Curious. C4 brakes adapted for late 2nd gen T/A?

[81Nascar]

With some of the talk about this with first gens I wonder how possible it would be? Ive heard talk about them fitting in 15" wheels front and rear. I 'm working with my 1981 WS6 Turbo Trans Am with stock turbo 15" wheels and factory discs. Thought about upgrading without spending for the Baer price. Wondering if there is a bracket out there for the spindles and I am aware of the B body spindle swap. I'm just looking at all my options or thoughts.

[Skip Fix]

I slid a 2004 GTO caliper(similar to the Baer/Vette PBR style) on the 12" 1LE rotor in a 15" snowflake and it fit easily(better offset on the rotor for wheels). I don't think anyone makes a bracket.

[Apogee]

I slid a 2004 GTO caliper(similar to the Baer/Vette PBR style) on the 12" 1LE rotor in a 15" snowflake and it fit easily(better offset on the rotor for wheels). I don't think anyone makes a bracket.

The C4 caliper is even smaller than the '04 GTO caliper, which more closely resembles a 98-02 LS1 F-body caliper, so that should only help with respect to wheel fitment. Plus, if you opted to run the 12" 1LE rotor, the thickness would match the design specification for the caliper at 28mm [1.1"], all you'd need to do is come up with a bracket.

Tobin
KORE3

[Skip Fix]

So Tobin what about a bracket??? I'd think there are enough guys wanting 15" wheels there would be some demand. Doesn't Baer make a 12" kit?

The Vette abutments swap on the GTO calipers to be able to use the current kits?

[81Nascar]

So a C4 caliper, 1LE rotor, and a braket and I'm good to go? Now I've heard of TWO different C4 calipers and they had different rotor sizes? I would assume I could adapt the C4 rear brakes as well to keep eveything balanced even though I already have factory disc brakes. I'm not sure what's different on the rearend. On another note I did notice some use the Wilwood GM III caliper for the 1LE upgrade. I'm not sure on wheel fitament, but I wonder how that design compares to the C4 technology?

[Apogee]

So Tobin what about a bracket??? I'd think there are enough guys wanting 15" wheels there would be some demand. Doesn't Baer make a 12" kit?

The Vette abutments swap on the GTO calipers to be able to use the current kits?

It's quite possible that the GTO calipers have the same pin spacing as the C5/C6/LS1 calipers, so swapping the caliper bodies onto C5/C6 PABs may be an option...but I'd want to confirm a few dimensions before committing to it.

There is some demand to be sure and I do believe that Baer does already make a "Sport" kit utilizing smaller diameter rotors and the C4 calipers for several applications. That said, we're plenty busy developing new C5/C6 applications such that the thought of jumping into the C4 arena seems like a step in the wrong direction...I'd rather be working towards the C6 ZR1 than the C4 ZR1 if you know what I mean.

We did co-develop a kit with a company called WX3 Performance based around the 91-96 B-body spindle (late-model GMTS) that utilizes the OE wide-body, single piston, cast iron calipers and the KORE3 GMTS HD hubs for the 9C1 police car fanatics that wanted to keep their 15" wheels. The beauty of that kit is that it uses a Ø11.88" x 1.25" thick 5000 Series DBA rotor with a custom aluminum hat and despite the increase in rotor mass at the swept surface, it actually weighs several pounds less than the OE setup.

The idea to offer this in a 5x4.75" configuration is currently being thrown around as we do understand that not everybody wants to run 17" or larger wheels. Depending on the level of demand, it wouldn't take much to leverage our existing designs into a solid 5x4.75" performance kit that would fit inside most 15" wheels. I believe the Wilwood GMIII caliper should be an option as well since it's a direct replacement for the wide-body OE B-body calipers.

Tobin
KORE3

[Skip Fix]

I'd think even using a C5 caliper with the 1LE rotor would be an easy kit to do and sell.

[Apogee]

I'd think even using a C5 caliper with the 1LE rotor would be an easy kit to do and sell.

That's a valid thought, but I think the issue there is that the C5 caliper on a 12" rotor definitely won't fit inside a 15" wheel...16's are even pushing it with that caliper, although an LS1 could fit inside a 16" wheel and have the larger 43mm pistons, offsetting some of the brake torque lost due to rotor diameter. Ruling out the 15" wheel crowd doesn't seem like such a great idea considering that was more or less the standard wheel size for several decades.

