I've been doing some searching and noticed in the brake FAQ, they mention the difference between na and turbo brake proportioning valves, that the turbo has more rear bias.
In my 91 MR2 non-abs, I've felt like the fronts are doing all the work when braking and I'd like to use the rears more. Also, like most people I'm running a much bigger tire than stock (215/245 stagger).
I initially thought of getting the 91 turbo non abs brake proportioning valve but I think the difference is very small and it won't move the brake bias to the rear enough to make a real difference.
That way you can adjust the brake bias however you want and costs about the same as a used 91 turbo brake prop valve.
Also, this is a really big deal with the mk1 guys, who often will "gut" their rear brake prop valve by removing the spring out of it and say this increases rear brake bias with decent real world results, the front still locks up first, but in theory this is a bad thing to do.
Anyone have experience with adjustable brake proportioning valves in a MKII?
Just adding the proportioning valve will not increase rear brake pressure. The valve alone will only allow you reduce pressure to the rears (which is what most people want to do when they are running bigger stickier tires and braking harder/deeper into corners going faster than most people). By definition the fronts will always do most of the braking becasue of the weight transfer forward under braking. Fast speeds into the corners mean more weight transfer forward and more need for more front bias. Proportioning valve (to reduce rear pressure) is a cheap way to keep the rears from locking up first (becasue they have been unloaded more with more weight transfer forward).
Just adding the proportioning valve will not increase rear brake pressure. The valve alone will only allow you reduce pressure to the rears (which is what most people want to do when they are running bigger stickier tires and braking harder/deeper into corners going faster than most people). By definition the fronts will always do most of the braking becasue of the weight transfer forward under braking. Fast speeds into the corners mean more weight transfer forward and more need for more front bias. Proportioning valve (to reduce rear pressure) is a cheap way to keep the rears from locking up first (becasue they have been unloaded more with more weight transfer forward).
I think his goal is to remove the factory valve and replace it with the after market one. True, it will not allow you to increase pressure to the rear, but it will allow you to decrease it less. The factory valve does this, but the theory presented here is that it reduces the rear pressure by too much. Ideal might be somewhere half way between the factory valve and no valve at all (equal pressure front and rear).
I do have to urge extreme caution on this however. I once had my rears lock first due to fronts that needed to be bled. It was quite the scary experience, resulting in a fishtail and spin at about 60mph. Make sure to leave yourself enough of a cushion on rear lockup when adjusting a valve like that.
I am aware of the risks of the rears locking up first, I would definitely do testing to make sure the fronts still lock up first and that the rears don't lock up first under any circumstances. Right now I'm just going to pick up a 91 turbo non abs prop valve and see if that makes any difference.
I mean the MK1 guys run with gutted prop valves and say its fine, I'm not that extreme however that is why I want the adjustable one.
I think the OP is meaning get rid of the oem and install an adjustable unit in place of it in which case you will DEFINITELY be able to increase rear bias. The knee point on a Turbo is roughly 850psi. Below that, there will be no change. Bowers N/A's knee point is around 430psi and from that point on, there is a significant reduction of rear braking ability on n/a's vs. turbos. With the adjustable, he could change the knee point to be even higher then the Turbo thus improving rear braking even more then the turbo should his setup allow.
Yes, more braking is don't by the fronts then the rears and yes weight transfer plays a significant roll but tires and suspension set play a major roll too. For a given load, a 245 will generate more traction then a 205 (all else remaining the same). The stagger plays an important roll too. If its increased more then stock, then the rears can generate a higher percentage of the total then stock would for any given condition thus rear bias should (well could) be increase for better overall braking performance.
Suspension (more accurately ride height / Cg height) plays significantly in weight transfer. If the Cg is lowered, less weight transfer occurs thus meaning more weight is staying over the rear wheels during braking.
To the OP, the knee point is roughly 400psi higher on turbos, this is pretty significant. As said above the rough knee point are 850 for the Turbo and 430 for the N/A. Below the knee point, the rears are receiving full line pressure so in the n/a case with a Turbo prop valve, rear pressure difference thus bias is pretty significantly different above 430psi.
That being said, I would still go with an adjustable cause you car might not like the knee point being that high cause you don't have as much weight in the rear compared to a Turbo.
Remember, rear bias is only changed above the knee point. Below it, the rears receive full line pressure thus the bias is only what the natural system bias is. This can only be changed by different pads (different coefficient of friction levels), different rotor diameters, and lastly, different piston areas (I.e. different calipers).
More food for thought, the natural bias of all mkii's (n/a, turbo, pre and post 93) is pretty much 60/40 f/r. There is slight differences from turbos to n/a's but its only like .1 to .2%. The same goes for pre and post 93 turbos.
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