924Board.org

[924 turbo]

I'm with Gohim on this. I converted my 924 about two years ago, and have had no problems. I used '84 944 brakes in the conversion, kept my original diagonal line routing, used stainless soft lines, and kept the original '80 924 master cylinder. I have driven very hard in that time, and only very occasionally have I locked the brakes. When it did happen (f*cking 4-Runner pulled out in front of me) the car remained very controllable (I had to steer a bit to avoid hitting him) even though I was going downhill AND around a turn.

I am very satisfied with the conversion, and would do it again on another 924, with the standard '80 or '81-up master cylinder and the diagonal routing.

[gohim]

I did my conversion over 4 1/2 years ago. I have been involved with more than a dozen other conversions in the past six years. Some other members of this board have used my instructions to collect the parts necessary, and install four wheel disc brake conversions. Not one person has had any problems. Not one of these conversions used the 944 master cylinder, and no one with a late (80-85) 924 or 931 rebent, or replumbed the brake lines.

As I wrote before, Porsche used the same parts that I used, and have recommended using on 80-85 924 and 931 cars, without brake bias problems. Porsche installed the same rear calipers on all 924, and 944 cars with four wheel disc brakes, and the same front calipers on 80-85 924, 80-85 931, and 82-83 944 cars. The 477 and 944 prefix front calipers all use the same caliper reseal kit, so the piston diameter is the same. I have measured the pistons, and the bores and pistons sizes and they are the same. I have compared the casting measurements, and they appear to be the same. The only difference that I have detected is a difference in the exterior finish.

If there was a problem with this configuration, how do you account for the fact that Porsche used both front calipers with both of the late (80 and 81>85) 924, and 944 master cylinders, and Porsche used the (early) the 477 prefix calipers with both diagonal and axle brake line arrangements?

What do you think the difference is between the 477 and 944 prefix front calipers is? They use the same caliper reseal kits, brake pads, caliper hardware, and mounting bolts. They bolt to the same steering knuckles, and use the same flex hoses.

What is wrong with doing, exactly what Porsche has already done, and what Porsche supplied in production cars twenty years ago?

By the way... I have never seen a normal production model 924 supplied by Porsche with a 944 brake booster. Even the privately (legally imported into the US) imported 85 924s that I have seen had brake boosters, and master cylinders that were the same as my 81 924.

Using the 944 master cylinder and brake booster are not necessary. I have seen 77 and 78 924s converted to 931/944 brakes still running the early (small) master cylinder, and early (smallest) brake booster, and the owners said the brakes worked fine (pedal effort was said to be higher (expected since master cylinder bore was smaller), but the brakes still worked fine.

Remember that braking and brake bias is affected by a car's weight distribution, and suspension settings. A 944 carries it's weight differently than a 924, coming with higher rate front springs, a larger front sway bar, and heavier/wider front and rear tires, but the same rear torsion bars as a late 924. So, the brake bias setting developed for a 944 probably would not be ideal for a 924.

While the brake bias requirements for a car driven on snow and ice may be different from those of a car driven of pavement. When you drive on ice or snow, the rear traction becomes more important. When driving on ice or snow, where do you put the chains? Where do you add the weight for more traction? Some requirments stay the same. Too much front brake can be just as bad as too much rear brake. What happens when you lock up the front brakes while the rears are still spinning? You don't get max braking for sure. What's more than that, you can do a simple experiment with a bicycle to see. If you lock up the front brakes (wheels), while the rears are still spinning free, the rear end of the bike will try to swing around the front. That can't be good.

[gohim]

To further my last thought from the previous message, slippery road surfaces that provide less traction, allow less braking force to be used, resulting in less weight transfer to the front of the car. This means that the front brakes may lock up prematurely, since there is less weight on the front end. This means that the rear of the car will have more weight on it, and the rear brakes may need more clamping force for them to work at/with maximum efficiency.

