So we have pad fade. No big surprise there. Going from here we know that you need a higher temperature pad. But, we need to keep in mind your brake balance. I am not sure why you have a proportioning valve plumbed in the front lines. If it was me, I would take it out. You indicated that with both valves all the way open the front lock just before the rear so the balance of the system seems to be good.
You indicated that you use porterfield pads but you did not mention the compound. I am unable to locate a good chart with temperature and torque (CF) for these pads but seems to me that they are around CF of .485. This is very close to the CF of the D compound that you are running in the front. This combined with the position of your proportioning valve and the front wheels locking just before the rears, but all four locking, while the brakes are at a moderate temperature, leads me a couple of places.
I would not go to a higher CF pad in the front then you are running now. This would increase the brake torque beyond its current value and and with the system being balanced you would not be getting the most out of your rear brakes (higher CF pad will lower the pressure required to achieve maximum braking which also reduces the amount of pressure going to the rears and without a balance bar setup you can not correct this except by cutting down on the front pressure using the proportioning valve but the problem with that is the pressure decrease is not linear so the harder you push the bigger the pressure difference from front to rear PHEW).
If any thing, you will want your new pads to have basically the same CF or the rear to have more. And obviously a higher temperature range. Did you order your temp paint and strips yet??????
Where to go from here. Since you do not know what temperatures that you are experiencing, new pads will be a shot in the dark. You need to find out the temperature range of you rear pads. I am not aware of Wilwood making pads for your rear calipers so that brings you back to a company such as Porterfield for the fronts. I have always heard good thing about them. Unfortuniatly, most manufacturers, except Wilwood, seem to keep the torque vs temperature information a deep dark secret making it very difficult for us to get the right set the first time and nearly impossible to design a braking system on paper. So you can try a higher temp front pad with a CF of .5 or less (not a lot less! say .45 as a bottom). Temperature range you are going to be guessing. Considering the current pads work for as long as they do you don't want a pad that doesn't start working until 600 degrees!
Cooling ducts. I am afraid those aren't really helping. How they are supposed to be made: make a plate out of say 18 gauge steel that will mount to the spindle behind the rotor, covering the open center section of the rear of the rotor with about 1/8" of clearence to the rotor. Cut a hole in it and make a rigid duct that will allow you to attach the flexable duct with out getting in the way of the suspension. Blowing air on one side of a rotor is asking for trouble. This is for ventelated rotors. Ventilated rotors work like a centrafuge. (I know, my spelling sucks!)
While you have those rotors off, you can have them slotted. No need for a pretty pattern. Just two, per side, .030 deep by .060 wide slot cut with a ball on a mill running parrallel to each other on the rotor face. Do not cut the slot where is leaves the ID of the rotor, the pad contact area. This will 'wipe the fire band' and help with pad fade.
Tire temps seem a bit low, push it man!!!! I just wanted to make sure you where not over heating the tires which make them like driving on
oil! Check for the proper range for that compound. If the temperature is even across the surface then you just need to push it harder!! Have fun!
Pad taper: When you pull the pads, measure the thickness at the leading and trailing edge of the pad, if the leading edge is thinner than the trailing, that is pad taper. This is caused by the leading running hotter than the trailing. If you notice some of the newer, higher performance calipers run two sized pistons leading to trailing with the smaller being in the leading edge. This is to combat pad taper. A badly tapered set of pads caused all sorts of things to happen, none of them good. If you have pad taper you can mill the pads flat and narrow the leading edge. How much you narrow it is trial and error. I start by figuring the difference between leading and trailing edge wear and remove that percentage from the leading edge tapering back to the trailing edge. This is only a starting point.
Just as an FYI, the 12.19" rotor is generally the largest that will fit in a fifteen inch wheel. Get the measurement from wilwood and measure first if you are thinking of doing this. But, if you do, realize that will all else being equal your system will now have a front bias issue that the only way currently available to you to get of is by using less of the front brakes. From what I am hearing, if you add any brake torque to the front, you are going to need to add some to the rear as well. What adds brake torque? Larger rotors, increase in clamping force, higher CF pads.
Rick
OK...........
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