Increasing Rear Bias
In this month's Pano there is an article by Kim John Crumb on brakes. Part of the article describes a modification in which the rear brake pressure regulator from late model Porsches can be replaced with ones from earlier years to boost rear brake pressure (ABS will continue to function). He has replaced the regulator in his GT (18 bar of pressure) with one allowing much higher pressure (55 bar), greatly increasing rear braking. I was considering doing this to my GTS, replacing the regulator (18 bar) with one from an early '86 (33 bar). The intent is to increase overall braking abliity, decrease wear and heat from the front brakes, and decrease front nosedive under heavy braking. Has anyone out there with a GTS tried this mod. at the track?
I did the 55 bar modification two years ago. I liked it so much that it's now on the GT. Regarding the benefits real or imagined here is what I have experienced. Mind you, I am not a dedicated track junkie, so my experiences are limited to about a dozen or so track events and about 100 autocross events.
The increased rear bias regulator works better that the 18 bar regulator in that it allows the rear calipers to contribute more toward the overall braking effort. This has the effect of reducing the overall braking contribution from the front calipers. The car is still braking at maximum efficiency, but it's doing so with more poise and balance. I find that the brakes have less fade than before. Best of all, the front pads and rotors are wearing twice as fast as the rears, where before they were wearing three or four times faster. YMMV.
Caution, the increased bias works well with ABS 928s. I do not recommend that you try this in a non-ABS car. The last thing I or anyone else would want is for the rear wheels to lock-up first. Such an occurance could easily lead to an oversteering condition that can then result in loss of control. (Gosh, I hate when that happens.)
Good luck and merry motoring, Ed.
Why did Porsche change the valve from a 55bar to an 18bar in the first place? This doesn't seem in line with Porsche's continuous improvement approach. Their must of been a reason for the change.
Actually, the 928 series started out with a 33 Bar regulator. (Those of you with very early models, please correct me as I'm not absolutely certain that thy even had rear brake regulators.) When the S4 brakes were added to the 86.5 MY, the regulator was changed to 18 bar. The 45 bar and 55 bar regulators are from the 911 & 930 series. (I'm not absolutely sure of this, so YMMV.)
Why Porsche reduced the rear brake bias probably has to do with safety and avoiding or reducing law suits. Having too much rear brake may cause them to lock-up first. This could easily lead to an oversteering condition.
It is much safer to drive a car with lots of understeer. However, it's also less fun. Correcting an understeering condition is very easy. To correct, whatever the driver is doing to cause the condition, he/she should do less of it. If the steering is turned too sharply, turn it less. If the brakes are applied too severely, ease off the brakes, etc.
Correcting an oversteering condition is far more complicated, and usually not very intuitive. For example, you may have heard that you should not brake in a curve, particularly if your car is an older 911. The main reason has to do with weight transfer. When braking the weight shifts forward and adds more load to the front tires and less load to the rear tires. If the car is also in a turn, it is possible (and in older 911s, very-likely) to have a very small rear tire contact patch. With the engine hanging beyond the rear axle, older 911s will behave just like a dart that is thrown with the feather end facing forward. As you can guess, the heavy end will want to go first. 928s are more evenly balanced than 911s, but they can be induced to oversteer, especially in a turn while braking or with a sudden lift of the throttle. To keep the back end from coming around it is important that the rear tires continue to maintain maximum contact patch. If the rear seems to be coming loose, the correction may be a combination of moves to keep it from coming around. Usually, some counter-steer and some light throttle application will keep the rear tires from losing contact and thus keep the backend from coming all the way around. (With major capitol letters; YMMV!) Moreover, quick reactions are paramount if a spin is to be avoided. That's where practice, and lots of it, helps make one better prepared for such a condition.
Merry motoring, Ed.
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