So, on to the brakes in this new car. It still has the OEM proportioning valve for the rears. As I understand when the brake pressure reaches a certain limit (500 plus psi as I recall) it stops sending fluid to the rear calipers. My first question is does it stop and hold at that psi leaving the rears engaged to that level while allowing fronts to get more input, or does it drop the rears to 0?
The other consideration is to use a manual valve such as the Tilton. If going that route how does one determine settings without chancing spins or other drama?
Thanks.

Not positive on the OEM system, but I think the pressure holds in the rear, while transitioning more to the fronts.

Both my track cars use the Tilton (or close version of). I have not messed with either setting from what the PO had previously set (its near neutral on both) but seem to have a nice front to rear balance.

The FAT -4 has Wide A vented fronts, M vented rears with a 19 mm MC, the -6 has 993 TT fronts and rears with a 21 mm MC, so calipers are well balanced from get go.

I’d recommend an adjustable if your class rules allow
(I used to run in club racing where the stock class did not allow)
I use a lot more rear pad with mine adjusted
I change the balance nearly every weekend
You can feel the difference depending on weather and even tires sometimes

To initially set your balance
Drive slow /30mph or so in a parking lot
Have 2 friends stand on each side of the car as you drive by and lock the brakes. See which axle is locking first. Then fine tune on the track

The factory regulator is far from optimal on a race car, but you can make it work better by installing a longer allen screw in the end and turning it in about a quarter inch further than the stock screw. Better to replace it with something designed for precise tuning.
On my first build I put a knob type bias control valve in the middle of the cockpit where I could easily adjust it while driving but that required making custom brake lines for the tunnel.

To clarify, both the stock prop valve and the Tilton-type regulators (whether screw or lever) work exactly the same way: they don't "stop and hold at that psi" but they change the rate at which the rear pressure rises in relation to the front.

See attached chart as an example.

Which way you go depends on your needs/desires. As Chris pointed out, you can increase the rear ratio by screwing in the threaded shaft a bit more; what he described is exactly what I did to my street car (per his instructions) to get more rear bias. The factory one is a bit too conservative for my driving, but is safer for the average driver on the street (and in the rain). That's a first step you could start with right now, with about 20 minutes' effort.

If you choose to re-plumb and add a manual valve in the cockpit, you'll remove the factory one entirely and insert the manual valve in a convenient location.

You could, technically, replace the stock one with a manual valve in its place, but brake proportion changes would require you to come into the pits and dive down deep under the car to change it. I'd suggest what Chris describes, and that's mounting it in an easy-to-the-hand location in the cockpit.

As for screw type versus lever? That's your personal preferences. The screw type will give you the option of much finer adjustments versus the 6-position lever.

I personally prefer the lever type because I can reach down and "feel" where I am on the prop valve setting without having to look at it. Plus, the fewer things I can mess up, the better... I doubt I can differentiate between 3 notches and 3.5 notches, let alone 3.25 notches.


Click to view attachment

On a racecar, I'd definitely want the Tilton type. If I'm not mistaken, the stock one, in addition to having other sub-optimal qualities mentioned above, has a valve that moves substantially as part of its normal action. I recall being able to hear it actuate when testing for firm pedal during brake bleeding on my street 914 years ago, and I could feel measurable pedal movement as it actuated. Not ideal in my opinion.

You bring up an excellent point that I learned long ago on a race car that I set up with 163 (exaggerating) way adjustable Konis....that just meant there were 162 wrong settings. After screwing with those for a season, I threw the old non adjustable bilstein group B struts back on the car and they worked better than I ever got the Konis working. lol. Simple is better in many cases.

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