993PET

Does anyone have knowledge of using the ABS Brain/Computer from a 964 Cup Car in an RS America Race Car with big red brakes? The upgrade is I'm told supposed to improve the problem of the stock ABS brain cutting in too much on bumpy tracks because it is tuned for low grip tires.

I'm getting conflicting information on whether the 964 Cup Car brain will work on the RS America

Thanks,

JF

ltc

Ask Geoffrey, I'm sure he'll know.
BTW, you can always install the ABS cutoff switch (as was installed in the Cups).

I tried it once at Lime Rock (not exactly a smooth track in its day).....yeah, that was a great idea.

993PET

Lewis:

I'm not sure who Geoffrey is, what's his user name. Also can you elaberate more on the ABS cutoff switch on your 964 Cup? Why would you need this if the Cup Car ABS functioned properly, except for by choice?

Thanks,

JF

Geoffrey

The 964 Cup ABS system uses a different master cylinder and a different power brake booster (called a brake servo) than the C2 turbo it shares in common with. The brake servo has less assist than the turbo but I'm not sure what the difference is in the master cylinder itself. The actual ABS hydraulic valve unit is shared between the C4/RS/Cup/Turbo and it is a 3 channel system meaning that if it pulses the rear circuit, it will pulse both rear calipers. The actual electronic control unit (brain) is also the same between the C2/RS/Cup/Turbo so you'll get no performance benefit from changing out the control unit. So, yes, the 964 Cup brain will work in an RSA because they are the same part.

These systems are not very good for a car with sticky tires and used on a race track, especially a bumpy one. They are simply a very early ABS design and they don't work well.

The cutoff switch removes power from the ABS brain so the ABS system can be turned off. The brakes continue to function normally with power assist, but with no ABS.

DrJupeman

I'm not sure the Cup ABS brain is anything special. It certainly isn't the "racing ABS" of the current Cups. I think better "racing ABS" started in the 993 Cups. Geoffrey will know (and has probably told me before and I'm just forgetting).

That said, there are two things. The 964 Cup has an ABS defeat switch which disables ABS entirely. It is there because the ABS on the 964 Cups apparently wasn't considered to be that great vs. the driver's ability. Some will say that it is there to hit if you're rolling backwards as the ABS in these cars doesn't like the brakes applied when you're rolling backwards.

Further, there is also an ABS push-through switch on the master cylinder which will deactivate ABS if you push the pedal with enough force (not all Cups had this, mine does).

Still want the 964 Cup ABS brain?

ltc

Yes, thankfully, he found this thread

http://rsamerica.net/articles/tech/A...rride_tech.htm

In a moment of stupidity, you might say to yourself...."I bet I can drive without ABS"....followed by a code brown moment.

Geoffrey

Actually, the ABS control unit doesn't care if the wheel is moving forwards or backwards, it still reads correctly. The cut off **** exists because with ABS you cannot put 2 feet in and spin the car, the ABS will activate and will go in the direction the wheels are pointed.

993PET

So optimal for my "NEW" RS America Race Car is an ABS cutoff switch for bumpy tracks! I do have experience racing an 86' 951 with big brakes and no abs, so hopefully the pucker factor should be somewhat reduced if I use the cutoff switch.

Thanks

JF

ltc

That will remain a secret between you and your suit's dry cleaner

NaroEscape

Jeez...you guys that can even THINK of ABS are sooo spoiled...

Real men don't do ABS....

924RACR

LOL... yeah, when things have gone pear-shaped is a poor time to learn to threshold-brake your suddenly non-ABS car...

Nader Fotouhi

I do not know if this is relevant to 964 let alone 964 cups, but on my 944S2 that has the same early ABS technology, when the ABS shuts off, the rears lock up under heavy braking. The 944 turbo cups had the ABS cut-off switch and I have put one on my S2. Another way that the cut-off switch works is when you turn it off and on, it re-sets the ABS similar to turning the ignition key off and on.

BTW, I use to get the ABS going off o lot. Someone suggested that it may be because I did not use the rotor screws. I have not had any problems with the ABS shutting off since I put the screws back on. May be coincidence, but it worked for me.

924RACR

Nader - does your ABS unit have the prop valve change (for the more agressive prop valve)? I'm guessing yes? That and/or you're running a more agressive pad compound on the rear...

However, you're barking up the right tree, in that later ABS software also does electronic proportioning of the rear brakes (either in addition to or in complete replacement of a prop valve), and obviously that function will be disabled if you shut off the ABS unit, which can result in locking the rears before the fronts! Woohoo!!!

Geoffrey

The ABS systems do NOT do proportining at all. All proportioning is done via the master cylinder internal sizing and the pressure limiting valve on the ABS output line.

924RACR

I was speaking in more general terms about ABS systems, not strictly about 944's or 964's... sorry if that wasn't clear.