I am running larger 4 piston Outlaw (read Willwood) calipers on the front of my v6 track car, with 911 M calipers on the rear. I installed a brake bias adjuster knob initially but my rears lockup under heavy braking no matter the adjustment. Switched to a Tilton bias adjustment with the notched rod with multiple positions and still no change. I cannot tell that there is much difference in the pressure to the rears. They will lockup no matter the position under heavy braking. They are not sticking. Not sure which direction to go from here. I am running Hawk blues front and rear. Perhaps a less aggressive rear pad? And yes the factory proportioning valve was removed.
thanks for any help guys.
LC

Quick test: Go buy the cheapest pads you can for the rear from your local parts house. Put them on and see if there is any difference. I went from Hawk to elcheapo and solved my lockup issue.

Thanks Randall. That certainly sounds like the "cheapest' place to start.
I'll let you know if it works.

I'm running 911 calipers on the front, stock calipers on the rear. If the elcheapo pads don't work, throw a stock set of calipers back on and see if you still have the problem. If it fixes the issue then you know 911 calipers on the rear are a bit much. Also took off the adjustable bias brake module as it never seemed to work as advertised and just went to a T fitting.

Strange that they keep locking up even with the bias dialed back. I would think that should do it, plus you get some natural proportioning wtih the front calipers being larger than the rears.

Silly question - is your aftermarket prop valve installed in the rear brake circuit like the stock one?

Scott

We took out the rear proportion valve and simply put in a T fitting. So no proportioning what so ever. Then started playing with the different pads, but actually the first attempt fixed the lockup, i.e., elcheapo pads instead of the Hawks.

And to be honest I really like the set up as I don't have to deal with lockup on a hill where a lockup could get ugly.

I wasn't clear in my questions! I meant to ask Spaceshuttle where his proportioning valve is ... if it's where the stock one was, I don't see why it wouldn't limit the rear braking force and keep wheel lockup under control.

..unless it's just simply too much clamping force between the larger caliper and larger pad. Look at how small a stock rear pad is. With good, grippy pads I can definitely see where the rear bias is too much to overcome. Also, I don't think that it's possible to completely dial out all rear bias with most adjustable prop-valves. I think it largely depends on how it's plumbed. I'm with the cheap and simple solution - try some really cheap pads next.

Interesting....I've got a similar setup, Wilwood fronts #120-11131 with 1.62 inch diameter pistons, and 38mm 911 rear calipers. Depending on which Wilwood piston diameter you are using, with 48mm "M" caliper pistons in the rear, I suspect that you may have too small Wilwood piston diameter. It's all in the ratios--I'm using Porterfield R4 compound pads all around and have had no rear lockup issues. The Wilwoods come with several sizes of piston diameters....here's the link to their page:
http://www.wilwood.com/Calipers/CaliperLis...lite%20Internal

Ok I have changed to a cheap pad and put the stock rear calipers back on. have not driven the car yet, but I will as soon as it warms up a little down here.
The Tilton valve does make a difference, but they were still locking up in any position.
I have another track car with the same setup, BUT that car is a V8 and it has a 930 tranny, so it's a couple hundred pounds heavier in the rear and I have had no problems. Thanks for all the info. I will try it out in a day or so.

Definitely odd. I have Carrera A front and M back and have never had lockup issues. Currently using Pagid Orange pads on both ends. Previously I had Blacks (more aggressive) in front but they could not hold-up to temps as well. With both pads I typically have the prop valve all the way open (max rear). My mechanic suggested just removing it but so far I have left it in the car mainly in case I ever drive in the rain or have some other weird situation where I need to dial-back the rears.

Maybe a more aggressive front pad is another option vs. a different rear pad. Also I wonder if weight distribution (significant rake or corner weight imbalance) could be a factor?

Tough to go wrong with the A/M set up. Porsche engineered that set up for the 911 that has similar F to R weight ratio, IMO, the ratio of clamping force of the front to rear brakes works fiine on a 914 also. I had S front calipers (Same piston size as the As). I did back off the rear pressue about 3/4 turn on the porp valve...as a personal prefference.

I suspect the OP has a unknown ammount of brakeing force (too little) on the FRONT calipers & that is the problem.

I don't think putting inferior parts on as a bandaid is a good idea.

I think I have identified the problem and it seems to be a weight transfer issue. If I apply the brake gradually I can press as hard as I like and no lockup till very slow speed. If I just jump on the brakes quick and hard, it locks up in a second or 2. When the nose goes down, it is the right rear that locks up. More weight on the left cause my arse is over there and no lockup. I put in larger torsion bars on the build (can't remember exact size at the moment) but I think I might have to correct spring ratios.
Running 250lb rears and I think it was a 21 or 22mm in the front. Car weighs 1900 dry. I will check the Torsion spring rate for sure on Monday.

Before doing anything I would corner balance with you in the car. My formula car experience has been that corner weights have a huge effect on things like this. Messed up corner weights can do really wacky things...

Big secret here. Use the weight balance to determine tire widths.