brake master cylinder swap

[timsz]

I have the bigger ZX front calipers for my 71 240Z and the stock drums in the rear. Would it make much of a difference in what master cylinder I put on? I have the stock 7/8 and I also have a 15/16. Thank you, TimsZ

[240260280z]

[tlorber]

Blue,

I'm trying to figure out exactly what those numbers for the MC's mean. Does "Deviation from Stock" imply travel distance of the pedal? Because from a volume standpoint, the larger the MC, the larger the cross sectional area and volume.

By my math, (I put in a 1" MC a couple weeks ago) a 1" diameter results in a radius of .5" Squaring this yields .25. A Stock MC of 7/8 has a radius of 7/16". Squaring this yields .1914". .25/.1914 = 1.306, or 30.6% larger. Are we measuring the same thing here?

Edited April 9, 2013 by tlorber

[LeonV]

Blue,

I'm trying to figure out exactly what those numbers for the MC's mean. Does "Deviation from Stock" imply travel distance of the pedal? Because from a volume standpoint, the larger the MC, the larger the cross sectional area and volume.

By my math, (I put in a 1" MC a couple weeks ago) a 1" diameter results in a radius of .5" Squaring this yields .25. A Stock MC of 7/8 has a radius of 7/16". Squaring this yields .1914". .25/.1914 = 1.306, or 30.6% larger. Are we measuring the same thing here?

Deviation from stock means .1914/.25 = .7656 (for the 1"), or 23% larger than the 7/8 MC.

Deviation from stock reflects how much LESS pedal travel there is and how much MORE pedal force is required. Using the 1" to 7/8" comparison (with stock calipers), there is 23% more force required but 23% less pedal travel.

Edited April 9, 2013 by LeonV

[240260280z]

Re-post related to the table above:

I decided to compare master cylinder area to total caliper piston areas for common Z part combinations and apply hydraulic principals.

Important Note: The total caliper area is the area summation for all pistons in both calipers.

From the data, it is clear to the Sumitomo 4 pot (Toyota S12-8) caliper upgrade has minimal area change from stock (2%) thus there is no significant change in required pedal pressure nor a significant change in pedal travel.

With reference to the table, the common upgrade to a 15/16" master and S12-8 calipers will increase pedal pressure by 11% but reduce pedal travel by 11%. For a 240z or 260z, this will reduce pedal travel from 140mm to 126mm (which is what an 82 280zx's stock travel is)

Negative values mean less travel and more pressure.

Positive values mean more travel and less pressure.

Stock brake pedal strokes

• 72 240z: 140mm (Longest)
  
• 74 260z: 140mm
  
• 75-78 280z: 123mm (Shortest)
  
• 82 280zx: 125mm
  

Pedal height from floor is 203mm

Examples:

• Adding a S12-8 calipers to a 72 240z will increase pedal travel from 140mm to 143mm (2% more travel)
  

• Adding a S12-8 calipers and 3/4" master to a 72 240z will increase pedal travel from 140mm to 195mm (39% more travel). This is not good as maximum travel is 203mm and the pedal may hit the floor if a slight loss of fluid occurs.
  

Data:

• Stock Datsun 240z to 280z caliper has two 53.98mm pistons (I could only find 72-78 data)
  
• S12-8 caliper has two 42.82mm & two 33.93mm pistons
  
• S12-W caliper has four 42.82mm pistons
  

Edited April 10, 2013 by Blue

[timsz]

Thank you very much, TimsZ

[tooquick260]

Great write up.... Thanks for the info!

[TomoHawk]

Unfortunately, you can't get the 15/16 cylinders any more, unless you have a secret cache.

[chaztg]

Junkyard availability?