Dust seal ID numbers from E92 M3 Brembo GT6/GT4 calipers - please?

[Captain Caveman]

Originally Posted by butte2butte

Attached picture is for the rear GT4 kit with a 345x28 rotor

Front pics didn't come in as clear so I won't attach but the part numbers end in 45, 43, and 41 for each piston. This is for a 365 gt6 front kit.

Top man. We now have definitive info on the piston sizes for the Brembo GT setup for the E92 M3. No great surprise that the piston sizes are very similar to other aftermarket setups, with slightly smaller overall piston area than stock. It's also worth noting that the piston sizes don't vary between their smaller or larger rotor kits.

There are quite a few OE Brembo setups with the same piston areas as the Brembo GT kit, so actually no real benefit of going for GT calipers other than convenience and possibly slightly less weight. Save 50% over a GT kit and go OE retrofit with upgraded discs/rotors. The future is bright

[patroklai]

Originally Posted by butte2butte

Attached picture is for the rear GT4 kit with a 345x28 rotor

Front pics didn't come in as clear so I won't attach but the part numbers end in 45, 43, and 41 for each piston. This is for a 365 gt6 front kit.

This happens to be the brand new kit that I currently have for sale, btw.

So are you saying that the last 2 digits of the deal part # is the piston diameter?

[patroklai]

Originally Posted by Captain Caveman

Top man. We now have definitive info on the piston sizes for the Brembo GT setup for the E92 M3. No great surprise that the piston sizes are very similar to other aftermarket setups, with slightly smaller overall piston area than stock. It's also worth noting that the piston sizes don't vary between their smaller or larger rotor kits.

There are quite a few OE Brembo setups with the same piston areas as the Brembo GT kit, so actually no real benefit of going for GT calipers other than convenience and possibly slightly less weight. Save 50% over a GT kit and go OE retrofit with upgraded discs/rotors. The future is bright

Not sure if this has been said or not, but according to my calculations, the total piston area for the front axle on a E9x M3 is 11,304mm squared. The rear is 6,644.24. This follows the logic that for floating / single piston calipers, you asume the piston extends twice the length of a multi piston fixed caliper so you double the piston area.

Assuming the 45/43/41 piston sizes shared here for the GT6 kit are correct, the total piston area would be 17,442.7 mm squared. Just eye balling it, that's like 150% from stock size. I would expect a very long pedal that is difficult to modulate with that setup.

[slowdown_dan]

I'm not saying those are piston sizes. I'm saying those are the last digits on the seals. I'm not certain what they all correlate to

[Captain Caveman]

Originally Posted by patroklai

So are you saying that the last 2 digits of the deal part # is the piston diameter?

No, I'm not saying that. The piston seal ID numbers DO NOT denote the piston sizes. However, they can be used to identify the piston sizes with a bit of digging.

E92 M3 Brembo GT calipers piston sizes are (irrespective of disc size):

Brembo GT BBK per caliper
Front 6 pot = 30/34/38mm - 2748mm x2
Rear 4 pot = 32/32mm - 1608mm x 2
Total 4356mm x 2 - 63% front bias

[wyatth]

Are you sure the GT6 calipers are the same for 365 and 380 mm kits? I’d always read they were different. They’re paired with 345 and 380 mm rears, respectively, so bias and relative torque would be all over the place if the calipers were the same between kits.

[Captain Caveman]

Originally Posted by wyatth

Are you sure the GT6 calipers are the same for 365 and 380 mm kits? I’d always read they were different. They’re paired with 345 and 380 mm rears, respectively, so bias and relative torque would be all over the place if the calipers were the same between kits.

100% sure. I got the piston sizes from a 380/380 Brembo GT kit and 'butte2butte' posted the piston sizes in his 365/345 kit earlier in this thread. They are the same.

I assume that it is more about maintaining the front/rear disc diameter ratio, so for example a 365/345 kit, a 380/380 kit or a 380/365 kit will have front/rear rotor diameter ratios within 4% of each other, so negligible difference.

[pbonsalb]

Stoptech has used different piston sizes on its 6 piston front calipers but I am not sure whether the change depends on rotor diameter or whether the calipers are Trophy or regular ST60.

[Captain Caveman]

Originally Posted by pbonsalb

Stoptech has used different piston sizes on its 6 piston front calipers but I am not sure whether the change depends on rotor diameter or whether the calipers are Trophy or regular ST60.

Yes the Trophy uses slightly different piston sizes, but the overall piston area is almost identical. I believe the piston sizes don't change irrespective of disc disc:

Stoptech non Trophy st60 = 36/36/30mm x 2 - 2741mm x 2
Stoptech Trophy ST60 32/34/36mm x 2 = 2729mm x 2

[wyatth]

Wild, I always thought ST pistons were the same, Trophy or not, but pistons varied by application/rotor size. Wonder why they would do that, or if those two ST60s were both from E9x 380mm kits.