trackbird,

Jegs has a Wilwood proportioning valve in their catalog for $40.00. Sounds like a good investment!

 

Yup....

The lever style are nice, adjustments are "repeatable" and it is easy to tune (no "how far to turn it" questions when setting it up).

Have fun!

 

the noob chimes in...
I'm familiar with Swagelok fittings. You're right, they can handle thousands of pounds of pressure, but the wall thickness and material used to make the stock brake lines was selected to work with flare fittings. It's a horrible feeling when the brake pedal goes right to the floor, and that usually happens when you're pressing the pedal real hard.

 

The only timeI would consider getting rid of my ABS is if I went to a custom pedal box with a double or tripple master cyl. ala Tilton style. In the hands of a good driver you can stop the car faster than an ABS set up will. This is only if your brakes are up to the task. Something I have been looking into though

Until then, I just take a big giant dump prior to hitting the track and skip breakfast.

 

I would LOVE to dump the ABS on my truck. The only time I found that it works reasonably well is on clean dry pavement. Any other time I hate it. Especially in the snow.

There have been countless times in the snow where I scared the hell out of myself when the ABS would go off and I cannot stop. Sure, I can sort of steer - but that RARELY does any good when I'm trying to STOP and the only thing my brain is telling me is to stand on the pedal with both feet.

There is a clear-as-day braking threshold in the wet and in the snow, where I can brake MUCH harder than the truck does with the ABS buzzing away.

 

Somebody called for a brake system engineer? Nice catch on the prop valve trackbird. Now if you're driving your car on the street with the same tire on all four corners, you need a prop valve. Modern cars have pretty strict regulations on the brake system and there's one in particular that concerns the prop valve. Cars are designed so that under light braking (~0.2 g's) the rear must not be underbraked and under high decel braking, the fronts must "tend towards lock" first. It basically means that OEM's MUST include proportioning of some type. In the f-bodies, the proportioning is done by EBD (electronic brake force distribution) within the ABS unit. If you take out the prop valve, or ABS unit in our case, the rears are now heavily overbraked during high decel and they WILL lock up before your fronts on any street car (look up the recall on the 1980 Citation). It's not a big deal if you lock up all four, but if you lock the rears and don't give enough pressure to lock the fronts, the car will spin (been there, done that.) It's particularly bad on our cars because the weight distribution is more rear-heavy than the average FWD, but under heavy braking, an F-body has a similar weight distribution as a grand am. This means our big rear brakes make us highly unstable without the prop valve.

Now, the highly debatable part... a track car running slicks out back and skinnies up front probably won't need the prop valve because the skinnies can't handle nearly as much braking as the slicks, not to mention a lot of brake pressure is wasted on the inertia of the bigger tires and if they're taller than the skinnies, you get a reduction in brake torque because of the longer lever arm. I can't say where to draw the line because the ideal brake force ratio depends on too many variables. Assuming the calipers, rotors, and master are all stock (or at least stock size), the ride height, tire radius, weight distribution, and tire size/type all still play a role.

Personally, I would pull the ABS system on a track car and toy around with a prop valve IF the car became a dedicated track car. But I've also got the advantage of a finely tuned assometer when it comes to braking, so I'd feel comfortable spinning out a few times while tuning the prop valve myself. As for a street car, I've had to brake at the ABS threshold too many times on the street to no fault of my own. I already drive smart, it's everyone else I can't trust...

 

There you go, an engineer has spoken!

I believe the LT1 cars had a mechanical proportioning valve, whereas the LS1 cars use the EBD to electronically perform a similar function. The problem I have with the EBD, is it doesn't always work correctly. I've mostly noticed this while autoX racing on Hoosier race tires, all four of the same size. I would prefer to have back the old system of no ABS and a mechanical proportioning valve, and have full control over my car's braking, rather than a computer that is trying to blindly take a guess at it.

And while I have the attention of an engineer, I would like to mention that both the ABS and ASR go offline at about 135 mph when making runs on the Salt Flats, every time. The system doesn't start working normally again until shutting down and restarting the engine. The Bonneville Salt Flats is actually a slippery surface since there is a layer of loose salt on the surface. This is one time I wish the ASR had worked correctly! Have you ever fish-tailed at 140 mph?

