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Old August 27th 2018, 09:55
Tzepesh Tzepesh is offline
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Join Date: Dec 2017
Posts: 5
I have 944NA brakes now on 1303S, with stock 19.05mm master cylinder. I have a set of 944T 4 pot brakes that I will swap on my car, so I've been reading a lot on brakes lately. This thread was a wealth of information. So I tried to plug in some data in the "formulas" used before:
1. 944NA brakes (54mm and 36mm slave cylinders) with 19.05mm pump.
Resulting ratio is 2.25. So huge front bias. Same result if using 23.8mm pump or stepped 23.8/19.05mm pump (explanation why stepped pump is not affecting bias is here: The stepped pump just increases the volume of brake fluid going to the front slave cylinders and makes the pedal harder. This explains what I saw during "MOT", a high efficiency of front brakes (80%) compared to rear (40-50%), this being correlated to front and rear weight. So I do not see any other option except bias bars (or dual pumps) to overcome this issue.
Pedal feel is great from my point of view, soft and long travel. I hate hard and short pedal.
2. Considering 944T brakes (40/36 front and 30/28 rear) with 19.05mm pump (same would be with any other pump):
Ratio is 1.717, so still front biased, but much less. Now, considering the same piston sizes were used in 930 with a 23.8mm pump (no step), and that the 930 is has a closer static/dynamic weight ratio to Bug (both rear engined), compared to 944 (front engined), it seems these brakes will be much better suited for a Bug, ratio-wise not power-wise. I'll try with the stock 19.05 pump first, I have read a lot of positive feedback on this setup.

But a question that still remains is why did 944T have a stepped pump with the 4 pot brakes and 930 straight pump, if the step in the pump actually does not affect bias. Just for volume?

Did I understand this correctly? Some older calculations took into consideration for the ratio the master cylinder different steps, but that seems to be false according hydrodynamic laws. That would hold true if the two pistons in the master cylinder would be hard connected to each other, which they are not.
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