924Board.org

[ideola]

Just curious what the relative pros and cons would be of converting to fully manual brakes, i.e. no booster. I have heard of guys doing this to improve the "brake feel" (along the lines of having manual vs. power-assisted steering). I know there was at one time a booster-delete kit available for the 944, but they don't seem to be readily available, and I've only seen it referenced once on a car listed over on Pelican.

Thoughts?
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erstwhile owner of just about every 924 variant ever made

[Rasta Monsta]

Find a hill to roll down and test them without assist. . .you may not like it, pedal effort is extremely high.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[bass gt]

rasta,

if you go boosterless, you need to change the master cylinder sizes to get the feel.
Dan, whats your thinking behind this? I have a Tilton pedal box in my race car, and loveit!! i need to size down the cylinders, as they feel a little wooden, but the feedback is great. You can really feel whats happening at the tyre. If you do this, how will you achieve it? What pedal/cylinder system will you employ?

Steve
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Front Wheel Drive is the Devil's work.

[StienbargerR]

I bought a project car that had a Vortec and they had to convert to manual brakes. They had an old 911 MC in it. The brakes seemed alright, I only drove it a handful of times, but it wasn't bad at all.

Ryan
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1978 924 NA
-250lb lowering springs, Euro Pistons

[Martijnus]

[ideola]

[bass gt]

Ok,

This is just my 10p, so take it as you will.
having removed the servo'd brakes in my racer, and replaced them with a non assisted set up comprising a Tilton pedal box and bias balance bar, i have to say the feeling is great. For example, under race conditions, i can tell when my slicks are just about to lock, whereas i find on many servo assisted systems, when you jump onto the middle pedal, the brakes will lock, which of course means you need to lift off and re apply. No weven though my cyinder sizes are less than ideal, ie they give too little pedal stroke and have a slightly wooden feel, this will be alleviated by fitting smaller cylinders, which increase the stroke, but will impart more scope for modulation. I also think it beneficial in left foot braking, where the ability to gauge initial pressure can be difficult to learn. Finally, i think it brings something to the art of driving, as it gives far more feedback as you are directly connected to each corner. So whether this is required on a road car is moot in my opinion. I think it comes down to whether you want to add a greater level of depth into the driving experience.
Finally, show me a true race car with servo'd brakes, and i'll eat a courgette!!

Steve
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Front Wheel Drive is the Devil's work.

[Martijnus]

[quote="ideola"]

[ideola]

So Steve, when you are referring to the "cylinders", can you clarify which cylinders you are referring to? in the caliper, or are you referring to the master? Which ones are you using? I have several to pick from as follows:
931 stock
944 early stock
951 early stock
951 brembos

All of these are sitting on the shelf, awaiting rebuild. My current line of thinking was to go with 944 booster & master cylinder + the brembos, but if I go boosterless, would I need to rethink those calipers?

And by the way, I really like the way you expressed it: "greater level of depth into the driving experience". That is ultimately what I'm striving for!

Oh, one more thing, eliminating the booster also gets rid of the tiny bit of concern I have over possible interference between my custom intake and the brake booster. I've measured it all up, but that was based on stock, worn engine mounts, and the tolerance will be close...I won't know until I get the engine back together and into the car just how much room is there! But I have other provisions in mind should that turn out to be an issue. I'm more interested in the "driving experience"
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erstwhile owner of just about every 924 variant ever made

[bass gt]

By cylinders. i mean the master cylinders. I currently have 7/8th of an inch for front and rear circuits, but those will be coming down 2 sizes. If you are looking to reain the stock cylinders from any Porsche model, be very careful, as these were sized to be used with a servo. Hence, with no vacuum assistance, they feel like a block of wood. If you are really serious about this, go to the Tilton website. They have a form you can fill out and the tech guys will recommend front & rear master cylinder sizes. Irrespective of whether you actually use a Tilton product, their recommendation will be pretty close. Then look at their catalogue and see the incremental step sizes. Each step gives a significant change. Now compare to the Porsche master cylinder sizes and see how close you get. I think Tilton offer cylinders in 1/8th" steps.
You will need to tell them everything, such as wheel sizes, brake sizes, car weight, brake piston sizes ect.
Don't forget, i have a complete Tilton pedal box installed. All original pedals have been removed

Steve
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Front Wheel Drive is the Devil's work.

[fiat22turbo]

You guys do know that all of this can be calculated out, right?

There were a couple of great brake articles written up over the years that go into depth on brake pedal ratio, master cylinder sizing, adjustable brake bias, caliper piston sizing, etc.

Grassroots Motorsports did a couple, so has Circle Track Magazine (don't laugh, there was a lot of great information there)

http://www.scirocco.org/faq/brakes/pulpfriction/pfpage1.html

http://www.ffcobra.com/FAQ/brakes3.html

http://circletrack.automotive.com/10835/139-0401-brake-system-tech-tips/index.html

http://circletrack.automotive.com/10856/brake-system-tech-terminology-setup/index.html

There are also some great books available from Pegasus Auto-racing, Steve smith Publishing, Motorbooks, etc about chassis and braking. I tend to prefer Carroll Smith's books first, before delving into books on a specific subject.
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Stefan
1979 924 Carrera GTS (clone-ish)
1988 944 Turbo S (Silver Rose)

[Raceboy]

[ideola]

Here's the info I posted on Tilton's online form:
Front Brake Information
Number of pistons within each front caliper: 4
Are pistons on one side or both sides of caliper: both
Diameter of pistons on one side of caliper (list all):
- Piston 1: 36 mm
- Piston 2: 36 mm
- Piston 3: 40 mm
- Piston 4: 40 mm
Front rotor diameter: 298 mm

Rear Brake Information
Number of pistons within each rear caliper: 4
Are pistons on one side or both sides of caliper: both
Diameter of pistons on one side of caliper (list all):
- Piston 1: 20 mm
- Piston 2: 20 mm
- Piston 3: 30 mm
- Piston 4: 30 mm
Rear rotor diameter: 299 mm

General Information
Tilton pedal assembly part number: 72-680
Vehicle weight (with driver and fuel): 2200 pounds
Vehicle Weight Distribution (with driver and fuel): 51 / 49
Vehicle wheelbase: 94.5 in.
Front tire diameter (outer): 24 in
Rear tire diameter (outer): 24 in
What is the vehicle being used for: road racing

The pedal assembly I listed is the floor mount version of the 600 series. For some reason, the 900 series wasn't available in the drop down list. I'll post back when I get their recommendations. These specs are based on stock 1986 951 brake setup as best I was able to determine. The tire diameter is the smallest allowable on 16" rims based on IMSA rules, so I used that measurement. Weight & weight distribution are estimates.
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erstwhile owner of just about every 924 variant ever made

[Martijnus]

[Raceboy]

No, I had VEMS installed on totally stock 924 euro engine, no ITB's if you mean that.
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'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
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