C7 Stingray Brake Analysis...are they track worthy?

10.31.2013

Our Stingray Z51 arrived about 10 days ago, and we've been busy digging into it. While the OEM brakes on both the base and Z51 appear to be better than what we've seen on Corvettes of the past, there's still a lot of room for improvement, particularly if one plans to track their car. Fortunately, we have a number of solutions already underway.:D

In this thread I'm going to post an analysis of the OEM brakes, specifically looking at changes in components vs. the C5 and C6 Corvettes. My analysis will include dimensions and weights on the calipers, discs, pads, etc., as well as my opinion on how I expect the OEM brakes to perform when pushed to the limit.


A Look Back: C5 & Base C6/Z51 OEM Brake Systems

If you owned and tracked a C5 or base C6, you probably view the front OEM brake system as a weakness of the car. They were more than adequate for street use, but didn't really live up to the performance potential of the platform overall, particularly on the Z06, or after the car was modified. While that isn't an uncommon situation with factory brakes, there simply wasn't much 'headroom' built into the system. The front calipers were heavy, two piston sliders with aluminum pistons and dust boots. They didn't respond well to heavy track use, and tended to fatigue and 'clamshell' (lose their stiffness and spread open like a clam) after repeated trips to several hundred degrees F. The pad shape these calipers used (shown below) had a reasonably adequate volume, surface area, and thickness, but the shortcomings of the overall system took their toll on pad longevity and effectiveness.

On all variants of the C5 and the base C6, a conventional one-piece 325x32mm, 19 lb. front disc was used. While on paper these front discs were large enough in mass to handle a significant amount of heat, their simplistic, cost-driven design prevented them from adequately doing the job on the track. The vane design on these discs was a pillar vane, which can be thought of as a group of posts or pillars connecting the two disc halves together. The pillars are not organized linearly from the outside to the inside of the disc, and turbulence is created as air flows through and among them. Pillar vane discs are therefore not particularly well suited to heat evacuation. The goals of a pillar vane design are disc face stability for low nvh (noise, vibration, harshness), and a low cost of production. The discs are non-directional, and the same part number is used on both sides of the car. For reference, below is a picture of an OEM pillar vane disc stacked with an AP Racing competition disc. You can see the considerable difference in air gap, and the number, shape, and direction of the vanes.

For the C6 Z51, the front disc size was increased to 340x32mm, weighing in at roughly 22 lbs. The added mass of the Z51 front disc provided a larger heat sink, but the same design deficiencies of the smaller disc were still present. It therefore wasn't much more efficient at removing heat from the system when being flogged.

Some of the common results of running the C5/Base C6 system hard on the track were severely tapered brake pads, pads quickly burning up, pad fade (firm pedal, but car won't slow), fluid fade (soft brake pedal due to boiled fluid), cooked dust boots, and quickly cracked discs. All of these problems are directly related to the overall brake system having inadequate heat capacity and cooling for heavy track use. When the OEM brake system was retained and tracked/raced, the system required considerable maintenance, and owners wasted a significant amount of time and money bleeding brakes, replacing pads and discs, and losing valuable track time.

Owners tried to work through these issues via several means: installing stainless steel pistons in the OEM calipers, stainless steel brake lines, high temperature race pads and brake fluid, and aftermarket brake discs. Ultimately though, most serious track enthusiasts were far better served by replacing the entire front brake system with an aftermarket kit designed specifically to handle the rigors of track use. Our Essex Designed AP Racing Competition Brake Kits have also been a popular choice for all of the reasons outlined above.

On the rear of the C5/Base C6, a simple one piston slider caliper and 305x26mm, 14.2 lb. discs were employed. Given their layout, modern Corvettes are not particularly hard on rear brakes, and many people found the rears adequate. These discs were on the small side for a car of this potential however, and again became more problematic as the car was modified and run on the track. A substantial number of owners did run into heat-related issues. Combined with the diminutive discs, the pads on the rear were quite small in terms of overall volume and surface area (see below), which can lead to heat dissipation issues. Also of note, drivers who left Competition Mode engaged ran into rear brake overheating from time-to-time, as the car tried to correct yaw by engaging the inner rear wheel brakes.

For the Z51 rear brakes, the caliper remained the same, but the rear disc diameter was increased to a more adequate 330x26mm, and 20 lbs., alleviating some of the heat issues. For those whose problems weren't solved, there are a number of options available, including a complete Essex Designed rear competition brake system we'll be releasing shortly.

