Porsche 944S

[JimLill]

Originally posted by RR@Dec 28 2005, 11:27 AM
I thought you were comparing the 944 to the 944S. If you want an interesting brake comparison, check the front double-piston RX7 swept/area size to the single piston 944. I think the RX7 has quite the advantage, would anyone disagree?

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You're right that a 2 piston fixed caliper has some advantage over a single piston floating type. Trying to assess that advantage via analysis is pretty tricky. In theory, the same force is available, but we all know that it practice it just doesn't work out that way.

The single piston floating design, whether it be street or track apps, can be less effective due to binding and twist. So for a 944 (and it is no accident that Porsche put different brakes on the Turbo models), it is pretty important to keep things cleaned and pads squared up.

When I used to have roadrace motorcycles in the early 80's, and had a few setups with floating calipers, I would either toss tapered pads or flat sand them square to prevent twist and thus binding.

So, you could take my swept/pad area analysis and apply a correction factor for caliper design, probably 10-15% less for cars with single piston/floating calipers.

[JimLill]

number of pistons has nothing to do with swept area..........

I add those cars to my Excel sheet and post more data later.........

[JimLill]

Originally posted by JimLill@Dec 28 2005, 12:51 PM
number of pistons has nothing to do with swept area..........

and.......... my calcs so far have only dealt with the swept area of the disc. I think pad length and piston type have to be considered too, but more for effectivity, temp rise, and lifetime.... get's complicated!

Here's the latest Disc Swept Area calcs. I scaled pad height off the Hawk drawings.

http://www.r-series.org/swpt.html

[JimLill]

Here's the math needed to calculate braking power:

http://www.thermoanalytics.com/appli...om/mom-02-2005

http://www.thermoanalytics.com/application...nsfer_Power.pdf

[JimLill]

The results are in!....

I don't think you can really accurately calculate the real braking power that can be generated based solely on dimensions etc.. Even ignoring friction material, things like caliper style and thermal differences make it tough to judge one system against another.

I redid my spreadsheet (as linked above). It calculates swept area and pad area for each axle, but for a single pad/rotor side per axle. Obviously, you have X4 at each end. The ranking would still be the same.

I devised my own scoring system using the best swept or pad area as a score of 100 and ranking others from there. The average is for the 4 scores for each car. Probably a poor scoring system, but you can look at the numbers and devise your own.

Before declaring a victor, you also need to consider vehicle weight

[Andy Bettencourt]

Jim,

Very cool stuff. Thanks for doing the excersize. IMHO, it refutes the contention that the 944's don't have excellent brakes compared to the class (E36 aside).

I would say that a 944 8V at 2575ish would be one hell of a car under braking.

AB

[924Guy]

Hmmm... I think your source numbers may not be as good as you'd hoped. The rear pad size for the 944 is definitely oversize; the 924S sitting in my garage right now has rear pads measuring 2.875"x1.625", for a pad area of 2993mm^2.

The rear rotor measures 289mm OD, but 200mm ID. The front rotor OD you likewise have already, but it looks like the ID may be as low as 167mm (manual lists active brake diameters as 224.6mm front, 242mm rear).

Not sure how this factors into your ratings.

Comparison, from the factory manual (since I don't have the front apart and don't plan to); it doesn't give individual pad sizes, but lists brake pad surface per wheel. The numbers are 92 cm^2 per wheel for the front, and 63cm^2 for the rear, for a total of 310cm^2.

Your front pad dims would give a pad surface per front wheel of 117.5cm^2, so it looks like they're a bit larger than is correct. This is directionally consistent with the rear, but not so drastic - I wonder if the rear 944 pad you picked might have been from the S2? The S2 shows an even 86cm^2 per wheel front and rear - pretty darn close to what you estimated, yes.

[JimLill]

Originally posted by 924Guy@Dec 29 2005, 12:06 PM
Hmmm... I think your source numbers may not be as good as you'd

Have any Hawk HB-nnn numbers I can use?

[JimLill]

Originally posted by 924Guy@Dec 29 2005, 12:06 PM
The rear rotor measures 289mm OD, but 200mm ID.

How are you geting the 200mm ID? Remember the only part of the disc that "works" is that rubbed by the pad frcition material. 289-200= 89mm, a pad with 3.6 inches of height.

I'll hold off on revisiting my math etc. until I get the correct pad dimensions via using Hawk or FMSI numbers.

[JimLill]

Originally posted by JimLill@Dec 29 2005, 12:33 PM
How are you geting the 200mm ID?

I found a dumb math error which I am now fixing.... so standby for a new chart.

[924Guy]

Originally posted by JimLill@Dec 29 2005, 11:33 AM
How are you geting the 200mm ID? Remember the only part of the disc that "works" is that rubbed by the pad frcition material. 289-200= 89mm, a pad with 3.6 inches of height.

I'll hold off on revisiting my math etc. until I get the correct pad dimensions via using Hawk or FMSI numbers.

