924Board.org

[imnotted]

Ok, we have a 924 that's been fitted with a full 944 setup. Now, we're sticking in a 351W, and we're thinking about getting a beefier master cylinder to stop the thing. I was thinking maybe 928 MC, but I don't know if it's any stronger. Anyone here know?

[emoore924]

There are several flavors of the 924 m/c that would bolt on to the current booster. You would have to consider moving the brake lines though and some have the brake light switch at the m/c and others have the brakelight switch at the pedal (inside the car). So some light fabricating might be required to switch depending on your situation. Check with your parts supplier but I vaguely recall available m/c bores of 17mm, 19mm and 21mm, or something like that.

I replace the m/c in one of my cars with the largest one (21mm??) and had to re-route the brake lines. I used the same booster. Due to the larger diameter bore, I noticed a distinctive change in the pedal travel (shorter) and am better able to modulate the brakes but the braking force is about the same. Stopping distances are about the same...

Which leads me to the suggestion that the place to start changes might be at the wheels by changing the calipers (which may necessitate a m/c change because of the greater displacement of more or larger pistons in the calipers), or changing of the pad material to a "tourquier" material, or moving to slotted rotors or something like that. You do need to be a little careful if you're changing around the calipers to be sure their needs don't exceed the capacity of the m/c to provide fluid, especially at the limits, but I think the easiest change might just be to install more serious pads.

Last, I don't know the 928 series that well but if I was looking for m/c interchangability, I'd probably start with a 944/951 m/c rather than a 928. However, I'm no expert here...

I'd start with improvements made at the wheels and work back from there.

YMMV

[imnotted]

the 944 booster is the same. I just talked with the other guy on this project, and it looks like we might have to go boosterless. we are, after all, cramming a 351W into a 924... since we're using a C4 (auto), we're going to move the whole braking system over, probably in place of the clutch. this is a GRM car, so it's tightly budgeted, and calipers are out of the question. I figure if I can find a decent non-power mc, I think it should work ok. the piston size and stroke for the 924/44 mc seems relatively small in comparison with others, so I don't think it'll be too hard to find a bigger one.

[Lizard]

the 928S3 or S4 have a large MC, which does have larger sizes than the regular 924, however iirc the 951 with the 4 pots is also bigger than a regular 944 MC,

I personally would NOT run the car w/o the booster, I am doing a engine swap in a 931 chassis I have, however I using a 928S 5.0L DOHC engine which is even taller in the valve cover areas, however to conteract it I am moving the engine forward instead, but this will leave NO room for the rad, so the rad is being relocated to the rear of the car, I am going to have to redo the rear ligths more like a 911RS stylle just round (prob LEDs) and widen it up and then put a black grate in the back kinda like some of the ferraris, but this is NOT something I would recommend for a daily driver, mine is going to be more of a weekend suicide machine
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3 928s,

[imnotted]

I guess I didn't mention that. this is NOT a daily driver. it's a grassroots challenge car ( http://www.grmotorsports.com ). we happened to have a 351W in the shop, and we thought it would be fun to put it in the 924. this will make our third entry for the 2005 challenge (we're bringing a crx and a rotary powered spitfire). I'll consider some of those MCs. I'm also going to change pedal geometry a little to make it easier on me, since we're already changing the whole assembly. would it be difficult to mate the 928S3 to the existing 944 brake system I have?

[gohim]

Taking out the brake booster would increase the amount of pedal effort required to stop the car. I thought you wrote you are thinking of increasing the braking ability of the car?

The 944 master cylinder is setup for front and rear dual braking circuits, while the 924 is diagonal braking. The late 924 (80-85) has the same size master cylinder as the 944 front section of the master cylinder(23MM+). The rear brake caliper section of the 944 master cylinder is about 21mm.

The 944 master cylinder and brake booster are shorter in length than the 924 parts. Many people that due engine conversions, or dual carbs switch to the 944 master cylinder and brake booster. The 944 brake booster is about 2 inches larger in diameter that the largest 924 brake booster.

911, 928, and 951 front brake calipers have different spacing between the mounting holes, and different offsets for the rotor hats. You might be able to use 951 steering knuckles and front calipers. 86 951 parts would be the easiest to adapt. You would have to drill the left front 951 steering knuckle for the 924 speedo cable.

Both the 928 and the 951 come with a proportioning valve in the rear brake lines. You will need to add one to your brake setup.

If I remember correctly, someone posted somewhere that the 951 rear calipers and rotors are a relatively easy conversion.

[924RACR]

351W, huh? Thinking about racing that beast, by any chance? GTS Challenge does allow engine swaps, including other manufacturers! We've been looking to expand to Texas, will likely happen for 2005... www.gtschallenge.com
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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