Hoping someone can shed some light on how to match brake boosters and MC
I have a very brake pedal throw on my car and want to change that.
Background:
I have a 350Z. It originally came with a single diaphram brake booster with this long skinny MC. It supposidly has a bore of 1"
The base brakes get the single diaphram, the track models with bigger calipers get a dual diaphram.
My car now has 6 pots in the front (stock is two tiny two pots) and 4 pots in the rear (stock is tiny single pot)
Should I be upgrading my MC and booster?
The GTR which has similar sized calipers to what I have now runs the MC and booster almost identical to the 370Z
The one on a 370z has a dual diaphram and the MC is short and fat. The ones for the 350Z are long and skinny. Oddly they both claim they use the same size (1") bore plunger. I find this hard to believe and think the bigger and fattter MC might give me shorter pedal travel and a nice stiff pedal
Does anyone know how you are supposed to match MCs/booster to calipers and can shed a little light on this topice?
I have added pictures of the skinny booster i have now and the stubby fat booster and MC from the 370Z I am considering.
I don't know that vehicle specifically, but I can say that in general the choice of single vs dual diaphragm comes down to packaging, cost, and weight. Then the MC is designed to package the required bore and stroke in the available space.
Two important and independent metrics for boosters are "boost ratio" and "helping force".
boost ratio is designed into the valve assembly inside the booster.
Helping force comes from the product of pressure (engine vacuum) and diaphragm area.
Area is directly proportional to diameter squared ( A = 1/4 * pi * D^2 ). Let's talk relative sizes so we can disregard the "pi over 4" term.
example: 10" diameter single diaphragm booster. 10^2 = 100.
compare to a commonly used "8+9" dual diaphragm: 8^2 + 9^2 = 64 + 81 = 145.
so the 8+9 has 45% more total helping force than a 10" single.
In reply to AngryCorvair (Forum Supporter) :
Good info.
How is it that all the MCs have the same bore but are shaped so differently? How does that work? Clearly the 370z and GTR MCs are made for much larger calipers. Why are they so short and fat?
In reply to Bmsluite :
Some masters have half their length hidden inside the booster.
From what I gather, the stock 2 piston front calipers had 45mm pistons, which is pretty big for a two piston caliper. I have not been able to find specs for the GT-R caliper, but I would imagine that they are going to not only be much smaller, but also three different sizes. That is to say, I have never seen a fixed caliper that had all pistons the same size, save for some cheap universal-fit aftermarket units.
AngryCorvair (Forum Supporter) said:
I don't know that vehicle specifically, but I can say that in general the choice of single vs dual diaphragm comes down to packaging, cost, and weight. Then the MC is designed to package the required bore and stroke in the available space.
Two important and independent metrics for boosters are "boost ratio" and "helping force".
boost ratio is designed into the valve assembly inside the booster.
Helping force comes from the product of pressure (engine vacuum) and diaphragm area.
Area is directly proportional to diameter squared ( A = 1/4 * pi * D^2 ). Let's talk relative sizes so we can disregard the "pi over 4" term.
example: 10" diameter single diaphragm booster. 10^2 = 100.
compare to a commonly used "8+9" dual diaphragm: 8^2 + 9^2 = 64 + 81 = 145.
so the 8+9 has 45% more total helping force than a 10" single.
How would you describe the difference in how boost ratio and helping force manifest themselves?
Keith Tanner said:
How would you describe the difference in how boost ratio and helping force manifest themselves?
boost ratio is the factor by which your input force to the booster is multiplied. Let's say you've got a 4:1 pedal ratio and a 5:1 boost ratio. If you apply 25 lbs to the pedal, you get 25 x 4 = 100 lbs input force to the booster, which is multiplied by the 5:1 boost ratio to give 500 lbs input force to the master cylinder.
Helping force is the total amount of force the booster can apply before you hit runout. Runout is literally the point at which your booster runs out of helping force (because pressure has been equalized on both sides of the booster diaphragm). Any input force above runout is not multiplied by the booster, and gives a hard pedal just like manual (non-boosted) brakes.
Pete. (l33t FS) said:
From what I gather, the stock 2 piston front calipers had 45mm pistons, which is pretty big for a two piston caliper.
for comparison, C4 and C5 have 2x 40mm pistons. S197 mustang 2x 44mm. 4th gen Camaro 2x 46mm. Honda Odyssey from 2002-ish to 2021-ish are 2x 48.
In reply to AngryCorvair (Forum Supporter) :
That is a lot bigger than the 2x38s that I used to see!
Also, thanks for the info on booster function. I assume this kind of info is not freely available. I have had cars that ran out of booster fairly early, which is fun. Power assist until you really want it, and then the pressure on the master flatlines.
In reply to AngryCorvair (Forum Supporter) :
That does not seem to be the case here. My buddy and I actually ordered a spare 350 and spare 370 MC and booster. We are going to tear these apart and see what is really going on as far as pistons go. No way can these have to same size bore.
In reply to Bmsluite :
They can have the same size bore, but a different stroke.
Thanks Angry. I know I've seen the boost ratio in Mazda spec manuals (or you can infer it from some of their tests) but never a reference to the helper force. At least, I don't think so - I'm going to go back and look :)
