Dr Diff 13" "Cobra" front disc brakes.....who has experience with them?

[Nxcoupe]

Here's it installed. I really decided on this one after a friend recommended this style, and the one from Dr. Diff was perfect for a manual brake car since the bore has the groove, and comes with the rubber grommet to fit the stock manual brake push rod. No issues at all, plug & play.

View attachment 1295437

Did you have to extend the pushrod to account for the adapter?

 

[Don67Satellite]

Did you have to extend the pushrod to account for the adapter?

No.

 

[Kern Dog]

I'm still considering this upgrade but I have some reservations.
Dr Diff's pricing is fine but I don't want to spend the money if it won't make a worthwhile difference. After seeing that video about that red '64 Mercury Comet that fried the brakes and crashed, I am even more interested in improving the brakes in this car.
Brake science starts to get confusing once you deviate from the factory stuff. Ma Mopar had engineers that figured out wheel cylinder sizing, master cylinder bore sizing, brake pedal ratios, front disc caliper bore diameters and all of that.
I've found that once you step out and away from a factory arrangement, your results can vary a LOT.
To recap....
Before I pulled the engine, trans and K member, I was using a stock 1975 Dart power booster and 15/16" master cylinder and the brake pedal pushrod was bolted directly to the pedal arm with none of the factory style power brake linkage that B body power brake cars used.
The front brakes are the Cordoba 11 3/4" rotors and 2.75" calipers with Carbon Metallic pads. The rear is the 10.7" Dr Diff disc brake setup with a 1 1/2" caliper. The brakes are okay but not spectacular. My '72 Duster has the 73-76 A body 11" front discs but with the bigger 2.75" calipers and 10" drums in the rear. 15/16" non power master cylinder. That car flat out S T O P S !! My "Jigsaw" Charger has 11 3/4" rotors, 2.75" calipers, 10" rear drums and a non power 15/16" master cylinder. That car stops great too.
One thing that I am unclear of is the bias that is preferred in terms of front to rear caliper bore sizing. If I have a 2.75 front caliper bore, what is the optimal caliper bore size for the rear?
Along the same theme, If the rear bore size is too big, what size do I need for the front to be?
This Dr Diff Cobra brake package does not list the size of the front caliper piston.

The picture shows 2 pistons though. What if the total surface area of the 2 pistons is smaller than my existing ones? Would that lead to a rear brake bias and make a swap like this an actual step backward?

Dr Diff did have this tech info on his website:

Master Cylinder/Caliper Bore Size​

All else being equal…

• Increasing the master cylinder bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.
• Decreasing the master cylinder bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
• Increasing the caliper bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
• Decreasing the caliper bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.

 

[Boris7K]

I'm still considering this upgrade but I have some reservations.
Dr Diff's pricing is fine but I don't want to spend the money if it won't make a worthwhile difference. After seeing that video about that red '64 Mercury Comet that fried the brakes and crashed, I am even more interested in improving the brakes in this car.
Brake science starts to get confusing once you deviate from the factory stuff. Ma Mopar had engineers that figured out wheel cylinder sizing, master cylinder bore sizing, brake pedal ratios, front disc caliper bore diameters and all of that.
I've found that once you step out and away from a factory arrangement, your results can vary a LOT.
To recap....
Before I pulled the engine, trans and K member, I was using a stock 1975 Dart power booster and 15/16" master cylinder and the brake pedal pushrod was bolted directly to the pedal arm with none of the factory style power brake linkage that B body power brake cars used.
The front brakes are the Cordoba 11 3/4" rotors and 2.75" calipers with Carbon Metallic pads. The rear is the 10.7" Dr Diff disc brake setup with a 1 1/2" caliper. The brakes are okay but not spectacular. My '72 Duster has the 73-76 A body 11" front discs but with the bigger 2.75" calipers and 10" drums in the rear. 15/16" non power master cylinder. That car flat out S T O P S !! My "Jigsaw" Charger has 11 3/4" rotors, 2.75" calipers, 10" rear drums and a non power 15/16" master cylinder. That car stops great too.
One thing that I am unclear of is the bias that is preferred in terms of front to rear caliper bore sizing. If I have a 2.75 front caliper bore, what is the optimal caliper bore size for the rear?
Along the same theme, If the rear bore size is too big, what size do I need for the front to be?
This Dr Diff Cobra brake package does not list the size of the front caliper piston.

View attachment 1312117
The picture shows 2 pistons though. What if the total surface area of the 2 pistons is smaller than my existing ones? Would that lead to a rear brake bias and make a swap like this an actual step backward?

