[s-cars] [urq] S-Car Brake system conversion- Read- no more PSpumpintegration <question>

[racingiron at comcast.net]

Cody Forbes wrote: 

> Actually I think you may be mistaken. A 996TT caliper has 4 pistons, 2 36mm and 
> 2 44mm. That would give a total piston surface area of 3553mm^2. A G60 has one 
> 60mm piston. A G60 would then have a total piston surface area of 2826mm^2 - 
> roughly 25% less than the Porsche part. 

We're comparing opposed piston calipers to sliding calipers here. Without getting into the details, to compare piston sizes between these two, you have to look at just one side of the opposed calipers (or double the numbers for the sliding caliper). I'm not sure how you came up with the 3553 number for the 996tt caliper, but I get 2538 (the area of one side of the caliper, so your calculation of all four pistons should have given double that). That's equivalent to a single piston of 56.85. 

The G60 caliper is a dual-piston sliding caliper. I don't have the piston sizes in front of me, but it works out to the same area as a single 60mm piston. The result is that a 996tt caliper has about 95% of the G60 piston area. That's why everyone can stick with their stock MC when doing all of these brake upgrades. Pedal travel actually decreases slightly when changing from the G60s. If the Porsche calipers were in fact 25% larger effective piston area, there would be a VERY noticeable increase in pedal travel. 

> If you factor in however much the pistons move (1mm each maybe?) you can see 
> that the used displacement in the Porsche caliper is indeed more. 

Not when you consider that the sliding caliper has to effectively move the pads on both sides of the caliper, so if the opposed caliper moves 1mm per side, then the slider piston has to move 2mm. That's why you halve the opposed pistons or double the slider's. 

> I can't find the info for a Boxster caliper, 

It's 40 and 36 for the Boxster, which is equivalent to a 53.8 sliding single piston caliper. 

Please note that I'm not saying the G60 caliper is in any way better than any of the typical Porsche options. This discussion was triggered by the claim of increasing the piston area with a big red brake upgrade and the effect on pedal leverage/effort. 


Eric R. 
'86 urq 
'92 urS 
'93 urS