roy -
you'd have to scale the pressure back proportionally to the area of the total piston size for the calipers.
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the fronts, if you put them on the back, will now have 40mm leading pistons and 36mm trailing pistons. the OEM rears have 30mm leading pistons & 28mm trailing. the difference in disk diameter is only 6mm and negligeble.
so, the new rears would have total area of 9100 square mm (remember, two of each piston per caliper) and the old rears would be 5300 square mm.
now let's consider the OLD system. the fronts were 9100, the rears were 5300 and the OEM proportioning valve has a .46 reduction factor. if we multiply the 5300 by .54, we get 2860 sq mm equivalent area from the rears.
back of the envelope to check our work: roughly 75% of the braking is done by the front. what is 9100/(9100+2860)? it's about 76%, so we are spot on.
now let's calculate the reduction factor required to give us the same proportion as OEM, front to rear.
our NEW boxster S calipers have, believe it or not, the SAME piston sizes as the base boxster. the disks are 20mm bigger, though, so first we need to calculate the EQUIVALENT area at the OLD disk diameter. let's assume that the center of the brake pad works on roughly the outer 40mm of the disk. this means that the OLD lever arm is 109 mm and the NEW lever arm is 119mm. the increase in braking torque is then 10/109 = 9% with equivalent piston sizes & system pressure.
so now we increase our 9100 by 9% to get the EQUIVALENT piston area in terms of the old system: 9920.
so what proportioning valve reduction will get us from 9920. to keep the same bias as before, we need our rears to be reduced to (9920 / .74)*.26 = 3485.
we need to reduce our new 9100 sq mm effective rear piston size to 3485. this is a reduction of 5615, or .62.
BTW, proportioning valves keep the F/R bias at 1.0 (NO reduction to rear force) until some pre-set pressure value. porsche's factory valve begins biasing at 25bar / 360PSI.
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you want: proportioning valve w/ a reduction factor of .62 and a divergence pressure of 25bar.
or an adjustable valve like this one:
http://www.amazon.com/Wilwood-260-8419-Adjustable-Proportioning-Valve/dp/tech-data/B002G37IEA
take care.