Don't some/many ice racing and rally cars have driver adjustable brake bias for just this reason? Changing road/traction conditions?

[-nick]

you're right about the booster, i just checked and there is a different pn for the 944 boosters. for some reason i thought they were the same. maybe the same diameter?

anyway, my big point is that a 944 uses 19mm MC bias in the rear and 23mm bias in the front. all the 924's use a 19mm MC (or 23mm, but they're both a single bore, not stepped which is the point).

if you switch in a 924 MC into a 944 do you agree that you're going to have more rear bias? this is essentially what you are doing. the routing of the lines doesn't change any of the biasing, so all else being equal, this is the difference.

i seemed to recall that 924 m471 front calipers did have a larger piston diameter than the 944's. this was a long time ago, and i didn't write the numbers down. the last time we went over this you said you didn't know. have you since measured some front calipers?

sure, some of it's a mystery to me, but regardless, the two MC's _will not_ give you the same bias. one possible resolution would be that the 944's are much more nose heavy than the 924's, but this isn't really the case either. i'm out of ideas.

why not keep parts together that work together? i haven't really heard a convincing argument not to.

you are right that slippery surfaces require more rearward bias. so that nulls my argument that we would have more trouble out here in the rain and snow. i don't know the answer to which setup is better. it makes sense to me not to mix and match brake parts without a definitive answer. that's just my opinion

-nick

[gohim]

I did not write down the front caliper piston diameters, since I was only checking to make sure that the 477 and 944 diameters were the same.

You can check the replacement part numbers yourself to verify that the calipers replacement/rebuild parts are all the same.

The bleed valve listed for late calipers is different from that listed for the early calipers. I think I looked at them, and they look different, but they all screw in the same.

The early (477) front calipers use the same rebuild kits as the later (84-88 944) calipers.

Therefore, the piston diameters must be the same. I compared the pistons and caliper bores at the time that I chose and rebuilt the calipers for my brake conversion. At the time, I had 3 sets of front calipers to choose from, and that is when I compared them. I had a set from a 82 931, a set of replacement 924/944 calipers from Automotion, and a set of 85/1 944 calipers. The casting numbers on the casting were all different, the pistons were all the same.

Yes, I would agree with you that if you took a car, kept the same front and rear calipers and rotors, and changed the master cylinder back and forth between the straight 19mm or 23mm 924 master cylinder, and the stepped 23mm/19mm 944 master cylinder, you would be changing the amount of brake fluid available, and the proportion of fluid available to the front and rear brakes.

Yes, I agree that this would/should affect the way that the brakes engage/work on the car.

BUT, without testing, who could say which configuration would yield a higher braking force, on a given surface without testing, or which combination work better overall, under all conditions, without testing?

I am saying that my car is configured in a way that Porsche originally tested and manufactured the cars, and is perfectly safe.

Your brake configuration in the one that Porsche used on the 924S. BUT your car weighs less, has a different balance point (more forward weight on the 924S/944), and has different suspension settings: front spring rate (higher on the 924S/944, rear torsion bar diameter larger on the late 924/944/924S, and ride height). You have a much higher chance of having an unsafe overall brake/handling setup than I, or anyone else that chooses the 924 configuration.

This is why I have suggested that after a car is converted, that the brakes be tested for function, then the pads broken in, and finally the brakes tested under severe braking stress, in a place with lots of free space (nothing to hit if the car gets out of shape) to make sure that the car will stop properly.

After testing, the driver/owner may very well decide that a change may be called for: larger master cylinder, larger brake booster, switched to straight or stepped master cylinder. Or a component that affects braking/handling should be changed: tire pressure, tire diameter, tire brand, front spring rate, rear torsion bar diameter, shock settings, ride height, etc...

