What about the GNIII 6 piston caliper?

 

The C3 is actually to rearward biased, the front should lock up just before the rear and in stock form it doesn't. Putting 13" rotors on a otherwise stock c3 with superlites is probably benificial to the brake bias.

Actually...this is not going to work as you'd think. The larger 13" rotors on the front would enhance the front bias to bring it back to a more neutral state but the loss of piston area is a huge negative. If the above statement is correct then it would go to support my comments on a poor choice of 13"BSL6 front and D8 Rear as it would push the bias to the rear even more.

*Probably on par with the 1.75 parts spoken of and the use of the prop valve for this same reason.

**Swap the 1.75 DLs for a 1.375 and I think you'd find both bias and response more to your liking.

 

Good caliper. Very old design. Lots of pad options, thick also. Big issue is lots of piston area (nearly stock at 5.2") and takes up a lot of room behind a wheel with the body size. For track use it would be an option I suppose.

 

Agreed, SL6 not a good choice.

Hey! I had them around, might as well use them. I wouldn't recommend them of course! The 1.38" pistons are the right ones.

Due to the way proportioning valves work I actually kinda like the way the rear digs in at first then as you press harder levels off until the fronts lock up.

I have a couple of sets of 1.38 pistons calipers here, I'm just too lazy to go through the whole bleeding hassle again.

 

Sheesh, who'd buy those pieces of junk ;-)

 

Just a brief follow up....

I personally don't feel the BSL6 is a bad choice at all. It's not a ***** out racing caliper as you may find the GNII to be but it has its advantages too.

I do feel the use of the BSL6 kits need to be properly paired with a rear kit for best results is all. Which is why Wilwood offers the rear kit....

 

With stock brakes and even my 315/35/17 rear tires. The rears would lock on hard braking even at 130 mph. It was usually one or the other rear and I would sense it and glance in the mirror and see a blue cloud of smoke and black stripe appear. It was never a big deal.

Now with 13 rotors and my 295/35/17 hoosier slicks the fronts lock first. Again no big deal. You just see a puff of smoke coming from the front wheel wells

 

Great thread! Keep it going. I went ahead and purchased my FSL4s and Rear FDL. Great deal. Im sure they will out perform the stockers! Thanks for all the help!

 

Yes. "H" is an excellent material, provided it has been properly bedded on rotors that have never had other pad compounds run on them. It has a tendency to be a bit fussy otherwise.

 

What would be the best rear caliper to pair with the GNIII in the front if you have a 13.5" front rotor and stock size rear?

 

As Todd stated, the biggest drawback to mixing the caliper styles is the funky look of the mis-matched parts. Style points will certainly be deducted...
But as for the clamping forces and bias changes, here's the skinny.
The stock system is right at 65% front static bias. Changing to a SL6 on the stock rotor size drops the front bias to about 57%, and that's too low for most any type of driving. Using the SL6 with the 13" rotor restores the front bias to just over 62%. That's still a little low, but it's back in a manageable range on a street car with an adjustable prop valve set low. For occassional time at the track with that same car, it will most likely be necessary to also find a good front to rear pad compound split. Pad compound splits can be tricky to manage though, because no two pad compounds change friction (Cf) at the same rate as they build heat and will sometimes produce inconsistent results. It's not impossible to achieve, but pad selection becomes very critical, so I always like to build the static bias into the car with the proper mechanical and hydraulic ratios so I can run the same pad compound on all four corners. Unfortunately, that's not always possible.
As for the differences in piston area and clamping forces between the stock calipers, the D8-4, SL4, and the SL6, the pedal effort required to produce a given amount of clamping force at the caliper with stock, SL4's with 1.875/1.75 pistons, or D8's with a 1" master cylinder is nearly identical to pedal efffort required to produce the same amount of clamping force with the SL6 and a 7/8" master.
(pedal effort) x (pedal ratio) / ( master cylinder bore area in sq.inches) = pressure. (pressure) x ( effective caliper piston area) = clamping force. As stated earlier, it is highly recommended, if not essential, that the master cylinder on a manual brake car be switched to a 7/8" when using the smaller piston area of the SL6 caliper kit.
These combinations will all produce similar results for comfortable street driving and "enthusuiast" level track time with an adjustable prop valve to take away rear brake pressure on hard braking...and probably the use of a stickier, higher temp front pad at the track.
BUT, I have never seen a real track-only car perform consistently well with front bias ratios that low. It is even more critical for a driver who relys heavily on downshifting to decelerate the car. My experience with good drivers in competitive cars has been in the area of 70% front bias, and higher, depending on the car's weight distribution, the driver's style and use of downshifting.
Anytime an attempt is made to set a car up for both street and track, there will be compromises. The bottom line is here that, you need to decide what your primary intention is for your car and set it up accordingly, with the understanding that other driving opportunities may require adjustment or compensation.

 

1 3/8" piston calipers should work well....

 

I'd sure like to drive a car with an SL6 kit, I am very skeptical....

 

With those calipers on those rotors I'd put on some dynalites with 1.75" pistons and a screw type proportioning valve. Mount it right by the seat so you can adjust it from within the car. Then you can make the brake bias perfect. Thats why I run the 1.75" Dyna's. I didn't know if the 1.38 pistons were going to be enough.