 

Grandamit,

Welcome. It's nice to have you aboard and thanks for providing lots of useful information.

Cal,

I'm suspecting that it's the tirespin that is causing your issues. The difference in speed between the front and back wheels is probably just enough to confuse the computer. You might try a slightly shorter tire on the front (speed up the wheel speed) as a creative band aid (if you can get the foot print you need), but the size required may be a guess at best.

 

trackbird, that's an interesting and creative solution. I think it's the wheel spin combined with the speed that's the problem though; almost as though there isn't enough processing power to keep up. I say that because it never threw lights on the 125 mph qualifying runs, with just as much wheel spin. My guess is GM didn't really test the system for wheel spin at 150 mph; who in their right mind would go that fast on a slippery road?

 

True. However, at 150, it is likley more difference in RPM than at 125. Now, if the system uses a % of difference to set a code, that's one thing. If it looks for a threshold (300 rpm?) then, at 150+ you may cross that amount between the extra tire spin and the higher rpm of the front tires. I'd bet a 3% or 5% smaller tire would be a good starting point.

Yea. And at 150+ on good pavement, the lights stay off.

 

Cal, the only thing I can tell you about the ABS/TCS shutting down at 135mph on the salt flats is to check the error code stored in the computer. Even though it resets the lights, the error should still be stored. I don't want to lead you down the wrong track by guessing what the problem could be when your car can just tell you. The problem with putting the smaller tires on the front is now you're could upset the ABS/TCS at low speeds...

I know what you're feeling here, but trust me, the limit braking performance with EBD (even with race tires) is far better than with a prop valve. EBD tries to mimic the ideal brake force ratio while keeping you on the safe side of it. A prop valve tries to mimic the ratio too, but the ratio is a curve (different value at different decel) and EBD can follow that curve very closely, even with race tires. A prop valve tries to follow the same curve with two (sometimes three) straight lines, so naturally it can't follow it too closely, which means your rears are underbraked. Underbraked rear axle = understeer... Unless of course your goal is to make the car push more Think of it this way, when you feel EBD, it's not telling you that you're braking too hard, just that you're perfectly balanced

 

GrandAmlt, thanks for the reply. The Salt Flats thing happened last year this time, and while I can't be certain, I thought I checked the codes and didn't find any.

As far as the EBD working better than a proportioning valve, I would like to believe that. I'll leave the system intact for another racing season and see how it goes. I do know that wheel hop while braking has become an issue with the Hoosiers, which leads to the ABS kicking in. When the racing starts again this spring, I'm going to an adjustable torque arm to address this.

The car already pushes while braking in a turn, so I try to get all my braking done before I enter the next turn, then add power if I'm slow.

 

I agree with the possibility of upsetting the abs at low speeds. My thinking was to keep it minimal (just a tiny percentage smaller in the front) to try to avoid crossing the "threshold" that sets a code/light (if that is what we determine to be the cause). If we could keep to a reasonable change (possibly not noticed by the computer at low speed) and enough to keep it a little closer at high speeds. It is a guessing game, but if we could get the specs for the system (or reverse engineer them), then we may have some luck, or we may waste some time.

 

I probably should have said more about the lights that were thrown . . . the pannel was lit up like a Christmas tree . . . most of them were stuck on, and the ASR switch did not function. It seemed like a complete computer failure, but cycling the ignition brought it back each time. As I said before, this happened on every run at over 150 mph, otherwise it would have seemed like a random computer glitch.

 

That's true about the wheel hop playing nasty games with the ABS. Only way to solve that is fix the wheel hop or go with a prop valve and no ABS...

The Salt Flats 150+ thing is puzzling. trackbird's idea about the small amount of slip on the rear axle causing the fault is possible, but I'm not sure how likely it could be. I'd imagine you would have to carry a decent amount of spin on one wheel for a long time without ever slowing down to the front wheel's speed to set it though. Since the fault doesn't occur on pavement, it makes sense that it's the wheel spin. I still wouldn't recommend trying to fix it with new tires without knowing the fault code.