3.2 to 930 brake conversion questions

[mnez]

I'm finishing off the 930 brake conversion on my '86 3.2. I've seen it recommended to remove the Carrera proportioning valve in the trunk and join the ends of the rear line. Why is this? Did the turbo not use one? Or a different one?

Are people leaving the rear line without a proportioning valve, or putting in an adjustable one, such as one of the Tilton units?

The car is tracked, with R-compound tires.

Thanks.

[bkreigsr]

Has to do with the specific proportioning for each model.
There's a lengthy thread on the subject that convinced me to gut the valve.
Couple of notes - if you do, be sure that all the internal components (springs, discs, o rings, and stoppers) are removed before you re-install. I left what I thought was a part of the valve inside - turns out it was supposed to be removed. (you should be able to see daylight from one end to the other)
Also bleeding is a bear. When all is said and done, it's best accomplished the old-fashioned way with a pedaler in the driver's seat an a bleeder at the corners.
Bill K

[KTL]

The proportioning valve is removed because it was a "band-aid" sort of fix that Porsche applied to the Carrera braking system. When the Carrera got its larger rear rotors (wider, more heat sink ability) the new wider caliper for some reason got larger pistons. The designers apparently decided that this caliper would provide too much rear brake and therefore installed the valve (actually a pressure reduction device) to limit rear brake pressure above a certain pressure threshold.

The 930 calipers front vs. rear have a good bias ratio and therefore the valve isn't needed.

You don't need to gut the valve. Just un-bend the lines and use the union that is already installed right there on the rear lines. You can "stretch" the lines quite easily w/out any worry of breakage and just take the valve out altogether.

[tshebib]

Hi Mike,
I've done the same upgrade to my car as well. I got tired of rebuilding my front calipers after every track weekend. Here's a link to show you exactly where the valve in question is. On my SC there is no such valve.
Turbo Calipers and Floating Discs
Hope to see you at Calabogie this July. How's Dave doing with his 930?
Tom

[KTL]

The link that tshebib actually has a picture of my car in it!



That piece the blue arrow is pointing to is not the pressure limiting valve. It's the union I mentioned. The line coming off the master cylinder looks different because I decided to bend my own line for some stupid reason when I removed the valve.

[mnez]

Tom, unforyunetly I have to give Calabogie a miss for the first time in years. My car has seen a lot of changes over the winter, I can PM you the story.
Bill and Kevin, thanks for the details.

Does anyone use an adjustable valve in its place? Or is rear lock-up on heavy braking never an issue with the 930 set-up?

Thanks
Mike

[KTL]

The 930 setup provides solid bias as-is. The adjustable valve is not needed. Also, keep in mind that the valve can only reduce pressure to a brake circuit. So by installing the valve on the rear of the 930 setup, you'd be adjusting yourself toward more front bias. We typically don't want that.

[mnez]

OK Thanks.