By far the most non-standard part of my car is the brake setup. I knew it was weird going into things because there are huge front calipers bolted on in the rear, so I had no emergency brake.

After sending pictures of the calipers to Eric at PMB, I now now know I have 48mm BMW calipers in the front, and some 48mm Girling front calipers in the rear. This gives me the suboptimal brake bias of 1:1.

Actually, suboptimal is an understatement. I believe Eric used phrases like "guaranteed disaster" and "death trap" when I talked to him on the phone. Any sort of panic stop should have led to the car spinning out of control. He assumed some sort of brake bias value was involved since no one had died driving the car.

So, I've actually removed the calipers and master cylinder out, I don't see any value! At first my dad (most immediate PO) was sure he had panic stopped without incident, but now he's not 100% sure. However, the PO before him raced this car to a PCA autocross class win. I find it hard to believe you could be that successful in autocross without really, really getting on the brakes. And you aren't going to win of you are occasionally flying off the track out of control.

So, what gives? I can see the lines from the cylinder to the tunnel, and from the back firewall to the calipers. All I see are two 90 degree fittings and a T. The cylinder is one fo the upgraded 19mm ones. Is there something that can done to a master cylinder to customize bias?

My plan, based on Eric's advice, is to replace my 48mm "rears" with actual 911 M rear calipers, of which I've already obtained a pair of cores. I will also be putting in the Porsche pressure regulator value thing to prevent the rears from locking up, also based on Eric's advice. I'm also going to get a new 19mm MC from 914Rubber. However, I want to make sure I understand what's going on with current system, just in case it will affect the new stuff I want to replace it with.

Here are some pictures:

Master Cylinder ports:

Cylinder to front of tunnel:

Rear firewall:

Rear T:

Don.t you just love it when names get dropped.

Reads like a trolling narrative.

#1 - Good call to follow Eric's advice. He's been doing this for a while and knows what works and what doesn't.

Speaking as a former OEM brake engineer.

You were likely running out of fluid displacement and bottoming the master cylinder stroke before you could build significant pressure in the rear brakes. A ratio of 1:1 between front brakes and rear brakes IS a recipie for disaster in a car without ABS or ESC to control the brake proportioning. Even with ABS and ESC, it would be considered professionally negligent to design a car this way and then to rely on the electronics to control it. It would not pass FMVSS 135 test procedures required to sell the vehicle as an OEM.

1st read this classic thread:
http://www.914world.com/bbs2/index.php?showtopic=55559

Stock 914/4 rear brake piston diameter is 33mm. 38mm for the 914/6

The area of the stock 33mm piston is 855 square mm (mm^2)
The area of a 48mm piston is 1809 mm^2

Volume of fluid displacment is proportional to area differing only by the stroke of the piston. Stroke of the piston doesn't change with piston diameter changes, so, really we can focus on the area of the pistons as the key topic.

Neither the stock 17mm master cylinder or the 19mm "upgrade" master cylinder are going to have enough fluid displacment to support the increased fluid displacement required by the rear 48mm pistons. The volume of displaced brake fluid needed to support a 48mm rear brake piston would be more than double the displacement required by the 33mm stock piston! That doubling of fluid displacement won't happen by going from a 17mm master cylinder to a 19mm master cylinder.

Consider it a blessing that someone didn't put a master cylinder in the car that would have had enough fluid displacment to actually build significant pressure to the rear brakes.

#1 - Good call to follow Eric's advice.

Great explanation !
Thank you.

So what's the plan for the e-brake?

Makes sense. Thanks.



My plan is to get the car on the road first, then put in the 911 drum brake system. If it just bolted in, I'd just do it as I'm doing the rest of brakes.

That was my thought as well as to why this setup hasn't killed anyone yet. MC was way to small to lock those huge calipers. Additional thoughts are along the lines of never having braked aggressively (or mid turn).

Either way, i agree that is a really sketchy setup. Curious to know what driving that would even feel like? Was the pedal even encountering resistance before hitting the floor?

Maybe the PO raced the car in reverse

I only drove the car extensively once in 2003, but I remember that it stopped just as well as it accelerated, and that was very, well. In fact, with wearing just the regular seatbelts (rather than the 4 point harness that were also in the car), you really had to be ready for a hard stop because it was borderline violent.

