Bearcatracing Posted November 29, 2009 Share Posted November 29, 2009 To keep brake bias correct I was looking at installing 1 7/8" piston four piston calipers on a new rear end, and installing 2 1/8" four pistons on the front. (I already have the parts) I'm no brake expert, but I've heard you can't run the same calipers front and rear due to bias issues even with a bias valve. Which I do have by the drivers seat. Does anyone know if this type 4 wheel disc set up would work properly with the larger pistons up front? These are OEM 1968 GM 4 Piston Calipers on a 1968 Firebird. Also, with 4 wheel disc, and calipers that big, what bore Master Cylinder might I need? Or use a twin MC Tilton type system? The car weighs 3380 with fuel and driver. Quote Link to comment Share on other sites More sharing options...
Lucid Moments Posted November 30, 2009 Share Posted November 30, 2009 The math to calculate that is more complicated than I want to go into tonight after several beers. But with a proportioning valve I really don't see why wouldn't be able to. I have never worked with a car that has a drum rear setup. Sorry I'm a modern car guy. Do you have any idea what size the stock rear piston is on the drums? Give us that info. We have the stock front piston size and the proposed front and rear piston size. With all of that information it shouldn't be too hard to calculate the differences and see if it should cause you any problems. IIRC typically rear drum setups use a smaller master cylinder, so if you go to a rear disc setup you may need to upgrade the mc, but get the piston dimensions and you should be able to get a better idea. Okay, I'm sitting here thinking about it. We also really need the size of the OEM drum pads, the OEM coefficient of friction, the proposed Cf. Well you see the issue. There are so many variables. The trick is that you can manage all of them to make the system balance. I realize that pad compound availability is an issue for you, but that is an easy way to work on your brake bias. If you have too much rear bias ( and rear brake temps aren't an issue) then go with a less aggressive rear brake pad. Thats leaving out entirely the Quote Link to comment Share on other sites More sharing options...
Bearcatracing Posted November 30, 2009 Author Share Posted November 30, 2009 I did a bunch of digging around for info on this, minus the beers but I think I need one now. It looks like I need the 69 Corvette manual Disc MC that has a 1" bore, larger volumn and longer stroke if I go to 4 wheel 4 piston disc. Different pad compounds might be a good option as well. Hawk has several options for me, they just don't stock much for it. I could really use a different MC for what I have, right now it's a 7/8 bore and equal volumn MC, basically a drum-drum MC, the correct I think is 1" bore and unequal volumn for manual disc-drum, but they are extremely hard to come by. But with the 1 7/8 pistons on it now it should be ok. With the 2 1/8 pistons I don't think I'd have enough volumn. It still may take a pedal pump before braking and brake check it, luckily Road Atlanta isn't that tough on brakes, at CMP I'd be more than a little concerned. Get RA behind me and I have a couple of months before CMP. Ahh, the joys of working on and parts swapping on a 40+ year old car using rare OEM parts. Quote Link to comment Share on other sites More sharing options...
cagedruss Posted December 9, 2009 Share Posted December 9, 2009 I have a good set up on our Chump car. I used the same spindles you have. I swapped them out for the originals our 81 came with. Also had to swapp out the upper control to work with the shorter spindle which you won't have too. I got a caliper bracket kit from Speedtech dealer here in town. It allows the use of a Corvette C4 rotor and a C5 caliper on a factory Drum brake spindle and hub. You will have to turn the OD of the hub down though to allow the rotor to slip on. I also used a master cylinder from a 74 Corvette non boosted. It has a larger piston and volume and bolts directly to the firewall. Need to add an external proportioning valve for the rear brakes. To make it easy I used caliper from a Metric Frame Chev Malibu ($9.00 rebuilt, $10 core) and made a simple bracket and bolted to the rear end axle flange. Rotors front and rear are around $400, master cyl was $99, brackets $140, proportioning valve $45. Brakes were awesome for our 24 hour race in Portland last month. Wear was negligible. Only draw back is I had to run a 17" wheel which worked out because tires were cheaper. If you are staying with 14"-15" you will have to get creative. Quote Link to comment Share on other sites More sharing options...
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