Tobin
KORE3

[81Nascar]

I'd think even using a C5 caliper with the 1LE rotor would be an easy kit to do and sell.

I was thinking the same thing too. Seems the C5 stuff is much more available and EVERYBODY is working with it. I just didn't know if they would fit. Tobin do you think the smaller C4 piston size would be a good match with my stock master cylinder since it is for discs in the front and back?

[Apogee]

The C4 calipers have dinky twin-38mm [1.5"] pistons and as such will require a smaller bore than an equivalent C5/C6 setup with twin-40.5mm [1.59"] pistons to achieve the same caliper pressures. IIRC, the early C4 Corvettes with 12" front rotors used a 13/16" bore master cylinder on either a 9" or 10" vacuum booster.

Your stock '81 WS6 master cylinder should have one of the early cast iron quick take-up master cylinders with a 1-9/16" primary bore and a 1-1/8" secondary bore...I don't think it would generate enough pressure barely stop the car, let alone inspire confidence in the brakes. Even with the dual 9" vac booster, it would be sketchy at best. Keep in mind that the OE WS6 calipers have 2-15/16" [74.6mm] pistons up front and 2-1/2" [63.5mm] pistons in the rear whereas the front C4 calipers have a piston area equivalent to a single piston caliper diameter of 2-1/8" [54mm] and the rear C4 caliper is ~1-3/4 [45mm], so significantly smaller.

Tobin
KORE3

[81Nascar]

The C4 calipers have dinky twin-38mm [1.5"] pistons and as such will require a smaller bore than an equivalent C5/C6 setup with twin-40.5mm [1.59"] pistons to achieve the same caliper pressures. IIRC, the early C4 Corvettes with 12" front rotors used a 13/16" bore master cylinder on either a 9" or 10" vacuum booster.

Your stock '81 WS6 master cylinder should have one of the early cast iron quick take-up master cylinders with a 1-9/16" primary bore and a 1-1/8" secondary bore...I don't think it would generate enough pressure barely stop the car, let alone inspire confidence in the brakes. Even with the dual 9" vac booster, it would be sketchy at best. Keep in mind that the OE WS6 calipers have 2-15/16" [74.6mm] pistons up front and 2-1/2" [63.5mm] pistons in the rear whereas the front C4 calipers have a piston area equivalent to a single piston caliper diameter of 2-1/8" [54mm] and the rear C4 caliper is ~1-3/4 [45mm], so significantly smaller.

Tobin
KORE3

Ok from what I gather my master cylinder is a plastic/aluminum piece like the third gen F body's , but I will admit that I don't know the specs on it. so from your post my current MC would be worthless if I went the C4 route. Are the C4HD's any bigger? Wonder if the would work with a 12" rotor?

[Skip Fix]

Tobin 79-80s had a cast iron non stepped 1 1/8". 81s had the aluminum for the quick take up when they went to the "low drag" big front calipers. Just spinning my 78 and my 81 you feel a difference.

The pedals also have less ratio than the non 4WD cars so if a little more leverage is needed you can pick up a "regular" brake pedal.

I'm guessing the new design(smaller pistons) needs higher pressure to achieve the same brake torque on the rotor. Why I always questioned Baers ads that their smaller piston sq in calipers had to improve the braking system since they often ignored the MC in the equation.

I'll measure the GTO calipers but I think they have the same pin width as the Vettes.

[Skip Fix]

Here are specs I had posted awhile back comparing them ans some wheels. Classic has started repoing the GTA 16" wheels again they are light!

"FYI

1LE rotor out of the GM box 11 13/16 diameter, 1.025 thick, wheel to disc offset 1 13/16"

79-81 TA rear disc 11 1/8 diameter, 0.982 thick(new non GM),1 3/16 offset

04 GTO used rotor hat type front 11 9/16, 1.082 thick, 2 1/16 offset

04 GTO rear 11 1/4 diameter 0.637 solid disc, 2 1/2offset

04 GTO caliper brackets both have 3" mounting centers. Front caliper 8" center, rear 5 11/16 center

My 78 8" snowflakes are 12 7/8 diameter at the hub level and 14 1/4 at the rotor level with the 1LE. Even has an extra 1/2 " diameter clearance with the GTO calipers on the 1LE rotor.

