NineMeister

If you check the Porsche spec book for the 1989 944 turbo SE and 1986 (only) 928S2 you will find that these models are fitted with calipers that have a difference in the small piston diameters, although I totally agree that the 928S4 has the same size as the 993 and big reds.

wjk_glynn

Hi Colin,

Given you and your company's excellent reputation, I'm 100% sure you're not one to make a mistake when quoting the "spec book".

And I would normally lend a lot of weight to the spec books regarding accuracy.

But there's a ton of other information which says the following 944 models used the same front calipers as the 928 S4, and hence must have the same 44/36 piston sizes:

* '87-'88 951 w/M030
* '88 951S
* '89 951
* '89-'91 944S2 w/M030
* '92-'95 968 w/M030

So what is that information? Its...

* Part number information
* Various articles
* "Cobalts" experience with the repair kits

Part Numbers

PET shows the 951 w/M030, 944S2 w/M030, 951 S and the 1989 951 used the following front brake calipers:

Left: 928-351-421-03
Right: 928-351-422-03

These are the same part numbers for the 928 S4 front calipers.

Various Articles

The following said (or strongly implied) that the 951 S used the 928 S4 brakes:

Original Porsche 924/944/968 by Peter Morgan
Page 84: "Four-piston brakes were taken from the 928S4..."

Porsche: Excellence Was Expected by Karl Ludvigsen
Vol 2, page 965: "The front brakes were enlarged and brought up to the standard of the 928S4."

Road & Track, June 1988 - Porsche 944 Turbo S by Ron Perry
"...front spindles fitted with 928S 4 front brakes..."

Motorsport, July 1988 - Porsche 944 Turbo SE
"...the SE gets a transplant from the 928S4 of the complete ABS system with its massive discs..."

Excellence, February 1988 - Porsche 944 Turbo: An Exercise In Zuffenhausen Nostalgia by Jerry Sloniger
"Brakes come right off the 928 S4"

Repair Kits

See Cobalts earlier postings in this thread


So I'm kinda mystified as to why the spec book quotes smaller piston sizes for the Turbo SE.

If you had time to spare, could you also look up the piston size specs for the following models, to see if there are any other discrepancies...

1987 928 S4
1988 951 w/M030
1988 951 S
1989 944 S2 w/M030

Thanks.

Karl.

NineMeister

As ever, I always base my comments on parts that I have worked on in the past. The last year of production of the 928S2 saw an upgrade to a black aluminium caliper of the same size as the S4 but it was not the exact same item. If I get a chance I'm sure I have one of these early calipers knocking around at work so it won't take a lot to dig it out and measure the pistons. Although I have never bothered looking up the part numbers on PET, I would not be surprised if the small piston calipers have been superceded with the later S4 part and that PET is in fact wrong in not listing the earlier version (I have found many errors and omissions elsewhere). I suppose that I could always dust it off and check my microfische machine.....

RajDatta

Cobalt and Karl are 100% accurate on this.
Raj

wjk_glynn

I agree. And from what I can tell, the 1986 928 used the following brake calipers (which according to PET have 42/36 pistons, and are different to the 928 S4)

Left: 928-351-421-00
Right: 928-351-422-00

Then the 928 S4 used the following variations over its production lifecycle (all had 44/36 pistons):

Left: 928-351-421-01, 928-351-421-02, 928-351-421-03
Right: 928-351-422-01, 928-351-421-02, 928-351-421-03

Then where our data parts way is on the 1989 951. Everything I've seen points to it using:

Left: 928-351-421-03
Right: 928-351-421-03

That would be great. It would also be interesting to see if those '86 caliper piston diameters agree with PET (42/36) or the spec book (44/32)

I also put in a request on the 944 Turbo board to see if someone has the opportunity to measure the piston diameters on a '88 951S or '89 951. See https://rennlist.com/forums/944-turbo-and-turbo-s-forum/320234-request-measure-the-front-brake-piston-diameters-on-a-88-951s-or-89-951-a.html for more details.

Yup. I've also seen PET only list the parts that superceeded earlier ones and not list the earlier parts. I've also seen errors and omissions. And as I said earlier, I would normally give a lot more creedence to the spec books than PET.

But in this specific case, there is such a large body of information (all sorts of different sources) that says those 951s used the 928 S4 brakes (and not the 928 S2 brakes), that I suspect a typo in the spec book.

Hopefully someone on the 951 board can make an actual measurement and it will help clear this all up.

Karl.

PS: Great discussion (for the hyper **** amongst us)

pete95zhn

The PET clearly states that 944 turbo M030 caliper's pistons are 44/36mm, like late 928 S4's, Big Red's / Blacks and 966 GT3 Mk I's.

I've done following front brake upgrades to my car:

944 turbo --> 928S4 --> 996 GT2 / GT3 MkII ( 6-piston ). With latest one I also installed 996 GT2 rear discs too.

A friend of mine has gone the similar way and, as our cars are '87 and '88, adapters had to be made. Another friend of mine installed Big Blacks ( = 928 GTS ) to his car, replacing M030s ( no need for adapters ). I also had 996 GT3 MkI's in my garage, but I sold them and bought those 6-piston ones. All mods are done with proper OEM discs.

PS. Pistons were also measured during installations.

wjk_glynn

In addition to "pete95zhn" previously making measurements, "samluke" has just measured the piston sizes on a set of 1988 951S calipers he currently has for sale and he confirmed they are 44/36. See https://rennlist.com/forums/showthread.php?t=320234 for more details.

So... it looks like the spec book has a typo.

Karl.