Swapping Big Brakes

[rbryant]

An lbody with a single diaphragm booster will require less pedal effort than a GHKHPA body with a single.
This is because the brake pedal lever has a longer fulcrum on an lbody than the other cars.


I spent some time figuring it out a while back.

It will take a little over 25 lbs less pressure on an lbody than on the other cars to get to 1000psi of brake pressure with the same brake booster.

Original Thread:

http://www.turbo-mopar.com/forums/sh...l=1#post709817

Chart:



-Rich

[cordes]

When you say harder, do you mean more firm or harder to apply?

You really have to stand on the pedal compared to the big booster.

An lbody with a single diaphragm booster will require less pedal effort than a GHKHPA body with a single.
This is because the brake pedal lever has a longer fulcrum on an lbody than the other cars.


I spent some time figuring it out a while back.

It will take a little over 25 lbs less pressure on an lbody than on the other cars to get to 1000psi of brake pressure with the same brake booster.

Original Thread:

http://www.turbo-mopar.com/forums/sh...l=1#post709817

Chart:



-Rich

That may be the case, but it's still a doozy with big brakes in an L body and the smaller booster compared to an 11" setup in it's natural habitat.

[supercrackerbox]

You really have to stand on the pedal compared to the big booster.

Yep. Now my car has absolutely no trouble slowing down quickly, and I'm quite certain it could stop in the same or less distance than my Daytona. But be it from the reduced power assist, the taller stickier tires, or both, I really can't lock the wheels up on the Charger. That's actually a good thing though when I'm trying to slow down and steer around a deer or raccoon on the highway home. I can- and have- locked up the wheels on the Daytona many times.


BTW, a raccoon took most of the front bumper off my girlfriend's Honda last weekend, so I'd rather not hit one if I can avoid it.

[Reaper1]

Rich, quick question, did you actually measure the pedal ratios? 3.2:1 or 3.3:1 seems REALLY low. I have grandeur ideas of one day going with a dual MC with a proportion bar on my car, but the typical pedal ratio I've found for that type of system in is the 7:1 range! That kind of kills the idea of maybe using the stock pedal as I don't know it could be modified that extensively.

I realize that the dual MC set-ups are not boosted (part of the reason I'm interested in it), so I'm thinking that's the main reason for the fairly large pedal ratio. On top of that they use small MC diameters, presumably to keep pedal feel and to build line pressure faster, BUT you need more travel to move the volume, hence the large pedal ratio. Am I on the right train of thought here? I also realize typically these systems are designed with multi-piston, fixed calipers, which (depending on how large they are) require less volume to actuate than floating calipers with single pistons.

[rbryant]

Rich, quick question, did you actually measure the pedal ratios? 3.2:1 or 3.3:1 seems REALLY low. I have grandeur ideas of one day going with a dual MC with a proportion bar on my car, but the typical pedal ratio I've found for that type of system in is the 7:1 range! That kind of kills the idea of maybe using the stock pedal as I don't know it could be modified that extensively.

I realize that the dual MC set-ups are not boosted (part of the reason I'm interested in it), so I'm thinking that's the main reason for the fairly large pedal ratio. On top of that they use small MC diameters, presumably to keep pedal feel and to build line pressure faster, BUT you need more travel to move the volume, hence the large pedal ratio. Am I on the right train of thought here? I also realize typically these systems are designed with multi-piston, fixed calipers, which (depending on how large they are) require less volume to actuate than floating calipers with single pistons.

The pedal ratios are listed in the FSMs.

Pg 5-44 of the '86 FSM has 4.2 for the Horizon/Omni and 3.2 for the others.
Pg 4-114 of the '91 FSM shows 3.3 for the cars so it did change slightly on the later cars.

My charts use 3.25 just to try and cover both 3.2 and 3.3 in a single chart given that the difference is so small.

The older style dual booster was also only 3690 N but I think I used the newer 4048 N dual booster in the chart so that could change things slightly.

I guess I never posted the updated chart that I did which had both. Here is a chart that has both and also a hypothetical 3000 N booster and also shows what happens with a 7/8" Master Cylinder.




Notice that the 7/8" Master Cyl can get you some of the way back at the expense of pedal travel so it is really all in what you like.

I am sure we can probably find a Booster that is 3000N if we look around a bit to make the feel be just like the GJKHPA 4 wheel setup.

A dual 150mm booster would probably be around 3000N depending on how it is made.

-Rich

[minigts]

I've got the 11" brakes on the front of my car (90 style) and they are better than the stock. Never swapped the MC, so I don't have a comparison to the difference as to how better or worse that upgrade is. I may swap it out when I put on the rears. I'm still concerned about locking the rears up on my car though.

[Reaper1]

The pedal ratios are listed in the FSMs.

Pg 5-44 of the '86 FSM has 4.2 for the Horizon/Omni and 3.2 for the others.
Pg 4-114 of the '91 FSM shows 3.3 for the cars so it did change slightly on the later cars.

My charts use 3.25 just to try and cover both 3.2 and 3.3 in a single chart given that the difference is so small.

The older style dual booster was also only 3690 N but I think I used the newer 4048 N dual booster in the chart so that could change things slightly.

I guess I never posted the updated chart that I did which had both. Here is a chart that has both and also a hypothetical 3000 N booster and also shows what happens with a 7/8" Master Cylinder.




