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Power to Manual Brakes - Master Cylinder Bore Size #2741278
02/09/20 03:41 PM
02/09/20 03:41 PM
Joined: Jan 2005
Posts: 2,474
USA, MI
MarkM Offline OP
Worthy
MarkM  Offline OP
Worthy

Joined: Jan 2005
Posts: 2,474
USA, MI
I'm converting my car over to manual brakes this season. Setup is already four wheel disc, but getting different front calipers.

  • Rear calipers are single piston, 2.544 in² area with 11" rotors
  • Front calipers are four piston Wilwoods, with 4.8 in² area with 11.75" rotors
  • Brake pedal is the factory power pedal with a 4.5:1 ratio


What bore master cylinder should I be looking at? Should I also be looking for a pedal with a higher ratio? These cars were offered with manual brakes earlier in the model line. I've got to assume the pedal ratio was different.

Thanks again Moparts!

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: MarkM] #2741331
02/09/20 05:51 PM
02/09/20 05:51 PM
Joined: Jan 2003
Posts: 26,649
Oregon
A
AndyF Offline
Too Many Posts
AndyF  Offline
Too Many Posts
A

Joined: Jan 2003
Posts: 26,649
Oregon
I think you'll want the smallest bore size that you can find. Might be 15/16 but I haven't looked for a while.

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: MarkM] #2741357
02/09/20 07:11 PM
02/09/20 07:11 PM
Joined: Jan 2003
Posts: 1,469
Lake Orion, MI
goldduster318 Offline
pro stock
goldduster318  Offline
pro stock

Joined: Jan 2003
Posts: 1,469
Lake Orion, MI
You're going to want more pedal ratio. I think original ones are typically 6-7:1 depending on the vehicle without measuring one.


'70 Duster 470hp 340/T56 Magnum/8 3/4 3.23 Sure-Grip
Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: MarkM] #2741415
02/09/20 09:28 PM
02/09/20 09:28 PM
Joined: Nov 2004
Posts: 24,865
Texas
GoodysGotaCuda Offline
5.7L Hemi, 6spd
GoodysGotaCuda  Offline
5.7L Hemi, 6spd

Joined: Nov 2004
Posts: 24,865
Texas
I attempted to calculate out a good fit out of the gate for my manual setup. It is a delicate balance of stroke vs force.

My first m/c was a 7/8", the feel was great but the stroke made me concerned, it just took too much pedal to dab the brake to slow the car quickly on the street. I have a 15/16, which is better, but I'd entertain a 1" test drive if the m/c showed up on my front porch. Everyone is going to have a different opinion on what is "right", or "best".


1972 Barracuda - 5.7L Hemi, T56 Magnum 6spd - https://www.facebook.com/GoodysGotaHemi
1976 Dodge Warlock - "Her Truck" - 360/727 2wd
2016 Subaru WRX - E30 Tune ~320whp
[img]https://i.imgur.com/v9yezP9.jpg[/img]
Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: GoodysGotaCuda] #2741536
02/10/20 11:23 AM
02/10/20 11:23 AM
Joined: Sep 2003
Posts: 12,361
north of coder
moparx Offline
"Butt Crack Bob"
moparx  Offline
"Butt Crack Bob"

Joined: Sep 2003
Posts: 12,361
north of coder
there is a calculator out there that when you enter the various items needed, will calculate the "effort in lbs" needed for leg input.
i thought i had it saved, but i can't find it.
it wasn't wilwood, but one of the others.
it may have been a chassis builder. i just plain forget. [CRS strikes again ! grin]
beer

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: moparx] #2741545
02/10/20 11:55 AM
02/10/20 11:55 AM
Joined: Apr 2012
Posts: 851
Phoenix, AZ
J
Jjs72D Offline
super stock
Jjs72D  Offline
super stock
J

Joined: Apr 2012
Posts: 851
Phoenix, AZ
I thought Feets made a spreadsheet awhile back.

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: Jjs72D] #2741858
02/11/20 08:52 AM
02/11/20 08:52 AM
Joined: Sep 2003
Posts: 12,361
north of coder
moparx Offline
"Butt Crack Bob"
moparx  Offline
"Butt Crack Bob"

Joined: Sep 2003
Posts: 12,361
north of coder
i forgot about that ! you're right !
however, the one i was thinking about was by mark williams. sorry i don't have a link, but should be easy to find. he's the axle and rear end guy.
beer

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: Jjs72D] #2741875
02/11/20 09:44 AM
02/11/20 09:44 AM
Joined: May 2019
Posts: 987
nowhere
S
Sniper Offline
super stock
Sniper  Offline
super stock
S

Joined: May 2019
Posts: 987
nowhere
Originally Posted by Jjs72D
I thought Feets made a spreadsheet awhile back.


