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Streetwiseguy MegaDork
5/23/22 7:52 a.m.

I'm converting a car to manual brakes, because engine and booster want to occupy the same physical space.

I feel like I read a pretty good article within the last few years on pedal ratios and master cylinder sizing, perhaps in everyone's favorite magazine, or Car Craft/Hot Rod etc, or online somewhere.

Any hints?  I have a pretty good idea of what should work, but the ability to do a bit of math to confirm would be nice.

Toyman! GRM+ Memberand MegaDork
5/23/22 8:48 a.m.

IIRC, 6:1 was the pedal ratio, and around 1200 psi max pressure was what you wanted.

I didn't find a calculator when I was doing the brakes on SanFord, but here is a link to an article about the math. 


tr8todd SuperDork
5/23/22 8:56 a.m.

Here is a good one.  https://www.markwilliams.com/braketech.html  Google brake master sizing, brake math, brake caliper sizing.  You can find a few others.  There was one posted by a Nascar team I looked at before, but now I can't find it.


Oapfu GRM+ Memberand New Reader
5/23/22 9:20 a.m.
maschinenbau GRM+ Memberand UltraDork
5/23/22 9:38 a.m.
79rex HalfDork
5/23/22 9:41 a.m.

when i did it i referenced this https://www.joesracing.com/master-cylinder-math/.  towards the bottom it goes into pedal ratio.  

Driven5 UberDork
5/23/22 10:39 a.m.
wawazat SuperDork
5/23/22 2:12 p.m.

Paging Angry to this thread.  Angry Corvair, please pick up on this thread.  

frenchyd GRM+ Memberand MegaDork
5/24/22 7:47 a.m.

In reply to Streetwiseguy :

Describe the projected drivers leg strength.  Yourself? Your wife? Maybe sons or daughters?   
       How long is the stock brake pedal from the center of the foot pad  to the brake assembly?   The longer that is the easier the pedal will be.   For an easy pedal use a smaller master cylinder.  
    Master cylinder size works opposite of what you'd think.  A smaller cylinder  gives an easier pedal and a bigger cylinder makes it  harder to press.  power brakes may use an 1 

  The actual volume required  depends on the size of the wheel cylinders. So you've likely got some options there.   In other words aftermarket calipers .   Plus there are lots of choices from people like WILWOOD who will help you sort it all out.  Maybe you need a tandem arrangement.  Or a dual master cylinder arrangement. 
   PS your best prices are from people like Jegs or Summit etc. not directly from Wilwood who will help you find the best solution. That's how they work.  

      Putting our guys in the Jaguar we  building, are mostly older geezers with less than muscle builders leg strength.  
      The car is easily capable of 150 mph speed. So I used the longest brake pedal I could get which has an 8 inch length to the brake cylinder. 
 Most pedal assemblies are 6 inch and nobody sells a single 8" to be used as a clutch.  Accordingly I accepted a 6" clutch pedal and a 8" Brake pedal.  Using the difference as a heel rest area. 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/21/23 12:40 p.m.

It's all ratios: master cylinder to caliper piston area, length of brake pedal overall to the length to the pushrod. A booster is a multiplier, so if you can find out the boost ratio of the booster (you can often find that in the factory manuals) you'll be able to figure out what you have to do to the master and the pedal to get the same overall assistance. The fact that you have some knowns (say, calipers and brake rotor diameter) makes the math simpler.

Hydraulic systems are just wet levers or gears :)

Nockenwelle New Reader
2/21/23 2:05 p.m.

This one is great and does advanced calcs very well:


2/21/23 2:18 p.m.
Streetwiseguy said:

I'm converting a car to manual brakes, because engine and booster want to occupy the same physical space.

Hydroboost perhaps for the easy button?

Ultimately unassisted brakes are going to have a longer pedal stroke to reach a given pressure. Longer stroke means more time delay for a given brake force. 

Also, if track car, racing can get tiring, so anything to make the drive less physically demanding lets the driver focus on driving.




frenchyd GRM+ Memberand MegaDork
2/21/23 2:32 p.m.

In reply to Streetwiseguy :

First how strong are your legs.   Numbers don't mean anything unless you can put the required amount of force  while maintaining the ability to finesse.   
      At age 75 I went to the 8"  pedal assembly length*. While I am stronger than that,  I'll be using this car as I get into my 80's.   Don't be fooled my legs are way stronger than most men my age.    To figure out your leg strength  run up a flight of stairs  timing yourself. Measure the height you raise by your weight and that formula can tell you your horsepower.  Compare that to  others and now you have a valid number to put in the pedal formula. 
   I can  always use less force. Finding  more force will get harder and harder.    How much?    Well? run up those stairs for a while and see how quickly you deteriorate.   
    Then there is the track you'll be racing at.   Mine has 3 long straights where you get to top speed  followed by really tight. ( more than 90 degree corners )  

   The smaller the wheel cylinder  the more force you can apply while  maintaining finesse.  Yet the bigger the brake piston(s)  is the more volume you need to move.     
         The size of the pistons  is determined by the weight of the car.  
   You can trade one for another.    But always leave some extra.  