Notice that the 7/8" Master Cyl can get you some of the way back at the expense of pedal travel so it is really all in what you like.

I am sure we can probably find a Booster that is 3000N if we look around a bit to make the feel be just like the GJKHPA 4 wheel setup.

A dual 150mm booster would probably be around 3000N depending on how it is made.

-Rich

I remember the first time I swapped brake systems on m car. I forgot about the MC needing to be changed. I had a system that was hydraulically the same as the 11" set-up, but I left the stock 10.25" Disc/Drum MC on the car. Now, the car WOULD stop, BUT the pedal was on the floor to do it! That was absolutely NO good!

So, would changing the pedal ratio to yield more MC travel per input end travel of the pedal make it so a smaller MC could be used to net the desired pedal feel and travel, even using the single booster?

Just a hypothetical question that might help somebody out. I still want to go the dual MC route myself....

[rbryant]

I remember the first time I swapped brake systems on m car. I forgot about the MC needing to be changed. I had a system that was hydraulically the same as the 11" set-up, but I left the stock 10.25" Disc/Drum MC on the car. Now, the car WOULD stop, BUT the pedal was on the floor to do it! That was absolutely NO good!

So, would changing the pedal ratio to yield more MC travel per input end travel of the pedal make it so a smaller MC could be used to net the desired pedal feel and travel, even using the single booster?

Just a hypothetical question that might help somebody out. I still want to go the dual MC route myself....

It might be possible to change the pedal ratio but I think it would take a bit of work. It would be much easier to change the booster and let that do the work. It should also be possible to put an adjustable regulator on the dual booster to only allow it to get partial vacuum which would allow for fine tuning of the brake feel but that is just a theory I have been thinking about.

You might have actually been using the 21mm MC which would indeed require more pedal travel. Do you know for sure if it was the 21mm or 7/8" MC?

I am curious to see how a 7/8" (22.225mm) master cylinder would work on an lbody with 11" fronts and non vented rears. The pressures look pretty good but I don't know about the travel. The 7/8" MC is just about half way between the 21mm and 24mm MCs.

If I feel like my setup has too stiff and has too little travel after the latest changes I think my first change will be to a 7/8" master cylinder because I am already running the smaller non vented rear brakes which means I have a little bit less fluid to push when compared to the vented rear setup.

I think overall I should be strong enough to push the pedal with a 24mm for a few more years though and it will probably give me more control anyway...

-Rich

[Reaper1]

I honestly don't remember which MC it was. I know it was the stock one for that car, but which one, I don't remember. According to Auto Zone the MC should have had a bore size of 0.875" which is 7/8". If you could find a way to change the pedal ratio to allow more piston travel it might solve the problem using tat MC. The only thing is, what is the maximum travel of the piston? It might be that we can't get enough volume displacement for it to actually work.

[NeverLow]

Sry to bust up an older thread, but just trying to figure out my big brake set up. Going with a brand new big brake factory replacement set up from R1 concepts. Waiting for them to let me know all that is included in their kit (hoping rotors, 60mm calipers, & pads). But I am trying to figure a few things out until they get back to me.

Even tho the 92 comes with both brake setups, since i have the 10" and drums in the back, I know i need the knuckles and hub assembly bearing (not the press in bearing) for the front, and backing plate for the back. Also lookin at prop valve and 24mm MC.

Thanks again for the help guys, im slowly learning...

[Force Fed Mopar]

91-up spindles are all the same, just need the 11" rotors and caliper brackets.

[NeverLow]

91-up spindles are all the same, just need the 11" rotors and caliper brackets.

So just need rotors and caliper brackets for front and backing plate and caliper brackets for the rear?

No knuckles, hub assembly, etc needed?

[wallace]

The pedal ratios are listed in the FSMs.

Pg 5-44 of the '86 FSM has 4.2 for the Horizon/Omni and 3.2 for the others.
Pg 4-114 of the '91 FSM shows 3.3 for the cars so it did change slightly on the later cars.

My charts use 3.25 just to try and cover both 3.2 and 3.3 in a single chart given that the difference is so small.

The older style dual booster was also only 3690 N but I think I used the newer 4048 N dual booster in the chart so that could change things slightly.

I guess I never posted the updated chart that I did which had both. Here is a chart that has both and also a hypothetical 3000 N booster and also shows what happens with a 7/8" Master Cylinder.




Notice that the 7/8" Master Cyl can get you some of the way back at the expense of pedal travel so it is really all in what you like.

I am sure we can probably find a Booster that is 3000N if we look around a bit to make the feel be just like the GJKHPA 4 wheel setup.

A dual 150mm booster would probably be around 3000N depending on how it is made.

-Rich

Rich, does it give the pedal ratio for the lbody with manual brakes?

[rbryant]

Rich, does it give the pedal ratio for the lbody with manual brakes?

Not in the '86 FSM.

What years did they have manual brakes? I assume that it had to be before '83 right? I don't think I have ever actually noticed them on an lbody....

-Rich

[Force Fed Mopar]

So just need rotors and caliper brackets for front and backing plate and caliper brackets for the rear?

No knuckles, hub assembly, etc needed?

You need the 11" calipers too. Hubs and knuckles are the same.

[NeverLow]

You need the 11" calipers too. Hubs and knuckles are the same.

Do the calipers come with brackets?

[Force Fed Mopar]

Do the calipers come with brackets?

Some stores sell calipers with brackets, some don't. Same for junkyards with used ones.