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As I said, that was a direct measurement of exactly the functioning area of the rotor, based on wear surface! Direct measurement of a rotor taken off a car.

Please, Jim, I do appreciate what you're doing here, but in order for this comparison to have any value, it must be accurate. Hawk clearly lists the applications for the 924/944 incorrectly. My values and data come either from direct measurement from a stock, unmodified car, or from the factory service manuals, as noted. There is NOWAYINHELL the pads you list for the rear of the car would ever fit. Wrong car, flat out. Those are clearly 944S2 pads, which has a completely different brake system. Wrong wrong wrong.

Your method for calculating rotor ID I agree with, where the pad dims are correct - that's appropriate and fairly close, if understood to still be an estimate.

I looked at the Hawk website - comes up with completely the wrong front and rear pads when searching on '86 944. Hmm, wonder if that has anything to do with why I don't run Hawks? It can't even find a listing for the '84 944. I recommend you stick with the factory sources. I've quoted the pad areas below. Your estimates are high for both front and rear. Please revise to reflect the true ITS configuration of this car.

I maintain that to do anything less would invalidate your comparison. I do hope that your info for the other cars is more accurate, but I have no way of checking.

[JimLill]

OK new files available....

- the areas calculated are now per wheel, 2 pads and 2 sides of disc
- I removed the score/ranking scheme, judge for yourself
- this is still based on Hawk HB-xxx pads as noted
- Hawk dwgs are pretty good, I used them to estimate actual friction material areas
- I have now included an Excel file you can play with

http://www.r-series.org/swpt-1.html
http://www.r-series.org/swpt-1.xls

[RR]

JIm how are you getting these numbers. The pad size (height and length) on the 944 is smallest of the group. The rotor is only couple cm's larger, and your final sweep on the 944 blows the comp away? When you say sweep, does this mean area covered by the pad, by percentage of rotor size?

[JimLill]

Originally posted by RR@Dec 29 2005, 03:20 PM
JIm how are you getting these numbers. The pad size (height and length) on the 944 is smallest of the group. The rotor is only couple cm's larger, and your final sweep on the 944 blows the comp away? When you say sweep, does this mean area covered by the pad, by percentage of rotor size?

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The rotor OD comes from the ITS tables starting on ITCS-17 and on

The rotor ID is the OD less 2X the pad height

OD and ID give you the swept area of the rotor, a ring

pad dims come from the Hawk info. Using a dial caliper and calculator, I worked to "guesstimate" the friction area dims.

I'm no math wiz, so may still have a math error in there. If you have Excel, you can look at my formulae on the XLS

[RR]

ok ill look at your files. But seat of the pants Ive driven944, RX7and E36. I felt the bmw had the best brakes, then RX7 then 944, not a huge diff just subtle mindyou. But your numbers would make one believe the 944 absolutly smokes everyone, which in track conditions it does not. just my thoughts. Good luck

[JimLill]

Originally posted by RR@Dec 29 2005, 06:16 PM
ok ill look at your files. But seat of the pants Ive driven944, RX7and E36. I felt the bmw had the best brakes, then RX7 then 944, not a huge diff just subtle mindyou. But your numbers would make one believe the 944 absolutly smokes everyone, which in track conditions it does not. just my thoughts. Good luck

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I think the numbers are hard to interpret. I feel that caliper-type and pad area contribute more to the braking power than the swept area of the disc. A larger disc diameter gives improved leverage, but the 944 is not that much bigger. Also, the 944 gains swept area by pad height but as you see the pad area is nothing special.

So your observations match the numbers if they are interpreted that way. I think there's a danger in looking at numbers to determine brake performance.

AND... we have not even looked at overheating and life on this...

Thanks for your input.....

[JimLill]

good feedback............ do you have the FMSI numbers for the correct pads?

[924Guy]

OK, correcting the 944 numbers based on factory data I supplied, we end up with the following:
Front pad area: 92 cm^2 (smallest of the bunch listed, 71% of the 240SX)
Front swept area: 811 cm^2 (1% larger than the BMW)

Rear pad area: 63 cm^2 (at the bottom end, 70% of the BMW)
Rear swept area: 684 cm^2 (greatest of the list, about 13% more than the BMW)

I'll see if I can find some online copy of the data I'm supplying, so I don't have to scan it myself...

[JimLill]

Let's start with Porsche Part #'s from the PET as it seems many online sources incorrectly show Turbo or S2 pads.

For late 944 and 944S NA, I show

F 944 351 951 02 and R 944 352 951 02

[924Guy]

Works for me. For the front, I show 944.351.951.02 for 924/931/944/924S/944S 79-88. 944T (and 944 S2) show either 964.351.939.02 or 965.351.939.03. For the rear, it's 944.352.951.02 for 924/931/944/924S/944S 79-88, and 964.351.939.02 for 944T/944S2. Yep, matches your #'s.

I'm still looking for a Hawk reference for the 924/931/944/924S/944S 79-88.