CincyScott

I’m curious why there only seems to be discussion of front caliper upgrades and never rears. Is there no bolt on upgrade for the rears on the 951?

(Spare me the tiresome “upgrade the driver” speeches)

V2Rocket

going by piston size, rotor size, and car weight etc you find the 944 turbo is very over-braked in the rear from the factory. something like 1.6:1 F/R if i remember my math. that's why the factory had to put the various proportioning valves on the turbos/S2/968.

all the "big brake kits" focus on the fronts and work well because they make the fronts more powerful, which actually helps the overall braking balance of the car. a 944 type car should be more balanced F/R braking around 2:1...

FWIW a stock 944NA is about 2.02:1 F/R and even those have a little too much rear brake when you really hit them hard (enough that Porsche put a proportioning valve on the 944S cars expecting they'd get driven harder and braked harder, despite having the same rotors/calipers as the base NA)

FWIW#2 - Porsche uses the same piston sizes in all their models up to the present day as they did in the 1986-1991 944 turbo...28/30mm pairs. even the cayenne! just with larger rotors (except the Boxster family which still uses a 299mm rotor in the back like a 944 turbo)

FWIW#3 if you really wanted to despite this info, your stock front calipers will bolt to the rear of the car...but all that will do is give you more brake pedal travel without any more brake power (because of the prop valve) OR youll just lock up the rear, spin and die faster.

CincyScott

Thanks, this explains the reasoning very well. FWIW from me, I enjoyed the #3’s spin and die faster comment quite a lot.

GPA951s

Pretty sure the idea when adding “ big brakes” in the front you're supposed to take your original fronts and move them to the rear.

Oddjob

You don't want the back end of the car nervous under heavy high speed braking.

Having had a track car that the previous owner had put front calipers on the rear, with M030s on the front - be aware that it will immediately lock up the rears and snap spin the car when the ABS is not working and doesn't catch the lockup. So doing this would need some setup and sorting with rear brake bias to make sure you don't create that scenario.

If you do want to play around with more rear brake, either start with installing the next higher pressure factory proportioning valve (18 to 33 to 45 to 55bar I recall are the intervals available), or go to an adjustable rear bias valve and work your way up.

GPA951s

I have MO30 setup, with the bigger rears, and i cant get enough rear brake.. even resorted to putting the most aggressive pads on the rear.. then changing the bias valve… still cant get enough rear bias.. over the course of 5 track seasons ive flat spotted 2 tires.. and let Van run my car in the fun race at wgi as a gt4 car, he ended up flat-spotting one of my fronts.. that made 3 …

Nowanker

Tried that....
Big reds F, stock F on rear same pad compounds.
NOT a good idea. Way too much rear brake.
Sticky square tire setup, no limiter valve, Wilwood prop valve for the rear.
Screwed all the way out, still way too much.
Big reds F, stock rear, well balanced at about 1/4 from max.

Switched to a dual master setup with balance bar.
Forget the sizes at the moment, but one size bigger for the rear.
Balance bar about 1/3 favoring fronts... seems just about right.
Larger rear tires than F would effect the balance, as will basic tire grip (more/less weight transfer to F...)

GPA951s

all good info but i have to run “ rules” so with a turbo-s i have to run the factory MO30.

CincyScott

Are the M030 brakes 928S4 calipers with stock 951 rears? I've never seen it definitively said. Thanks.

zmf

If you want the rear brakes more involved, just change to the 928 proportioning valve. This is fine with a street setup, but can be scary with a track setup with the car leveled and lowered. I quickly switched back to the stock valve.

V2Rocket

Yes

GPA951s

As i understand it.. mo 30 / s4 brakes utilize the same size pistons on the rear as the front of say. An 86 turbo

so if you wanted the bigger brakes and have a “ stock” turbo , move your front calipers to the rear and put the bigger brakes on the front.. take your rear calipers and put them on e bay

Oddjob

So front regular turbo calipers were 38/36mm pistons in 1986; 87+ calipers were 40/36. The M030 are 44/36. Rears are all 30/28mm

The stock proportioning valve for Turbos was 18 bar for all MYs. The 944S came with a 33 bar valve (this is the 928 valve). 964 and 930 valves can be used, and those are 45 bar and 55 bar. All of them have a nominal 50% pressure reduction above the set pressure (I think the factor is actually 0.46 depending on technical document source).

I believe there was also a master cylinder change between 1986 and 1987, where the rear system piston was slightly larger diameter, so same pedal effort would generate somewhat higher rear line pressure comparatively. Approximately 24mm front, 19mm rear (early MC), 20.5mm rear (late MC). Maybe someone can look that up to verify.

I did a lot of brake testing on various 944s in the 10 to 20 years ago timeframe, unfortunately long enough ago that I've forgot most of it...

I ended up going with the 45 bar valve in my old 944S PCA club race car, the higher 55 bar valve would allow rear lockup (car did not have ABS).

I've run the 33 bar valves in regular and M030 Turbos going back to the mid '90's.

The Escort car came out of Steinel's shop in Ohio with front brakes on the rear. That's the car that would spin under braking when the ABS faulted out. Braking was great when the ABS was functioning fine. Best of my recollection, it had the 33 bar prop valve. It was relatively light, under 2600 lbs w/o driver, and didn't have extremely stiff springs at the time (500 lbs/in).

I did run the cup-spec G car without a prop valve for awhile with standard rears (& M030 fronts), but went back to the 33 bar to try to reduce rear end tail wagging in high speed heavy braking corners. That car had/has ABS, and I don't remember it faulting out and having rear end lockup without the prop valve.

Looking at an old spreadsheet, M030 fronts with stock rears & no prop valve has about the same front/rear balance (ratio of pad force at 125 bar line pressure) compared to M030 fronts with 40/36 front calipers on the rear with the stock 18 bar prop valve.

As touched on above, understand that car weight, spring rates and shock settings (front compression, rear rebound), and tire/size will all have significant impact on what you can run for brake bias. The stiffer the car is, the more rear brake you can utilize.

CincyScott

I guess part of why I ask is that my usual modus operandi is to upgrade the braking system when I upgrade the engine. Since my target is 300-350 whp, it made sense to have greater stopping power to me to scrub speed off.

Perhaps the front 993tt setup is my new target with stock rears or just upgrade to M030 and call it close enough.

Oddjob

So a couple questions for your application:

Street car only, DE use, dedicated track/race car?

Does the car have ABS?

Size of wheels you have or intend to use? M030's fit with most 16" wheels, anything larger requires 17"+.