It's the top pinned thread in this forum.

Link is in the last post in the first page of the thread.

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: moparx] #2742170
02/12/20 07:22 AM
02/12/20 07:22 AM
Joined: Nov 2004
Posts: 24,865
Texas
GoodysGotaCuda Offline
5.7L Hemi, 6spd
GoodysGotaCuda  Offline
5.7L Hemi, 6spd

Joined: Nov 2004
Posts: 24,865
Texas
Originally Posted by moparx
there is a calculator out there that when you enter the various items needed, will calculate the "effort in lbs" needed for leg input.
i thought i had it saved, but i can't find it.
it wasn't wilwood, but one of the others.
it may have been a chassis builder. i just plain forget. [CRS strikes again ! grin]
beer


Correlating caliper pressure to stopping power is still an unknown. So if the calculator says 100lb leg force gives you 3,000psi at the caliper, then what?

Vehicle inertia, rotor size and pad coefficient of friction are still out there..


1972 Barracuda - 5.7L Hemi, T56 Magnum 6spd - https://www.facebook.com/GoodysGotaHemi
1976 Dodge Warlock - "Her Truck" - 360/727 2wd
2016 Subaru WRX - E30 Tune ~320whp
[img]https://i.imgur.com/v9yezP9.jpg[/img]
Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: GoodysGotaCuda] #2742240
02/12/20 11:31 AM
02/12/20 11:31 AM
Joined: May 2019
Posts: 987
nowhere
S
Sniper Offline
super stock
Sniper  Offline
super stock
S

Joined: May 2019
Posts: 987
nowhere
A good experiment only changes one thing at a time.

All else being the same, and it should be, you can use the math to decide what to do with your M/C bore size.

So start with what you have, put the numbers in get a value. test it as configured and if you want more stopping power, or less leg effort, you can substitute whatever values are available to get where you want to be.

If you like the leg effort but want more stopping power then you fool with pad friction coefficient or rotor sizing.

But you already know all this.

Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: Sniper] #2742414
02/12/20 07:28 PM
02/12/20 07:28 PM
Joined: Nov 2004
Posts: 24,865
Texas
GoodysGotaCuda Offline
5.7L Hemi, 6spd
GoodysGotaCuda  Offline
5.7L Hemi, 6spd

Joined: Nov 2004
Posts: 24,865
Texas
Originally Posted by Sniper
A good experiment only changes one thing at a time.

All else being the same, and it should be, you can use the math to decide what to do with your M/C bore size.

So start with what you have, put the numbers in get a value. test it as configured and if you want more stopping power, or less leg effort, you can substitute whatever values are available to get where you want to be.

If you like the leg effort but want more stopping power then you fool with pad friction coefficient or rotor sizing.

But you already know all this.


Sure, if you are starting with a blank slate. Most of these situations come up with everything other than the master cylinder already being defined and purchased. Swapping pad coefficients can get you somewhere, but not all of it depending on your need. The intended use can very much dictate what pad will be needed and what pad will survive.

Even with all of that, the subjective feeling of a extended pedal stroke to obtain the calculated pressure is also often ignored. The numbers can say that 100lb force gets you 3000psi at the caliper, but if it takes 4" of pedal stroke to get there, it is not comfortable to drive with on the street. It's too long and most people won't go for it.


My point is, that you can try to pencil whip this into an answer, but you may very well find that you don't like what you thought you would like once it's all installed and running. Buy what you think is right and be ready to adjust up/down a size, if needed.


1972 Barracuda - 5.7L Hemi, T56 Magnum 6spd - https://www.facebook.com/GoodysGotaHemi
1976 Dodge Warlock - "Her Truck" - 360/727 2wd
2016 Subaru WRX - E30 Tune ~320whp
[img]https://i.imgur.com/v9yezP9.jpg[/img]
Re: Power to Manual Brakes - Master Cylinder Bore Size [Re: AndyF] #2742876
02/14/20 01:46 PM
02/14/20 01:46 PM
Joined: Dec 2004
Posts: 2,143
Wheatfield, NY
Cuda340 Offline
top fuel
Cuda340  Offline
top fuel

Joined: Dec 2004
Posts: 2,143
Wheatfield, NY
Originally Posted by AndyF
I think you'll want the smallest bore size that you can find. Might be 15/16 but I haven't looked for a while.


If you are looking for a 15/16" bore size master a 1975 Duster With Power Disc brakes uses a 15'16" bore.


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