If any R53 is over 80k miles and hasn't had a new crank pulley put on, I'd do it the next time you have the car in for service... especially when putting on a smaller pulley. When it goes, the car goes into limp mode... you lose the supercharger, the alternator, the AC compressor and, most importantly, the coolant pump. When my crank pulley let go at 85K miles, the AC went warm... followed in 10 seconds by the check engine light from low electrical. It went into limp mode about 15 seconds later. I made it home in 2-4 minutes, just as the coolant got hot and blew out the top of the resivoir tank, spewing coolant onto the header and making for a dramatic, steamy entrance as I coasted into the garage. Luckily I got home with seconds to spare, but it would have been dead on the side of the road. With the OEM crank pulley (engine damper) it isn't a question of IF it will die, but WHEN.
I put on a 16% supercharger pulley on my R53... I did mostly spirited street driving w/ the occasional DE weekend. If you do Auto-X where you see sustained high revs, stick with 15% or lower. With my 16% pulley, I changed the belt every 2 years or 20k miles to be safe. It's easy to do in 1 hour or less and costs $25... cheap insurance, IMHO. I also did a lot of regular preventative maintenance, so I felt comfortable stretching the bounds a bit. If I remember from my research, the 17% pulleys are stretching the limits of the supercharger's ability to draw in air and, at high RPMs are churning/cavitating more than pumping air.
PS: the crank pulley can be done yourself too, if you have a few specialized tools and are handy in the garage. I did a write-up that is in the library section of Motoring Alliance, along with a write-up on the belt change.
I am not sure what the mileage of your car is but make sure you inspect the crank damper / pulley while you are working on your car. If it is old they tend to split and the aftermarket makes better replacement options.
Bought at 97 now at 109.. around 104000 I replaced pistons/ crankshaft/spark plugs/....not sure about crank damper? I will check it out when I go to install.. I have to cross a couple more things off the to do list before I get there
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