brake prop valve adusting, any experience doing this? Options

[Trekkor]

I still have the stock proportioning valve in the car.

I think the brakes favor the fronts a little too much.

I smoke the fronts, never the rears.

I'd like to put on some junk tires and see if I can get to the point where when I apply max brake pressure, the rears lock just after the fronts. (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

Anybody try this experiment?


KT

[Jeroen]

Believe me, you DON'T want your rears to lock...
But some more rear bias (or less front) can be good

[JWest]

QUOTE (trekkor @ Aug 23 2005, 01:05 AM)

I'd like to put on some junk tires and see if I can get to the point where when I apply max brake pressure, the rears lock just after the fronts. (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

That is a bad set of control conditions. The good tires you normally run will transfer more weight to the front (more grip), leaving less vertical force on the rear tires (less rear grip and more front grip). Your rear brakes will lock first if you had the bias set perfectly with the lesser tires.

You probably do want more rear bias than stock, just don't test it this way.

[914werke]

(W/O checking first) do any of the usual manuals describe how to ADJUST the OE prop valve? (IMG:http://www.914world.com/bbs2/html/emoticons/huh.gif) I thought it was fixed?

[URY914]

The only way to adjust the stock valve is to remove it and toss it in the trash.
Get a Tilton valve if you want to play with it. (I got one I'll sell ya)

Paul

[Trekkor]

Thanks for the responses.

James, I'll listen to that. I hadn't considered the tire factor. (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)

The stock valve has a allen headed adjuster with a jamb nut. Just like adjusting a valve on the rocker arm.

I want to know which way to turn the screw to give a slight bit more brake to the rears.

I would guess, " Lefty Lucy "... (IMG:http://www.914world.com/bbs2/html/emoticons/lol2.gif)

KT

[pete-stevers]

Eric is a good guy to ask about this....perhaps he'll chime in
I my rears lock slightly before the fronts, kinda thows the car sideways when they ( the rears) lock up first...somthing i gotta fix (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif) I would say it might be a bit of a balancing act

[brant]

The tires comment is absolutely correct.
you want to do this test with the tires/brakes/all variables just like you intend to run it.

there is a lengthy article on the stock valve.
the article says that the adjusting bolt is not long enough to cause the change that you are looking for.

It is theorized that you could remove the adjusting bolt completely and buy one that is 1/8th longer and then get more than is possible from stock.

However... before you go to that extreme. I agree with paul, you should buy a different valve. The only reason for not buying a different valve would be some rules classification.

I know the local pca chapter would not allow an aftermarket valve in the stock class, and that would be the only time I would even consider fiddling with the stock valve.

Instead, buy it from james. His price is as good or better than anyones, and he will sell you the correct bubble flare convesion fittings you are going to need and won't be able to find anywhere else. I know this because when doing the new car I bought a valve somewhere else and then months later when I realized I needed those conversion fittings I went to james anyways.

Now onto testing.
for a ROUGH estimate of setting the new valve.
take a ride in the country on a dirt road....
you can easily judge lock up that way.
even better to have a friend stand outside the car and watch for wheel lock up.

this will not take into account weight transfer.
but it will give you a rough estimate.

seat time, practice (settup) sessions, and a private track rental will get you the rest of the way.

brant

[zymurgist]

This is a bit OT... but I saw a 914 advertised on Ebay where the seller said "proportioning valve replaced with a T fitting." That didn't seem quite right to me, but is there any reason that someone would want to do this?

Not planning on making this mod to Babydoll... I'm just curious.

[lapuwali]

If you put bigger front calipers on the car, replacing the prop valve with a tee fitting will probably help move the brake bias to the back, as just upsizing the fronts will move it forward. It's pretty clear the stock setup has too much front bias even with stock calipers. Even with a tee-fitting, you'll still have some static front bias, as the front caliper pistons are quite a bit larger than the rear pistons.

[URY914]

I've also been told several times over the years that the stock p-valve holds air and make is difficut to bleed the system completey.

So a installed a Tilton and ran it for several years. Set it and never adjusted it.

I now have a T fitting.

P

[brant]

James,

I know this isn't a "T" thread...
but just the piston size of front versus rear is not the whole picture. Weight transfer plays into it as well as age/variance in pad compount/tire compound/etc.

And putting a T on a stock system is an argueably bad piece of advice. my own personal experience of a stock race car with a T resulted in leaving the track at 75mph backwards.

not something I would want to recommend to many people.

But overall I do agree that a T allows more rear brake...
just too much more for a stock set up.

brant

[KenH]

Most 914s I know about have replaced the valve with the "T".

Bias was added by running agressive pads up front and the next lowest rate pad in the rear.

Have run this combination with 2 914s without incident.

If you want to play then add an adjustable valve.

Not hard, but you have to make up new brake lines to connect it.

Ken

[lapuwali]

QUOTE (brant @ Aug 23 2005, 12:50 PM)

James, I know this isn't a "T" thread... but just the piston size of front versus rear is not the whole picture. Weight transfer plays into it as well as age/variance in pad compount/tire compound/etc. And putting a T on a stock system is an argueably bad piece of advice. my own personal experience of a stock race car with a T resulted in leaving the track at 75mph backwards. not something I would want to recommend to many people. But overall I do agree that a T allows more rear brake... just too much more for a stock set up. brant

Well, I did say "static front bias". I'm not advocating anyone fit a tee, only that you will have front bias with one. Whether or not you can use one depends on a huge array of factors, including your tires, your pads, your ride height, what suspension you run, how much gas you have in the car, and your driving style. All due respect, but lots of people run tees with otherwise near-stock brakes successfully. As always, one needs to be careful when making any changes substantial setup changes to brakes or suspension.

I would advocate running an adjustable prop valve over the stock unit. Even if you have to run it open, they're much smaller and lighter than the stock unit, and a tee isn't a whole lot lighter or smaller.

[brant]

james,

thats fair.
your right.. my set up may not have been exactly the same as the next persons... or even exactly stock.

I wasn't trying to re-open the debate, but wanted to let any new person reading along know that its at least a debateable topic.

I agree with you.
use caution.. everyone's mileage may vary in regards to the T.

and for the record, I have a T and an adjustable valve in my new car. (mostly because I couldn't afford a dual mc adjustable set up, plus this is lighter than that)

brant

[airsix]

I don't advocate running a 'T'. I have vented 'A's on front and back and with no bias I can lock the rears too early. I run a Wilwood proprotioning valve and all is well. Throw the stock valve in the trash. It's a POS. It displaces a HUGE volume of fluid and contributes to mushy pedal feel. The Wilwood valve serves the same purpose but displaces very little fluid and gives much better pedal feel.

-Ben M.