Big Reds - Do I need prop valves??

[mmasse]

I recently installed a Big Red kit in my converted ´76.


I have Brembo F40 calipers in the front with 14" rotors, and 993 TT calipers/rotors in the rear.

I have notice that front tires (225-40-18) tend to lock much in advance of rear tires (295-30-18).

Do I need prop valves for this setup? If so could you please enlight me on where-how to install and also which will be the appropriate size?

Thanks

[EarlyPorsche]

An adjustable proportioning valve wouldn't hurt things. Wilwood makes a nice one. However, remember that in most cars the front wheels lock before the rear. Now put the engine in the rear and I think that its safe to say the front will easily lock before the rears.

[89911]

You absoulutely should have a proportioning valve going to the rear brakes on this setup. The chances of getting this 100% correct with none is very slim. The last thing you would want is a car with too much rear brake.

[mmasse]

Coming for the second round.

I have now done some adjustments to the suspension. I installed 22F and 30R torsion bars and exchange the front tires for 235 (following Bill's advice). this was done in order to solve some serious understeering that I was having in my car.

I still have to work a little on suspension set up since there is still some understeering but the situation is much better. I think that installing a fron adjust SRP sway bar will solve the problem.

Now, I still have the problem of front bias brakes. Front wheels still lock too much earlier than rear wheels.

Could I solve this by installing a propotioning valve on the front brake line?

Thanks in advance for you inputs.

[Bill Verburg]

Originally posted by mmasse
I recently installed a Big Red kit in my converted ´76.


I have Brembo F40 calipers in the front with 14" rotors, and 993 TT calipers/rotors in the rear.

I have notice that front tires (225-40-18) tend to lock much in advance of rear tires (295-30-18).

Do I need prop valves for this setup? If so could you please enlight me on where-how to install and also which will be the appropriate size?

Thanks

Absolutely not, A p/v can only be used to limit rear bias.

Assuming your alignment is reasonable(too much front camber promotes front lockup) what shoud be done depends on how bad the situation is
cas1- not tooo bad; try a pad w/ a higher m on the back

case 2- pretty bad; use 993 RS rear calipers(that's what I have on both of my cars) this is all assuming that you have the correct, modified, 322x28mm 993RS/tt rear rotors.

[klaucke]

A p/v should never be installed on the front brakes. The p/v only allows a certain amount of pressure through. Once you are over this limit, you cannot modulate the front brakes, so if they lock you have to drop the line pressure back down below the p/v's limit in order to unlock and modulate the fronts. This is not adventageous. A p/v on the back does not give rise to the same problems.

If you need more rear bias, you need larger pistons in the rear or smaller in the front.

I'm sure Bill V. will weigh in w/ a more detailed solution (edit: apparently he just did).

[mmasse]

Ok, I got it. Thanks

If i go with thinner tires in the back, will that help?

I was thinking of going with 275s to solve the understeering problem .

Actually, after I changed to 235 in the front the situation is not that bad, but on extreme braking, front does lock before rear and I feel that there is more potential to extract from the brakes.

[Craig 930 RS]

Give some thought also to a higher friction pad in the rear - generally speaking, perhaps not in your particular case.

[Bill Verburg]

Originally posted by mmasse
Ok, I got it. Thanks

If i go with thinner tires in the back, will that help?

I was thinking of going with 275s to solve the understeering problem .

Actually, after I changed to 235 in the front the situation is not that bad, but on extreme braking, front does lock before rear and I feel that there is more potential to extract from the brakes.

Probably a little. I dislike that much of a diparity front/rear, 235/275, 225/265, 245/285 are all good selections