (34093 bytes)        `    post-tin-trimming.jpg (49687 bytes)

NOTE:    If you have a motor which is drilled and tapped for a full flow oil filtration system, you MAY need to clearance the fitting and tin there as well; I did.  Here's a couple shots of where I had to clearance it and how it looked when finished.

pre-FF-fitting-trim.jpg (45826 bytes)            post-FF-fitting-trim.jpg (46416 bytes)            FF-fitting-clearance.jpg (12099 bytes)

    With everything trimmed and clearanced, slide on the new pulley and torque the nut to the proper specs.  I always like to add a little anti-seize to the area between the crankshaft and the pulley so that dis-similar metal corrosion won't "weld" the two parts together over time.  The fan belt is a tight fit around the bottom of the pulley as is goes around the sheet-metal.  I had to use a screw driver to wedge it into place, so eventual replacement should be "fun".

    Here's a before and after shot of the motor with a stock sized pulley and with the new over-sized pulley.

old-pulley.jpg (45852 bytes)            new-pulley.jpg (49168 bytes)

WARNING:  If you have an engine which makes frequent revs above 5000 RPMs, I WOULD NOT RECOMMEND FITTING ONE OF THESE PULLEYS TO YOUR ENGINE.  The added RPMs at the fan could cause your fan to explode or otherwise come apart.  You've been fore warned!!

10/17/02 - I've had the pulley installed for 2 months and here's my non-scientific findings:

Did it work?? - Yes, but not as well as promised or hoped.  

How was the high speed operation?? - I hoped for at lease 10F - 15F lower oil temps while on the freeway driving 65mph.  Well I only got 5F - 7F lower oil temps.  This thing does turn the pulley a full 20% faster right??  Kind of a let down, but progress none the less.

How was the slower engine speed operation?? - I have to say that this thing was great when running around town and right after getting off the freeway.  My engine oil temps would go from 215F down to about 195F in a matter of about 2-3 minutes after a freeway run.  Oil temps around town never exceeded 195F after a freeway run or if never on the freeway, they would never exceed 190F.  

Was it noisy?? - Yes.  Fan noise increased significantly.  The fan almost sounded like a turbo charger spooling up.  This was likely due to the faster speed coupled with the quicker reving-up of the fan.

Did you notice a power loss from turning the fan quicker?? - Yes.  The loss was slight,.  I'm guessing 2-4 hp, but I don't have dyno results to back that claim up, but it was noticeable from the driver's seat.

Any other problems?? - Yes.  My alt pulley wore out from the stress.  The outer pulley started to spin on the hub causing the two flat spots to elongate and groove the hub.  I would expect this from a cheap Taiwan pulley, but this one was a quality German pulley from the 70s.  I can only assume that the added stresses from quicker spinning with faster ramp-ups caused it to twist on the hub.  This was NOT due to running the pulley loose or without the correct amount of shims.


In my opinion, the claims by the manufacturer just didn't pan out as hoped.  Realistically, it was claimed that temps would drop 15% - 20% (that's 35F - 40F).  I was only hoping for about 5% - 10%, but got 2% - 3%.  I would gladly live with the increased noise and decreased power for 10% - 15%, but not for a mere 2% - 3%. . .  This could be a benefit to folks that drive around town a lot since that's where this pulley really shined.  It truly helped in town but have a spare alt. pulley handy just in case of failure.

In all fairness, this was tested in my bus and your results at freeway speeds may be better in a bug.  Buss engines typically heat-up quickly above 60mph and it could be that the heat generated at that speed with that amount of drag would be too much for even a pulley 50% bigger.  Without transplanting this motor into a bug, I have no way to test this theory.

I've removed the pulley from my engine.  I can't see risking another alt. pulley at this time or any other time for that matter.  I have a spare pulley, but I hate road side repairs and refuse to run something I know "could" strand me.


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Revised: August 27, 2003 .