My dad drove the car for 5 years, and he still describes this setup as extremely dialed in. The PO before him (Mike Z, or JimBob on this website) raced it to a class win somewhere around the 1999 or 2000 season.

Now that I'm thinking of things, after my dad bough the car we saw Mike at a 914 meetup at house in OC around 2001, and he drove me around that neighborhood. I still remember that ride almost 20 years later, and "aggressively" doesn't even begin to describe how he was accelerating and braking.

So, I already have access to the shift console and rear tunnel access ports because I'm replacing the plastic fuel lines, but I'll take off the front tunnel access panel and make sure I put eyes on the entire brake line front to back to absolutely rule out a valve somewhere.

While "MC displacement was too small to lock up brakes" sounds a like possibility, "brakes would have felt like complete hell" is 100% not the case.

I had a girlfriend who was a 12 (on a 1-10 scale) back in the day..... saw her about a year ago and she is now a 1. Age has a way of ruining great things. I dodged the bullet on that one for sure.

Ah, so the front/rear bias of 1:1 would become a rear/front bias of 1:1.

Move On...

Never said they would feel like hell, just curious what they did feel like as with that amount of imbalance in the size of the MC to the calipers I imagine there was a lot of pedal travel and not a lot of resistance. No matter how nice you think the car stopped though, if the system was unable to fully lock (and specifically fronts before the rears) than any properly working stock 914 brake setup would be able to stop you faster.

Ah Mike Z.... Havent heard that name come up in awhile. Assuming its the same Mike Z, his name was still on the title of my very first 914 I picked up in ~1999 from one of his friends, actually went over and talked to Mike for a bit before making the purchase. The "Freshly rebuilt 2L" then dropped a valve seat literally on my drive home from Santa Barbra to Huntington Beach after buying buying the car. You want to talk about missmatched parts, tearing that engine down i found it had a mix of 3 different colored fuel injectors, some of which didnt even have injector tip seals installed. I may have only been 19 and new to working on cars, but even i knew that wasn't right. If i recall the case was the only bit of that motor the machine shop found to be acceptable for re-use. Pretty massive unexpected bill for a 19 year old to take on and unfortunately the engine was far from the only issue. Literally my first 914 experience. Oh well it helped provide the 914 education i have today. Every bit on that car though was very far from being what anyone would call "dialed in" though being new to 914s at the time i still found the car to be amazing.

Not blaming anyone, but I will say that if your car came from the same crowd around the same time as mine, you may want to take a REALLY close look at everything. Or better yet get the opinion of an experienced professional (Like Eric).

Surely if the piston size was the same front and rear and the pads the same pad friction area that would be rather dangerous and worse with the bias valve deleted ?,can't see that ever being anything less than scary.
I have always assumed it was a 70/30 split front to rear which is governed by piston size or pad friction area,wondering if its not also based on front to rear weight ratio.
I'am running 944 bremdo 4 pots front and rear and 24mm vented rotors and it appears the caliper piston size is the same front and rear however the rear pads are smaller by a reasonable margin,I have 19mm m/c and have great stopping power for a non-assisted system.....this thread has got me thinking .

Just a side note: Pad friction area has no effect on brake torque. It has everything to do with wear and fade but no effect upon brake torque assuming equivalent effective radius is maintained.

Friction = coefficient of friction x normal force. There is no term to account for friction area.

Not sure what calipers your running but I'd be very surprised if you're running equivalent piston areas on both front and rear. From what I can see on 944 4 pots that is likely the M030 option package. 36mm/44mm front calipers & 28mm/30mm rear calipers. So in this OEM configurarion, there is still a front bias coming from piston sizing alone excluding other effects like effective radius and/or pad compound difference (which could vary the coefficient of friction).

Thanks for the info on pad size (area) I had no idea that was the case.
Mine are early 4 pots as used on S2 and early turbo 944's and thanks for the correction,pistons are 40/36mm front and 30/28 for the rears.
Appears the early Boxster calipers have the same piston sizes but went to different calipers later.
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