* it's nearly impossible to find 8* clutch pedal.  So I raised the brake pedal assembly top pivot point as high as possible.  While lowering  the  clutch pedal.   As long as the center of the pedals are nearly the same  you can drive.  Clutch pedal with a triple 7&1:4" disk  is still pretty light work compared to braking. 

obsolete GRM+ Memberand HalfDork
2/21/23 2:48 p.m.

You guys all realize this is a thread from last year that got canoed back up to the top, right?

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/21/23 3:22 p.m.

In reply to obsolete :

Doesn't change the quality of the real responses, though :)

AngryCorvair (Forum Supporter)
AngryCorvair (Forum Supporter) GRM+ Memberand MegaDork
2/21/23 4:00 p.m.

and lets remember that you always want your master cylinder to bottom out before the pedal hits the floor.  a pretty typical master cylinder stroke is 36 mm (about 1-3/8"), but there are others.

combine that 36 mm stroke with a 6:1 pedal and you need 216 mm (about 8-1/2") of available pedal travel.

gotta be able to use 100% of MC stroke in the event of pad and/or fluid fade.

real answers are real.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/21/23 5:15 p.m.

I was always told 6.8 or 7:1 for a manual pedal.



Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/21/23 5:30 p.m.

In reply to Pete. (l33t FS) :

Probably just a rule of thumb to work with commonly used master and caliper piston sizes.

I used 6:1 on my Locost, but that's because that's the ratio of the floor mounted pedals I bought from Wilwood :) I remember having to relocate the pivot on the clutch pedal on Miata-based Westfields because Westfield got the math wrong and there was no clutch master in existence that would work with the random ratio they came up with. Move the point by (I forget) and it worked like stock. It was all just working the ratios.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/21/23 5:35 p.m.

In reply to Keith Tanner :

While true, there is also something to be said for the modulation ability of having high leverage on the hydraulics, vs. having low leverage and a tiny master cylinder bore.

I can't really put into good words why it feels that way, but the closest analogy I can think of is steering feel is better with a 16" steering wheel and fast steering than a 10" wheel and normal steering.  Technically the same hand distance motion but one definitely feels better than the other.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/21/23 5:49 p.m.

In reply to Pete. (l33t FS) :

Meanwhile, the Miata pedal ratio is in the 4-something range and it's got boost in the hydraulics :)

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/21/23 6:57 p.m.

In reply to Keith Tanner :

Speaking of manual brakes of course smiley

RX-7s are 4.5:1, I would not doubt that Miatas are the same, given how much other engineering and engineering team they share.

When I had my '81 apart, I measured the brake pedal at 315mm from pivot to center of pedal pad, 70mm from pivot to pushrod.  I drilled a new hole at 45mm to futureproof a conversion to manual brakes, as removing the pedals requires removing the steering column/box which is easiest done with the engine and front suspension out of the car.  Not gonna do that again if I don't have to smiley

But the power brakes on it feel so "right" that I may never do it.


I noted that some newer cars have absolutely absurd pedal ratios like 2:1.  I have a feeling that there used to be a standard for brake function if the engine stalled, but modern engines are so reliable that they relaxed that requirement.  Most cars seem to have brake vacuum pumps anyway.

GaryC83 Reader
2/21/23 7:17 p.m.

In reply to Pete. (l33t FS) :

A lot of new stuff is going to electric masters. We have been running conversions on our hot rods for quite a few years now. I still recommend about a 4:1 ratio on the pedal on those.  But, you'd ve surprised how many cars nowadays are running an electric booster of some sort. They just...work. and tend to work better for a myriad of reasons. 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/21/23 7:20 p.m.

More blathering as I wait for the garage to warm up a bit before I start beating my head against BMW wiring diagrams.  (I am sure someone in the world likes them, and that person should be isolated from society and studied closely)

Could it be possible to stick the master cylinder/booster on a standoff like old shocktower Fords had?  Although those had a lever ratio to convert a manual pedal to a power friendly ratio, so maybe nevermind.

Is it possible to modify the valve cover for clearance?  I did that one once, oddly enough on a shocktower Ford...

GaryC83 Reader
2/21/23 7:32 p.m.

In reply to Pete. (l33t FS) :

You can also always spin it sideways and stick it under the dash. We do it on a ton of stuff....

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
2/21/23 8:22 p.m.

Remote boosters were a thing in the Little British Car world occasionally as well. I am not so much endorsing this as acknowledging it :)

I'd be looking at hydro boost myself in this situation. I've seen it done in a LS3 powered 944. 

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