And for a final note to those who care about aesthetics, the C5 & Base C6 brakes were just plain ugly!

C6Z06- An upgraded brake system?

Everyone knows that the C6 Z06 is an absolute monster on track. The LS7 and improved chassis turned the platform into a bullet, and it's not uncommon for the C6 Z to dominate the time sheets on any given day. To match the prodigious output of the LS7, GM 'upgraded' the brakes to a completely new system. While the general design direction of the new brake system was correct, the implementation was not, resulting again in an inadequate system for a car with such an extreme performance envelope.

The front and rear calipers were fixed six and four piston units by PBR, a step in the proper direction. The goal was to provide a firmer pedal feel, more total pad volume, and more leading pad edges to bite into the disc. The final product didn't work out quite as planned however. Despite looking the business, the front and rear calipers were heavy, not terribly stiff, and overly complex featuring the now dreaded brake 'padlets,' which provided inadequate volume for heat absorption (most owners run a one-piece pad when available).:smash:

While the calipers presented a list of problems, the discs were an even bigger problem. At 355x32mm, the front disc size was an excellent choice. It's been proven over and over since the C6 Z's release that a quality, optimized, floating 2-piece racing 355x32mm disc is a perfect choice for even the fastest cars and drivers. A one-piece, drilled disc that has vanes pointing in the wrong direction isn't such a great choice however, but that's what came on the car from the factory. If you drive your car on the track, 'just say no' to drilled discs! They will crack more readily than a slotted disc, regardless of whether or not the holes were cast into the disc (a popular myth propagated by Porsche road car owners worldwide, but refuted by just about every winning racecar in the world). Ultimately, someone in marketing at GM won the battle for the drilled discs because they looked really cool. Even more egregious than the drilled holes however, was the fact that the same directional disc was used on both front corners of the car. In other words, the directional vanes were actually spinning in the wrong direction on one side of the car!

Many C5 and Base C6 owners 'upgraded' to the C6 Z system, but I personally would have stuck with an aftermarket system, or the C6 Z51 setup. Back in 2006 I posted along those lines on the Corvette Forum you can see the original post here. I won't say, "I told you so"...actually, yes I will!

Enter the 2014 Stingray

So did GM take a step in the right direction with the brakes on the new Stingray? Yes...and no.

Calipers

The factory Brembo calipers used on both the base and Z51Stingray are a step in the right direction. They are four piston front and rear. The base and Z51 calipers use the same exact pad shape, but we're uncertain at this time if the calipers use the same piston sizes. They almost certainly do not (I'm assuming the master cylinder is the same on both cars)

These calipers will certainly be stiffer than the C5/Base C6 calipers, and likely stiffer than the C6 Z06's PBR's. Stiffer calipers allow for less pad taper, and superior pedal feel. The pad shape is a better choice as well. The shape is a D1001 FMSI, and is identical to what has been used in many OEM Brembo's on other performance platforms such as the Mitsubishi Lancer Evo, Subaru WRX STI, Cadillac CTS-V, 350Z Track, Camaro SS, Acura TL Type S, Hundai Genesis Coupe, etc. The pad has a good deal of radial depth and surface area. It's also 16mm thick, and has an acceptable overall volume for heat absorption.
There are a few downsides to these calipers however. The front calipers in particular are extremely heavy. Each front caliper weighs 9.8lbs. They also have dust boots, which are great for the street, but turn into a burnt mess when tracked hard. The painted finish will be fine on the street, but again, when these calipers are hit hard on track, expect to see some lovely shades of brown and grey, flaking paint, etc. One of the biggest downsides to the calipers on our Z51 is that they have square piston bores. When brakes are applied, the leading edge of the brake pad is pulled into the disc by its friction against the disc face. At the same time, the trailing edge of the pad lifts away from the disc surface. Pad material from the front edge of the pad is scraped off and travels to the back of the pad, and the back edge of the pad rides on top of that layer of material. To combat this phenomena, aftermarket and racing brake calipers employ a smaller leading piston and a larger trailing piston to 'even out' the forces on the back of the brake pad. Unfortunately, this strategy was not applied to the C7, and each front caliper has four pistons of the exact same size. It's likely that we will see some pad taper issues. The rear caliper will likely be more than adequate on these cars. At roughly 5 lbs. each, they aren't terribly heavy. The rear pad shape is a new one, and it has a decent surface area and volume that I expect will be adequate.

