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Discussion Starter · #1 ·
My Si came with a 7/8" master cylinder, I'm sure some of you have a 13/16" also.
I wanted to upgrade to a 15/16" Master so I went to my local parts store here in Brampton, Ontario, Canada and asked for a 91 Civic EX(US model) master. I checked the box and sure enough it's a 15/16" master cylinder. I installed it on my car and to no surprise, my brakes feel great!! I've heard of all kinds of different master cylinder swaps, all requiring line bending or some kind of modifying. This setup is a straight bolt-on.

Very sweet.

D.
 

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Discussion Starter · #3 ·
no, the prelude MC requires some bending of the lines, this MC bolts in EXACTLY the same as the stock piece. It also works on EG's too. I used one on my EG with my big brake setup and it worked amazing.
 

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Masta said:
My Si came with a 7/8" master cylinder, I'm sure some of you have a 13/16" also.
I wanted to upgrade to a 15/16" Master so I went to my local parts store here in Brampton, Ontario, Canada and asked for a 91 Civic EX(US model) master. I checked the box and sure enough it's a 15/16" master cylinder. I installed it on my car and to no surprise, my brakes feel great!! I've heard of all kinds of different master cylinder swaps, all requiring line bending or some kind of modifying. This setup is a straight bolt-on.

Very sweet.

D.
Do you have stock Discs? I was told that going to larger bore increases peddle efort and not to do it unless you got bigger callipers.
 

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I just used a 1989 prelude si MC 15/16 on my 91 crx..
Bending the line is a peice of cake.

alos, how much did you pay for the MC ?
What is the part # ? and the phone number to the shop ?

I wonder if it will fit a 1990 civic si with rear drums ?
 

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jfrolang said:
The larger bore MC just moves more fluid with an equal movement of the pedal, so the effect is that you don't have to press as hard to stop as hard.
Other way around actually. The larger bore MC will require more pedal pressure, but less travel to get the same braking effect. The larger bore means that a given amount of pedal force is distributed over a larger area of brake fluid. That means you get lower PSI of brake fluid pressure for a given pedal effort, but more flow. The caliper pistons and/or wheel cylinders don't move much in normal operation, so flow isn't all that important. The caliper/cylinder bore hasn't changed, so lower PSI means less force is applied to the brake pads/shoes.
The brakes feel more solid because you've reduced your leverage over the pads/shoes... It's like trying to remove a lugnut with 6" ratchet versus a 24" breaker bar. Ok, maybe not that dramatic, but it's the same idea.
 

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Very true bobski. Although just for analogy's sake I'd draw the parallel to tightening a bolt with a 3/8" ratchet or a 1/2" ratchet. You put the 3/8" on there and crank as hard as you can. Feels nice and tight. but then you put the 1/2" on there and find that with the same effort you can torque it down another 1/4-1/2 a turn because the lever arm is longer.

Putting the 15/16 bore mc on would reduce pedal travel, reduce brake feel since it doesn't move as far, and ultimately reduce braking force. I'm not sure if brake feel is a priority, but as long as you aren't running out of pedal travel with your stock mc then ultimately the bigger bore will make your brakes weaker when you really squeeze them becasue like bobski said, you have a bigger area over which to apply your foot force.

P=pressure
F=force
A=area

P=F/A and P(mc)=P(piston)

=> F(foot)/A(mc)=F(caliper)/A(piston)

if F(foot) and A(piston) are constant, say 1, then A(mc) and F(caliper) are inversely proportional.

=> 1/A(mc)=F(caliper)

So as A(mc) increases, then F(caliper) decreases.

This means that when you jump on the brake using your max foot force, you won't be generating as much force at the caliper with the 15/16" as you would with your stock master cylinder.

I porbably won't swap my mc out with anything but the stock bore.
 

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Discussion Starter · #12 ·
That's odd. I find the opposite has occured. I have better feel, less travel and just better braking.

I'm perplexed now. When I had my EG, I ran the same MC in place of the stock 13/16" MC, braking force at my foot went wayyy down, and overall feel was much improved. At the track I found the new 15/16" MC ideal, and believe me, brake feel was a huge priority as I raced the car in Solo 1. The brakes were the best part of the car.

