993 calipers on 930T

[dane89]

Hi all,
I looking for some advice on my braking circuit, I have 993 ( non Turbo ) calipers on the front of my 89 930Turbo which I have recently matched up with 965 turbo calipers on the rear.

I thought this was according to bills brake chart a good pairing in terms of the piston sizes and the bias being reasonably correct.

However this conversion has left my brake pedal to hot to handle. I get about a quarter of pedal travel before the car locks up the front, it's not very safe or nice to drive like this nor could anyone else jump in and drive my car as they wouldn't be used to the pedal.

I've trailed through a few posts on the here, and from what I can see some people recommend putting a tilton brake bias control valve in the system to take the edge off the front and send a little more to the rear, it was my understanding though that wouldn't be the case as the bias was matched via the piston size.

Any help is much appreciated as it's annoying the hell out of me ��.

Cheers Dane.

[Bill Verburg]

Originally Posted by dane89

Hi all,
I looking for some advice on my braking circuit, I have 993 ( non Turbo ) calipers on the front of my 89 930Turbo which I have recently matched up with 965 turbo calipers on the rear.

I thought this was according to bills brake chart a good pairing in terms of the piston sizes and the bias being reasonably correct.

However this conversion has left my brake pedal to hot to handle. I get about a quarter of pedal travel before the car locks up the front, it's not very safe or nice to drive like this nor could anyone else jump in and drive my car as they wouldn't be used to the pedal.

I've trailed through a few posts on the here, and from what I can see some people recommend putting a tilton brake bias control valve in the system to take the edge off the front and send a little more to the rear, it was my understanding though that wouldn't be the case as the bias was matched via the piston size.

Any help is much appreciated as it's annoying the hell out of me ��.

Cheers Dane.

our bias is almost exactly the same as stock,

biggest change unless you also changed m/c is in pedal ratio, which is now 37.342, 33.105 previously

this will make the pedal more touchy but I suspect that there is some other issue in play, most likely in back.

check for stuck pistons swollen/clogged lines in back.

possibly some air in the rear circuit

[dane89]

Hi Bill.
I have serviced the rear 965 calipers with new pistons / seals. The fronts had the same when I put them on a few years back.

The pedal is touchy, just the minimal amount of pressure put the brakes on hard and a touch more and the fronts will lock up.

I'd say there is about less than 6 inches of pedal movement from nothing to complete lock up.

I have kept the standard 930 master cylinder.

Cheers Dane.

[Broday679]

Thanks for sharing this. A friend of mine was also same difficulty recently

[dane89]

I was under the impression that once all bolted on it would work quite well, I know this is a bit of an old school setup but it seemed to work for people back in the day. And who also want to keep standard size wheels.

[Mustangkev]

Do you have pictures of how the rear calipers were modified to fit the 930 trailing arms?

I’m in a similar situation. I have 993 turbo front brakes and stock 930 rear brakes. I bought a set of 993 rear calipers that should correct the brake bias.

[Bill Verburg]

Originally Posted by Mustangkev

Do you have pictures of how the rear calipers were modified to fit the 930 trailing arms?

I’m in a similar situation. I have 993 turbo front brakes and stock 930 rear brakes. I bought a set of 993 rear calipers that should correct the brake bias.

while 993 rear and 965 rear calipers have the same 30/34 pistons and 97mm(from memory so i'd need to look this up for sure) transverse mounts they're different in throat width and transverse o/s.

The 965s are wider and designed for a 28mm thick rotor, the 993s for a 24mm rotor.

due to the wide 3.5 inch 930 rear mounts the best way to mount them is to convert to radial mounts, not easy or cheap.

some 930s do have the 3" mounting tabs if using these then you fill the stock mounting holes w/ stock and then redrill a little wider and a little lower to fit the rotor.

I used to have a template but would need to look for it.

[Bill Verburg]

Originally Posted by dane89

I was under the impression that once all bolted on it would work quite well, I know this is a bit of an old school setup but it seemed to work for people back in the day. And who also want to keep standard size wheels.

it should work well; the biggest negative is the mushier pedal w/ longer throw induced by the larger pistons.

If the fronts are locking that easily there is an issue related to the rear, could be anywhere from the m/c back.

I'm assuming that you kept the stock 930 rotors too.

[dane89]

I would love a mushier pedal compared to the pedal I have now, all the rest of the braking system is the stock 930 Turbo setup.

I can re-bleed the rears and see if that makes a difference, but I cant see it doing a lot.

before I changed to this set up the brakes worked ok and the pedal had a nice gradual feel to it which in my eyes is what you'd want ideally.

[dane89]

Originally Posted by Mustangkev

Do you have pictures of how the rear calipers were modified to fit the 930 trailing arms?

I’m in a similar situation. I have 993 turbo front brakes and stock 930 rear brakes. I bought a set of 993 rear calipers that should correct the brake bias.

I do , but as Bill sais I have used 965 calipers, had the ears machined of and adaptor brackets made to be able to mount them

this is an expensive process (unless your best friend is a machinist ) which he is its also a time consuming as we had to fabricate the brackets from scratch as no one sells them anymore.

[Bill Verburg]

This one is a 964 rear modified to fit a 911 rear using 930 28x309mm rotors, the existing axial mount holes are filled w/ billet al. rod and redrilled for the 3" rear 911 mounts, it looks like there is room to move each over .25" for 930 mounts, remember that the bleeds and feed need to be swapped for 930 trailing mounting. The 964s are only wide enough for 24mm rotors, the throat area needed to be clearanced for the 28mm 930 rotors


This one is a pic of a 964 front which has had the mounting ears machined off to convert to radial mount and the homemade adaptor to mount on a 911/930 front strut



lastly these aren't 930 but are 993RS the rears have been modified similarly to the 964 fronts above, ie ears off and drilled to accapt an axial mount adaptor.

[dane89]

Seemed to have sussed out this problem. having traced it to a faulty master cylinder, and also a loose front right wheel bearing.

There was a lot of air going to the front right Caliper, checked everything and re bled the whole braking system last week, checked on the weekend and there is air in the right side again this would cause the right hand caliper to be not working to its full potential and the left side working fully in theory clamping harder under braking and pulling the car that way. only possible cause is the MC.

Have one on order now, also some pictures for those who were asking of the 965 rear caliper adaption. its quite a bit of work but when your best Friend is a machinist its pretty cheap

[Bill Verburg]

Originally Posted by dane89

Seemed to have sussed out this problem. having traced it to a faulty master cylinder, and also a loose front right wheel bearing.

There was a lot of air going to the front right Caliper, checked everything and re bled the whole braking system last week, checked on the weekend and there is air in the right side again this would cause the right hand caliper to be not working to its full potential and the left side working fully in theory clamping harder under braking and pulling the car that way. only possible cause is the MC.

Have one on order now, also some pictures for those who were asking of the 965 rear caliper adaption. its quite a bit of work but when your best Friend is a machinist its pretty cheap

Nice job!

For even more rear brake you could do the same o a 993rS rear caliper. Thats what I have on my car

on left stock 993RS rear on right modified to radial mount for use on a 911


left to right, stock 993rS rear, mod 993RS rear, stock 993RS front


This should work w/ the stock 930 rear rotor, I use a modified 993RS rear rotor but that's a whole world harder