And then keep an eye on the cars in the mirror, Objects will not be stopping as fast as you!

wweast, why do you keep talking about SL6's??

 

IF you were to run the GNII up front you'd want a caliper with more piston area than "ideal" for the rear. Only to match it. Most rear kits run about 1.25" pistons or upwards to 1.375. For your GNIII combo you could do 1.625 bores without much problem. (I'd remain a bit skeptical on 1.75 parts regardless of the fronts)

Caliper wise, I'd do the FSL over a DL (now Lug Mount Dyna Pro) mainly for pad area. Piston area being the same effort will be also but the FSL is a much beefier caliper and the added pad materials will be a benefit for both life and heat rejection.

FWIW...all the talk of running the GNIII is for what? You'll probably desire to move to a larger bore mc to keep any reasonable pedal feel in it (especially if paired with the 1.625 rears) and that's only going to lower the operating psi. It's tit for tat here on "work". Just because it has more area doesn't mean it's going to stop better. If you lock up the rotor at 60k lbs of torque it's not going to matter how you got there- big pistons low pressure or smaller pistons higher pressure. (within reasonable values of course, not 4 x 1.00 front or some crap)

 

Post for a test drive. There are hundreds of the in service I'm sure. Granted not all on C2/3 but the parts remain the same, only the mc size may be different. The SL6 stuff will be a firmer feel over the D8 all things being equal.

 

wweast, why do you keep talking about SL6's??[/QUOTE]

The SL6 is what comes in the Wilwood 13 and 14" Big Brake kits...someone else brought them up earlier. The comparisons offered included all of ther calipers being discussed, and their compatibility with master cylinder sizes.

 

The GNIII has 5.37 sq inches of effective piston area ... a little less compared to the 5.52 sq inches of the stock caliper. There is no real need to change rear caliper piston size or master cylinder bore size to use them. The bigger rotor diameter on the front will add back a little extra front bias that is preferred on a hard driving track car.

 

You have your numbers backwards...it's 5.73", but in any case I don't totally agree or disagree---we have not talked about what caliper he'd be running on the rear (without a change) nor the size of either rotor that either caliper would be used upon.

I only make note of it as we're swaying all over the road map on this discussion and possibly making assumptions, or others are, that would need to be factored before I'd agree fully. For a lot of readers the confusion is growing maybe rather than becoming more clear.

For me, the GNIII is just not the ideal caliper for anything but what runs them the most: off road trucks. There are what I feel are better options currently that would be a better choice for both appearance sake and compatibility to larger rotors. I don't see an increase in piston area combined with larger rotor an ideal BBK design. If it was to be done I'd want to check the balance to what I'd plan to convert the rear to before I dove in. Fitting a GNIII to the front on a 13" rotor and running an iron D8 on a stock rear rotor (dia?) seems somewhat pointless. I'd much rather take advantage of the two piece rear offerings for added mass and other reasons.

 

No, the numbers are not backwards... the GNIII is a combination of 1 x 1.75" piston and 2 x 1.375" piston per side..that's 2.404 sq.In + 1.484 sq in + 1.484 sq in = 5.372 sq in.

As for the GNIII being an ideal caliper, I agree that it is not. It is NOT my first choice for anything. It's only attributes are that it carries a big pad and it's comparitively low cost relative to more modern machined billet or forged body calipers. There is far better technology available. There are also some secondary mechanical considerations or questions with trying to use a GNIII on a 13.5" rotor. Two of which include caliper bridge radial capacity and the subsequent pad radius compatibility with a rotor that size, and the match between the pad width and rotor swept face width. And since Wilwood doesn't make a 13.5" rotor, I am sure that compatibility recommendation didn't come from them. Most of the "vintage" Wilwood combinations paired the GNIII caliper with a 12.72" rotor. But James question wasn't asking for a judgement on his choice to use the GNIII. He was looking for a rear caliper recommendation on a stock rear size rotor.

As for the rear caliper selection, my apologies for maybe not being more clear. My recommendation was for stock ( 1.375") piston sizes, and not necessarily to use the stock iron calipers. The only place I would ever truly encourage the use of the stock iron calipers would be on a NCRS car. A forged or billet aluminum caliper with "same as stock size" 1.375 stainless steel pistons would be a far better choice for a rear caliper over the OE iron calipers. But if James did choose to try racing with the stock rear calipers, it would work fine for balance. Not so great for unsprung weight, heat dissipation, or style, but fine for balance. The GNIII fronts with 1.375" SL rears and 1" master cylinders was a mainstay in NASCAR and road course racing two - three decades ago before the better technology started trickling in. That combination worked pretty good for a couple guys named Petty, Earnhardt, Allison, and a few others. But even though technology has changed substantially in caliper design, heat management, and pad compound technology, NASCAR and a large segment of other styles of oval and road course race cars still run relatively the same basic bias balance ratios, including GT1 and Trans-Am. Physics is physics.
The other advantage to staying with 1.375 in the rear is that the pedal heoght won't change. Stepping up to 1.625 or even worse, the 1.75, and then using a prop valve or split pad compound to take rear bis away will have a noticeable increasae in required pedal travel, which is usually an uncomfortable feeling for the driver.