[-nick]

i gotta say, you make some pretty good points. i didn't think the 944/924S's had that much more frontward weight. with the Al block and all, i thought it was about the same. it would certainly make sense if the 944's had heavier fronts. this giving more dive and requiring more front bias. heavier front springs should offset this effect though.

you've just about made me into a convert. i'm running non stock spring rates front and rear which changes all the rules. plus i'm running a 20/23mm 951 MC, which gives me a little more rearward bias.

i've run the brakes in with lots of threshold stops and the balance has been great. hitting them hard in the rain i found that all four wheels will lock just as they should, with the fronts first and rears soon after with a little more pedal pressure.

it's much safer to have the fronts lock before the rears. if you're turning with the rears locked the car will spin. with the fronts locked the car will just push straight ahead, which is a much safer scenario. i'm not sure if your bicycle example captured this point.

thanks for the exchange. you brought up a couple points i didn't even think about. it's probably more dumb luck than anything that got my brakes to work out just right. of course anyone will tell you suspension work is 90% black magic.

final verdict- give the 924 MC a try. honestly, i never did the math before but these 4mm we're talking about only change the bias by ~5% anyway (if the clamping pressure is directly proportional to the MC area which i believe it is (the area being pi*r^2)).

would any of us be able to notice it? especially with all the other suspension variables? that might make this whole argument a moot point anyway.

regards,
-nick

[gohim]

I didn't know that the 951 master cylinder is a stepped 21mm/23mm. Of course I never checked, because I never planned on running one.

951s run with proportioning valve that can be reset by changing a part. I have read numerous messages about this on the RennList. You haven't mentioned that you are running the 951 proportioning valve, so I take it that you aren't.

If you were to install one, which calipers, and master cylinder you are using would be less important, because you could adjust the balance by swapping a single part inside of the proportioning valve.

[John Brown]

The 944 rear bore is the larger 23mm; the front is the smaller 19mm.

The 924 was/is NOT plumbed with 3 corners to the front chamber and a single rear to the back chamber.

Running 944 hard lines in a 924 is doable (I have) but not a 'drop in'. Look closely at the routing.

All M471 924 or 931 work just fine with the 'X' plumbing. How bad can it be?

If you want to re-plumb with 944 stuff it undoubtedly is a (minor) improvement. I wouldn't sweat it one way or 'tother, personally don't think you will ever know the difference on the street. If brake bias becomes an issue use different pads front to rear. If snowy climate is the issue get some winter tires on all 4 corners. More than one way to skin that cat.

[eturbo924]

Thanks guys. I will run the 924 lines and see how it is. One other thing to cosider. I am running 205 front and 225 rear tires so it will take more friction on the pads to lock the rear tires . Man those rear fenders are filled out now! Car looks like a monster from behind. Now the 924S spoiler looks lost. Might need to get another 944 spoiler since I already put the one I had on the 1980. I will post pictures as soon as I can. Still job hunting.

Eric

[-nick]

hey John,
that's news to me that the 944 MC runs the larger bore for the rears! i'm not even going to make conjectures on this topic anymore. boy do i feel like a dummy! this has definately been informative.

regards,
-nick

[baldwin]

Whoa John Brown! I thought that after the very informative discussion between -nick and gohim I thought I knew what I had to know...

I've asked two 944 type 1 owners about their brake plumbing, and I'll post their findings as soon as I get them.

[baldwin]

The 23mm goes to the front, the 19mm goes to the back. As one would expect.

[!tom]

AGGGH!

I wish I had read this post before starting my current project.

I'm in the middle of a 5-lug swap. I've read many threads about this over the years, and I recall many points being made about being careful to preserve the camber setting while swapping steering knuckles, but I don't recall ever seeing a post mentioning the swap affecting toe.

I purchased a digital level (it gives measurements in degrees from horizontal or plumb). I stuck my car on jack stands and measured the machined surface of each knuckle (where the backing plate bolts on) with respect to plumb. It was easy to preserve these angles, so my camber setting is unchanged with the new knuckles.