Dr Diff did have this tech info on his website:

Master Cylinder/Caliper Bore Size​

All else being equal…

• Increasing the master cylinder bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.
• Decreasing the master cylinder bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
• Increasing the caliper bore diameter, yields a longer, softer pedal stroke with increased clamping force at brake pads.
• Decreasing the caliper bore diameter yields a shorter, harder pedal stroke with decreased clamping force at the brake pads.

Hey Kern, I wish I could be more help but on Dr Diff's site there is a comparison chart for the different stages. It says the Cobra calipers are 2 piston 1 1/2 inch bore. I ordered the stage 3 plus the 11.7 rear discs, 15/16 manual master cylinder. I haven't installed them yet but Cass seemed to believe that this will be a good combo.

 

[mgoblue9798]

I am no braking expert, but I think you stumbled on your own answer in your post. The two braking systems you like are rear drums. The one you don't has rear discs.

Point is the rear discs take a lot more fluid capacity and movement than the wheel cylinders on a drum set up.

Ask Cal what master cylinder he recommends for his setup. Also find out if you should still have the proportioning valve installed or not, and if so do you need an adjustable one?

When you do a panic stop in the disc/disc car, which wheels lock up (if any) front or rear?

If you like the braking in the other two cars then why spend the money on the new stuff. Sort what you have and get it working.

 

[Kern Dog]

I've messed around a bit online and at Moparts in their "Cornering" forum, a member there (Feets) had some interesting comments about brake setups.
He plugged some numbers into his setup and reported on them.
The surface area of the 2.75" caliper bore I have is 5.93". This is the area of the piston as defined by Pi R 2, AKA Pi x Radius squared.
the 2.75" caliper bore divided by 2 is 1.375, the radius. 3.14 x 1.375 x 1.375 = 5.93"
Taking the Cobra 2 piston caliper into the mix....
1.5" caliper bores x 2 equals...... 3.14 x .75 x .75 which = 3.53" of surface area. That is less than half of the single 2.75" piston in my iron calipers.
I'm trying to understand how a smaller surface area amounts to better stopping force.
Maybe....the smaller clamping surface area requires less fluid pressure and volume to work?
Maybe with the same pedal movement, the smaller twin piston caliper moves further and clamps harder?

 

[Kern Dog]

I am no braking expert, but I think you stumbled on your own answer in your post. The two braking systems you like are rear drums. The one you don't has rear discs.

Point is the rear discs take a lot more fluid capacity and movement than the wheel cylinders on a drum set up.

Ask Cal what master cylinder he recommends for his setup. Also find out if you should still have the proportioning valve installed or not, and if so do you need an adjustable one?

When you do a panic stop in the disc/disc car, which wheels lock up (if any) front or rear?

If you like the braking in the other two cars then why spend the money on the new stuff. Sort what you have and get it working.

Thank you.

First, I am not putting drum brakes back on this car. People have made 4 wheel disc systems work well and I am sure I will find a way as well. When I do get this right, the 4 wheel discs will eventually outperform a disc-drum setup by a nice margin.
Cass does suggest the 15/16" MC for most applications including his 13" Cobra kit.
Panic stops....I rarely get any lockup and I don't recall if the rears do at all.

 

[mgoblue9798]

Thank you.

First, I am not putting drum brakes back on this car. People have made 4 wheel disc systems work well and I am sure I will find a way as well. When I do get this right, the 4 wheel discs will eventually outperform a disc-drum setup by a nice margin.
Cass does suggest the 15/16" MC for most applications including his 13" Cobra kit.
Panic stops....I rarely get any lockup and I don't recall if the rears do at all.

I wasn't suggesting you go back to drums. What I was doing was pointing out the different fluid volume requirements for drum v/s disc as a good place to start investigating why the new setup wasn't working for you.

Do you still have the factory proportioning valve in the system? Do you have new brake lines run? An old line with a restriction from corrosion or trash could have you chasing your tail.

Another though- what about the brake pedal arm ratio? Power is 4 to 1, manual is 6 to 1

 

[ckessel]

As the gent with the Comet points out the hard way, if you increase the speed of acceleration, you need to do the same with the deceleration. Even on a basic ride, improving upon what it came with is a win win.

 

[Kern Dog]

I wasn't suggesting you go back to drums. What I was doing was pointing out the different fluid volume requirements for drum v/s disc as a good place to start investigating why the new setup wasn't working for you.