A 89 GTA mesh is 13 7/8 at the hub and 15 1/2 the outer area. Rear wheel has a 5 5/16 back space."

[81Nascar]

Here are specs I had posted awhile back comparing them ans some wheels. Classic has started repoing the GTA 16" wheels again they are light!

"FYI

1LE rotor out of the GM box 11 13/16 diameter, 1.025 thick, wheel to disc offset 1 13/16"

79-81 TA rear disc 11 1/8 diameter, 0.982 thick(new non GM),1 3/16 offset

04 GTO used rotor hat type front 11 9/16, 1.082 thick, 2 1/16 offset

04 GTO rear 11 1/4 diameter 0.637 solid disc, 2 1/2offset

04 GTO caliper brackets both have 3" mounting centers. Front caliper 8" center, rear 5 11/16 center

My 78 8" snowflakes are 12 7/8 diameter at the hub level and 14 1/4 at the rotor level with the 1LE. Even has an extra 1/2 " diameter clearance with the GTO calipers on the 1LE rotor.

A 89 GTA mesh is 13 7/8 at the hub and 15 1/2 the outer area. Rear wheel has a 5 5/16 back space."

Hmmm. So are you thinking that the GTO calipers would work with a 1LE rotor?

[ebmiller88]

One thing to consider is that a 12" C4 rotor is .81" thick, not 1.1" as are 1LE rotors. On the other hand, the larger J55 C4 HD rotors are 13" x 1.1" thick and only work with the C4 J55 caliper. Pairing a C5 caliper with a 12" C4 rotor would not be a good idea, the C5 caliper is for a 1.25" thick rotor.

That said I have installed the larger 13" C4 brakes to a 2nd gen without issue but the pics are at home. They will fit inside 16" wheels. The 12"s will fit most if not all 15" and larger wheels but may be a bit harder to adapt to the spindle since the abutment holes are moved inward a bit but slight clearancing of the spindle should do the trick. I ran the 12" C4s on my RS for a while before I upgraded to J55s and the 12"s are a good setup for spirited street cars, very good performance from a smaller kit.

Ed

[Skip Fix]

Ed (or Tobin) have any dimensions for the offset on the C4 rotors? I think the offset on the rotos unless you use a hub and a offset hat is what kills the clerance for alot of wheels.

I think the GTO and the Vette are pretty close dimensions. More Vettes around-although alot of '04 GTO guys upgraded to '05 calipers and rotors (caliper bracket different to go with larger rotor) so they are aorund fairly cheap.

Ed-C5 caliper and a 1.25 rotor? or 1LE thickeness?

So any reason to use the twin pistons PBR style over the big single factory other than looks and maybe weight(assuming not using a Wilwood aluminum single piston)??

[ebmiller88]

Not off the top of my head, I'd have to measure. I used the C5 caliper with the 1.1" C4 13" rotor, it works well. As for preference, the dual pistons will give you better clamping force over the single piston as I understand it hydraulically (force over area). Please correct me if I'm wrong. And the larger rotors give you better leverage over the smaller OD stock rotors.

Ed

[Apogee]

The 12" and 13" C4 rotors both have the same overall heights at 2.205" with a .300" hat thickness. The smaller 12" rotor therefore should have less caliper overhang due to the fact that the rotor is only 20mm thick versus 28mm for the 13" HD rotor. These definitely allow for more options with respect to wheels when compared to C5/C6 rotors which are much flatter (1.790") and thicker (32mm).

Since most 2-piston PBR calipers are have less combined piston area than most of the GM single piston calipers that come to mind, they actually create less brake torque given the same pressures, hence the use of smaller master cylinder bores to make up the difference. The 2-piston design however allows for better brake pad support and more uniform contact pressure between the rotor and the pads. It's quite common to back-bend the pads over the pistons in a single piston application when you push them hard for extended periods of time.

2-piston calipers inherently tend to be wider across the face, which yields a more rigid design since there is more material in the bridge. This tends to be more critical with aluminum calipers where the material is less rigid than steel/iron. Also, smaller piston diameters allow for tighter packaging between the wheel hub, rotor hat, and wheel hoop as well while maximizing pad size given the swept area of the rotor.

Tobin
KORE3

[Skip Fix]

Tobin thanks for explanation on caliper difference in design.