However, I didn't make any measurements of the new knuckles. A couple measurements of 5-lug knuckles should make it easy to determine the change in tie rod length necessary to preserve the existing alignment. I would like to avoid pulling the calliper and hub that I just put on, so I'm hoping someone else out there can measure an existing knuckle so we can determine the length of tire rod change necessary in the swap.

Here are what I believe the necessary measurements are to determine the change in length of the tie rods. It may be that more are necessary, but I hope not.

1. Distance from tie rod end to reference face
   
   Measurement is 19mm
   
2. Distance from ball joint to reference face
   
   Measurement is 16mm
   

I'm going to measure more carefully if we do get a measurement from a 5-lug knuckle; I used the tape measure for clarity. In retrospect, callipers would have been far more clear, as they would have shown more clearly the reference point I've used. In both cases, it's the closest edge of the hole to the reference plane, and the ball joint hole was the top hole, or the smaller diameter end.

Note that it may very well be the case that the reference plane that I am using is in a different location on different knuckles relative to everything else, as it is a non-critical surface. What really matters though is the difference between the two measurements, so the fact that two knuckles don't give the exact same measurements is irrelevant; the difference between the two should always be the same amongst the same type.

I also see from this discussion that the 5-lug cars have a spacer on the rear, and this was missing from my kit. That explains why my rear studs are too long, and why I ordered extra deep lug nuts to clear them. I guess we'll see how it looks when it's all mounted, but I might be on the lookout for spacers soon!
_________________
78 924 NA
5-lug

[!tom]

Nobody has come through with measurements, so here's where I am at.

I had reasonable toe settings previously, but I don't know what the toe setting was.

After changing to 5-lug spindles, I have around 6 degrees of toe in per side, for a total of 12 degrees, at full droop.

I know the 924 has a reputation for terrible bump steer, so the setting at full droop is going to be a ways away from rest ride height, but it's going to be closer than it is now!

I also worked out that the 5-lug spindles will give about 0.72 degrees of toe adjustment per turn per side. So, I'm going to lengthen each tie rod 8.5 turns to get the wheels around parallel at full droop. This means I'll be in the right ballpark (I'm not sure if I'll end up with toe in or out), but wrong nonetheless.

Once I get my rear wheel spacers, I'll be able to put the car on the ground and take another measurement, and I'll see how close things are.
_________________
78 924 NA
5-lug

[!tom]

Wow!

Finally the spacers arrived so I could put the rear wheels on and take it for a spin.

The toe is off, and it looks like there's way too much camber in the left front wheel. The car suffered a pretty big prang in the left front corner in its previous life; the OE strut was bent at the top (since replaced), the fender was a different colour (hence the horrendous respray on the whole car), the wheel was bent (both fronts were), and I discovered (hadn't noticed before) as I was playing with the toe that the tie rod is bent. I wouldn't be surprised if the spindle was bent as well; this would explain the tendency to eat left front wheel bearings. I'm guessing this is why my method of preserving the camber didn't work for the left front wheel.

Impressions after taking it for its first spin with the 205/55/R16's (as opposed to the 185/70/R14's that it had); way lighter steering wheel. I really don't understand why it's so much easier. The only thing I can think of is that there's so much camber on the left front that there is less tire to scrub.

The alignment is obviously off; it feels as though it might have a fair bit of toe out, as it's not terribly stable on the road. But what a difference the new tires make; previously it had worn out Yokohama all season fronts, and worn out Yokohama snow tires on the rear. It's a great feeling with the same tires all the way around! I also noticed another difference which may be related to the toe, or it may be related to different spindles. With the old setup, there was so much ackerman that there was significant tire scrub when driving around a parking lot at full lock. With my first spin with the new spindles, this didn't occur at all. Again, this could be because the toe is so far out, or maybe there is a bit different steering geometry with the 5-lug spindles.

I'm really looking forward to running it after getting a proper alignment!
_________________
78 924 NA
5-lug