Do you still have the factory proportioning valve in the system? Do you have new brake lines run? An old line with a restriction from corrosion or trash could have you chasing your tail.

Another though- what about the brake pedal arm ratio? Power is 4 to 1, manual is 6 to 1

The system worked fairly well but wasn't awe inspiring. I had the Dart booster and disc/drum MC with a 4 wheel drum distribution block that has no proportioning to it. Yeah, a disc/drum master cylinder did work with 4 wheel discs but it wasn't "excellent".

Regarding the pedal ratio...
I have heard a number of responses on pedal ratio from several sources. The numbers seem to vary from what I have read.
I found that mine was 6.6 to 1. This was determined by measuring from the center of the brake pedal to the center of the hinge bolt, then measuring the distance between the hinge bolt and the center of the pushrod. Divide the two. The Dart booster had reduction linkage on the firewall side, the B body cars had it under the dash.
When I still had the automatic, I drilled a new hole above the original one to increase the ratio. That did help. When I converted to this Tremec, I lost that additional leverage since the new pedal had the hole in the stock position.

 

[Kern Dog]

To expand a bit on the last post:

I forgot to mention that at some point, I removed the brake pedal and drilled a new pushrod hole above the stock one.

Going by what I have read, the working length is from the top bolt from which it swings....

To the center of the pedal where the lever is welded.

With that, I measured 12 inches.

The next measurement is again from the center of the top hole to the center of the pushrod hole.
Stock seems to be somewhere around 1 13/16"....call it 1.81.
I modified this one and it measures 1.46 center to center.
Stock pedal ratio......12" divided by 1.81 = 6.62.
Modified pedal ratio...12" divided by 1.46 = 8.21.
Complicating things further is that on the engine side, the booster had reduction linkage so the gains I had on the pedal ratio are not fully realized.

 

[Nxcoupe]

Maybe....the smaller clamping surface area requires less fluid pressure and volume to work?
Maybe with the same pedal movement, the smaller twin piston caliper moves further and clamps harder?

Yes and yes. Smaller piston with same pressure applied will move further. It may make the pedal feel firmer as it requires more effort to apply a smaller piston. Look at a floor jack, 1/2" piston applies a 2" piston. You need a long handle to generate enough force to make it go up.

 

[Kern Dog]

My 64 has the following Dr. Diff brake parts / systems.
Front - stage 2 - 11.75 discs with 2 piston calipers
Rear - 10.7" discs / Cobra calipers
In a conversation with Cass, he recommended that this system may need to be boosted either by a brake booster or by a hydro boost system. Since my engine has a "large" cam with not a lot of vacuum, hydro boost seems the best option for me.

I'm still wrestling with my current system.

Through emails, Cass told me that this 13" front disc kit works well with the 11.7 kit he offers. He also added that if the pedal effort was too high, he is working on a hydroboost unit that uses a master cylinder with a 1 1/8" bore spec'd for a 1985 Dodge D-150 truck. Those master cylinders are around $30 from Rock Auto....that is C H E A P !!

 

[70ChargerRT]

Through emails, Cass told me that this 13" front disc kit works well with the 11.7 kit he offers. He also added that if the pedal effort was too high, he is working on a hydroboost unit that uses a master cylinder with a 1 1/8" bore spec'd for a 1985 Dodge D-150 truck. Those master cylinders are around $30 from Rock Auto....that is C H E A P !!

Cass was wanting me to test his Hydroboost for him but I have my car tore back apart again and it’ll be a few months before I get it back to together. I do only have 7-8” of vacuum but my brakes still work pretty good but will get hard with excessive use. So I added a silent electric vacuum pump and it works awesome. I tied my vacuum coming from my intake in to a T with my vacuum pump going in to my booster and added a check valve in it also. This way if my vacuum pump fails I still have brakes from my intake or if my motor fails I still have brakes from my electric vacuum pump. Do you have this option of having brakes no matter what if you had a hydroboost or engine failure when running hydroboost?

 

[Kern Dog]

From what I recall, you get a few pumps after shutoff like with vacuum assisted brakes.

 

[Nxcoupe]

From what I recall, you get a few pumps after shutoff like with vacuum assisted brakes.

Up to three.

 

[70ChargerRT]

Up to three.

And then it’s, Jesus please forgive me for…………

 

[Nxcoupe]

And then it’s, Jesus please forgive me for…………

Took me a minute. Lol

 

[70ChargerRT]

Took me a minute. Lol

Lol