Turbo brake dilema

[John Sims]

I find myself in a bit of a quandry.

As some may know I swapped my 87 Turbo callipers and discs for SE callipers and 928 S4 discs (i.e. late model Turbo) some time last year.

This conversion wasn't without problems and required an addapter block (from Lindsey Racing) to mount the callipers, but all lined up OK. The discs and callipers were second hand and a little worn.

I now come to replace the discs and have remebered how close the steering rod knuckle is to the disc - as in the rubber boot is almost touching. With new unworn discs the gap is almost non existant.

During the first swap I assumed this was as per Porsche design and should be OK. Now, having interogated PET, it would seem that there are 8 allternative types of steering knuckle (thats the axel/hub carrier that mounts on the bottom of the strut). Great, whist I expected there to be a difference for the calliper mounting I am now assuming the the steering arm element has different lengths/andgles to accomodate the different discs.

My options as far as I can see it:-

1. Dump the SE set up, go back to my original callipers with drilled 28mm thick discs and uprate the pads/hoses. These are lighter which is a good thing.

2. See if I can get a set of second hand SE steering knuckles - a total pain as I've just had the geometry checked.

3. Fit the new 928 discs and hope all is OK - they havn't been a problem for the last year. As an indication of how close the worn disc is to the rod end - if you fold up a bit of card in half, the rubber will hold the card against the side of the disc.

What do you think gentlemen?

 

[Diver944]

John,

According to www.clarks-garage.com the SE disks should be 32mm thick when new and the minimum thickness allowed is 30mm (I haven't double checked this in the workshop manaual but Clarks is normally right). This means that if your disks are right at the minimum and you have more than 1mm clearance to your knuckle then you should be okay (1mm wear on each side of the disk). Just how thick is your sophisticated cardboard measuring device []

It's amazing how little wear is allowed for disk thickness before it needs replacing - in this case its only 6%

How desperate is your need to change them? Because I need to change mine in a few months or after one more trackday. You could wait until then, buy your disks, fit them and if you don't have the clearance then I'll have them off you for what you paid (Strasse have the best prices at the mo and I have a 10% off Christmas Card from them [])

 

[John Sims]

Just how thick is your sophisticated cardboard measuring device

It must be at least 1mm. Well maybe just about 1mm. [8|]

I do have a set of new drilled Zimmermann 928 discs in my hall, but haven't fitted them as they are wrong (the polite term). One disc has two rows of hoes the other disc has four, so the surface area on each is completely different. They are supposed to be replacing them but, at the current rate of service, I could have changed cars before that happens [] Not the happiest of bunnys.

I am still considering flinging the 928's and going back to the original set up. The original discs themselves are only slightly smaller in diameter, but are thinner at 28mm. The 928s are better in so far as the vanes are handed and spiral so they suck air through the disc where as the originals the vanes are just straight out from the center.

The later callipers have bigger pistons and larger surface areas on the pads, so all of this is obviously much better.

Noting all of the above, when I swapped to my current set up expecting retina detaching improvements I was very disapointed. Granted I was using second hand pads on second hand discs, so the bedding in requirement was even greater than normal, but even now they don't feel like I could flip the car onto it's roof under heavy braking.

 

[Fen]

I was having a chat with my friendly local OPC part manager last week - finding out scary stuff like a set of rings is £113 per piston and a set of main bearings £273 (both plus VAT) []

Anyhow I asked about brakes. I'n not a fan of red callipers so I got a price for 928GTS versions ('big blacks'?). I was quoted £300 each for callipers and £171 each for discs but that he could do 20% off that. Pads are extra obviously but I make that a 'big red' conversion in subtler black for £753+VAT - not too bad.

Still, the even bigger discs probably still wouldn't fit you car any more easily...

 

[Diver944]

It's my belief that the SE callipers and disks will not stop you any quicker than the original ones (thats down to the tyres), it's just that they have a bigger heat soak and venting capacity so will last longer until you get fade. My S2 has the same brake setup as your original one and I do find it much crisper and with more pedal feel and less pedal effort than the SE ones on my Turbo (but it is a lighter and less powerful car).

I asked about this at JZ Machtech when I first got the Turbo, thinking that something was wrong. Their possible explanation after checking everything to be in good order was that you still have the same servo pushing fluid into larger callipers so it will feel less direct.

 

[Diver944]

Fen those callipers always have seemed good value [] and with Zimmerman disks it could be even cheaper.

Would that be Chapelgate? I always find their parts prices very competitive

 

[Fen]

Nope. I boycott Chapelgate after they tried to sell me a heater control unit for £850+VAT when all that was wrong with my air-con was a plastic clip at the side of the glover box (72p IIRC). They were paid to diagnose this during a service too, not just giving advice over the parts counter.

It's Exeter OPC, where the cheap M030 castor bushes originated a couple of years ago.

 

[John Sims]

Still, the even bigger discs probably still wouldn't fit you car any more easily...

They are the same thickness, so they shouldn't be any worse than 928's.

I take your point about red calipers. I had considered re-painting my original callipers red as a precurser to puting them back on, but my red SE callipers are such a pain to keep clean I'm inclined not to bother.

 

[John Sims]

You know how it is when you feel like you are standing under a waterfall? You get to the stage when you think "well blow it, we might just as well poor that cup over as well!"

I am soon to be the proud owner of an 87 Turbo with full SE front suspension !!!

I came to the conclusion (and managed to convince my finacial adviser) that Porsche changed the front brakes on the SE (and 968CS) for a reason, and as such, to go back to the original brakes would be a retrograde step.

Being concerned about the steering rod ends, I now have the hub assemblys off an SE en route. Looks like another geometry check then! [&o].

At least I can do away with my calliper mounting conversion blocks though. []