You seem to know what you're talking about so I'm hesitant to debate, but I think you may be missing something, or maybe it's me..... :?:

Now, my EG had Type R brakes, that could be where I'm getting lost.
 

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Basicall think about it this way: you're increasing the volume of fluid you're pushing through the master cylinder, so you don't need as much travel to move the same fluid. Volume = Area x Length, and Area is pi*radius squared. Obviously, you're increasing the overall volume with the larger radius, so to move the same fluid, you don't need as much length (pedal travel). I wouldn't say the force is necessarily changing, but you don't reach the end of the travel as quickly.

does that help?
 

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Discussion Starter · #14 ·
Yes I understand that part of it, what he is saying is that the braking force is REDUCED with a larger MC. That I don't get. The pedal travel makes sense, that's a big part of why I did the swap in the first place.

It seems to me that now I am moving more fluid with the same travel, meaning more force apllied at the caliper. I may need to push a bit harder, but more fluid is displaced, so there is more pushing on the piston when the pedal distace is identical. When I stomp on the brakes, in which case my pedal would hit the floor before(at the track anyway),
I would be moving more fluid which in turns applies more force at the caliper.

Can you tell I don't have a University degree in this stuff? :oops:
 

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downest said:
Basicall think about it this way: you're increasing the volume of fluid you're pushing through the master cylinder, so you don't need as much travel to move the same fluid. Volume = Area x Length, and Area is pi*radius squared. Obviously, you're increasing the overall volume with the larger radius, so to move the same fluid, you don't need as much length (pedal travel). I wouldn't say the force is necessarily changing, but you don't reach the end of the travel as quickly.

does that help?
This is what I experienced: Less pedal travel and a stiffer pedal. I had to exert more pressure to get the pedal to travel, but because I didn't have to move it as far, my brakes were far more responsive. Once I got used to how touchy the new setup was, I found that I could brake much faster than with the sloppy long travel experienced with the smaller master cylinder. However, there is the danger of being able to lock up the brakes much easier when doing some emergency braking.

However, my plan to install the 15/16" master cylinder was not to change the pedal pressure or travel. I did it with the intentions of installed larger calipers. I now have EX calipers on the front. My pedal feels just a tad stiffer than stock now... no where near as noticeable as when I had the smaller CRX Si calipers on there.
 

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I was told that unless you increase the caliper size it justs makes it harder to press and not any better for braking. After all Honda is pretty good at engineering and usually know what is right. Now if you went to multi-piston or just larger, then it would make sense. Many of the guys with big block dragsters here try to find a SMALLER one so the pedal effort is less when they take the boost off the brakes.
 

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Masta said:
Yes I understand that part of it, what he is saying is that the braking force is REDUCED with a larger MC. That I don't get. The pedal travel makes sense, that's a big part of why I did the swap in the first place.

It seems to me that now I am moving more fluid with the same travel, meaning more force apllied at the caliper. I may need to push a bit harder, but more fluid is displaced, so there is more pushing on the piston when the pedal distace is identical. When I stomp on the brakes, in which case my pedal would hit the floor before(at the track anyway),
I would be moving more fluid which in turns applies more force at the caliper.

Can you tell I don't have a University degree in this stuff? :oops:
This is where you would have benefited huge from the larger diameter. When your pedal hits the floor you arn't pushing anymore fluid, BUT with the larger cylinder you have the ability to push more fluid. It is going to be harder to push it all the way in because your moving more fluid over the same distance.
 

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I think part of the problem is that people may be comparing an old stock mc with worn leaky seals to a new rebuilt mc with fresh seals. If your stock mc has good seals it should not hit the floor. My last si would lock up the brakes before coming close to hitting the floor. The pressure exerted has nothing to do with the movement of fluid per se. it has everythign to do with the area of fluid that is acted upon at each end. Also, since the you can lock up the brakes easily with the stock mc, it is possible that you can still exert the maximum braking force at the caliper with the bigger mc exerting a relatively lower range of forces for a given person. Does that make sense?
 

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Discussion Starter · #19 ·
You're absolutely right, my bad. I went and spoke to my buddy tonight who is a Speed World challenge competitor/Engineer and he confirmed what you said. Good call. My car brakes are much better now though, and the reduction in pedal travel makes it sooo much easier to heel toe, which is a huge benefit with my size 12's.
 
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