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      01-21-2010, 03:33 PM   #15
JamesClay
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Drives: BMW 3
Join Date: May 2005
Location: Virginia

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1996 320iT  [0.00]
2003 M3  [0.00]
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2008 M3  [0.00]
They should run cooler than stock, but to me this isn't a huge issue. I really don't have a strong issue with stock rotors as I ran them on our car with a proper pad with zero issues. High temps are rarely the ultimate issue as long as that temp is within the operating range of the pad you are using. Some places we work hard to get our brakes up to this temp, although that is less the case in a higher power heavy production car.

Shorter stopping distances are a result of more precision and control - just like when you upgrade your suspension to take out slop it works better. So everything along this route helps. There are some absolute junk brake kits out there that I don't feel will perform as well as the stock brakes even - a BBK is not a checkbox with one flavor. So better rotor tolerances and quality, less slop in the caliper (fixed helps when it isn't paired with junk calipers), stiffer calipers (and lighter - a balancing act), etc all do this.

Given an average BBK with a cast 2-piece caliper (fixed) and Chinese made rotors, the stopping distances will decrease. Put on a PFC kit with a 1-piece forged caliper with less flex and tighter tolerances on all components, it will decrease more. Amounts depend on the rest of the car too since it is translated through the suspension. We did a lot of back to back testing on our World Challenge cars which are somewhat close to production engineering but with almost all slop removed that you can from a production platform, and you see the differences. In a street car - not quite as much. I did a test for Grassroots Motorsports recently with data on our M3 and a stock M3 and our car with a PFC 4-wheel kit went a tick deeper - definitely worth about 1 second per lap, but the stock car wasn't that shabby.

A PFC rotor (for a BBK or jsut the Direc Drive street upgrade) has some pretty insane tolerances on metalurgy, design, finishing, etc. Just an example (one of many and I don't have time to write a book) - PFC rotors have no balancing weights or machining. The rotor is cast oversized, EVERY external surface is finished (including OD and ID of the vein area), and a very smart system of machines places the rotor casting in the CNC so that when the machining is done the balance is perfect. This means that when it heats up - the balance doesn't change even slightly.

I wish the parts cost less as well, but it is a big casting and a lot of machining, so its in there. If even the OEM stuff did just the machining component, the cost would be higher and on par. You can buy OEM brake pads for less than a performance pad, OEM shocks are by far cheaper than our Moton sets, etc, etc. This rotor IS a performance rotor with real definable performance and quality characteristics, so maybe it is the lack of buy-in on that concept that makes you compare them to stock part price? But after initial purchase with includes a reusable center hat, the incremental replacement cost is less than $300:

http://store.bimmerworld.com/Product907