To give an idea of caliper size, here are the front Z51 four piston calipers next to AP Racing's six piston CP5060. The OEM caliper weighs 9.8 lbs., vs. 6.5 lbs...over 3 lbs. difference per caliper!

Here are the rear Z51 calipers next to the CP5040. They're actually dimensionally similar, and both weigh in the 5 lbs. range.

Discs

The discs on the Stingray are certainly interesting, and a definite departure from what we've seen on previous Corvettes.

Base Model Discs


The base model has front discs sized 320x30mm, and 339x26mm in the rear. There is absolutely nothing remarkable about these discs. They are an ordinary one-piece, cast iron, pillar vane disc.
Vane design- The base model vane design is similar to the C5 and C6 pillar vane designs. They simply aren't optimized for high airflow, and won't do a very good job at removing heat from the disc.
Air gap- The air gap on the OEM discs is tiny. As you can see in the pics below, the air gap is partially obscured by the disc hat, which is going to severely limit airflow.

Face grooves
There are no drill holes or slots in the disc face of the base model. Slotted would be better for improved pad bite, but plain face is far better for crack resistance vs. drilled.

Weight/thermal mass
The fronts weigh in at 20 lbs., and the rears are 19.4 lbs. You heard it here first: The front discs on the base model are not adequate for anything other than street or autoX use. I guarantee that with a good driver, on the right track, with sticky tires, they'll be lucky to last a couple of laps before severe brake fade sets in (regardless of what brake pads and fluid are used). They are woefully undersized for that type of use. In fact, when the C5 Corvette was introduced in 1997 with 340hp, the front discs were 325x32mm. Now we have a car that has an extra 100+ hp, wider tires, and vastly different performance envelope...yet the discs are 5mm smaller in diameter, and 2mm thinner! If C5's and the C6 Base models had brake problems under heavy use, what do you think is going to happen on this car? The rear base model discs weigh 19.4 lbs., a little over a lb. more than the Z51 rear discs, which have the same dimensions.

Z51 Discs

The Z51 front discs are 345x30mm, and have the same rear disc dimensions as the base model at 339x26mm.

Vane design
The front discs are a pillar vane, just like the base model. The rears are actually a straight vane design, which is odd given that the fronts are not!

Below is the front base C7 disc on top, followed by the Z51 front disc, with an AP Racing heavy duty J Hook racing disc on bottom supporting the stack.

The base model rear disc is on top in this pic, with the Z51 rear below it. Notice the rear Z51 disc has straight vanes...no idea why they didn't do that on the others!

Below are the Base model, Z51, and AP Racing J Hook disc air gaps. This should give you a good idea of how limited airflow will be on the OEM discs.

Dual casting
The Z51 discs are a dual-cast design, which essentially means a separate hat and iron disc ring are formed as two separate pieces, and then cast together. The goal with these discs is to provide some of the benefits of a two-piece floating/racing disc, while still being able to resist the road salt/corrosion issues aluminum disc hats can have. Other performance cars have used similar designs over the years, with the BMW M3 being the most common one that comes to mind. The amount of float in these discs will be more limited than a true racing design, and as you can see in the pics, the 'arms' used to attach the hat to the disc partially obscure airflow into some of the vanes. The dual-cast design also weighs more than then typical two-piece aftermarket discs we're used to seeing, which I'll address in more detail below.



Slotted instead of drilled
GM made the right choice by going with slotted discs instead of drilled. They will be less prone to cracking than a drilled disc. That said, the slot pattern is not optimized, and it is non-directional. As with the C6 Z06, the same exact disc is used on both sides of the car, leaving some performance on the table.

Weight/thermal mass
The Z51 discs are definitely superior to the base model fronts. The separate hat keeps weight down a bit, and the dual-cast design will give them a little bit of float. However, these discs are still several pounds heavier than a billet aluminum hat on a racing disc. The front Z51discs weigh 20.2 lbs., which is essentially the same as the dimensionally smaller base model front discs. For reference on that weight, the larger 355x32mm, 72mm, two-piece AP Racing discs we use in our big brake kits weigh about 17 and a half pounds. There is definitely an opportunity to drop a few pounds per corner on the disc alone. I believe the Z51discs are still on the smaller side for heavy track use. They are only very slightly larger in diameter than C6 Z51 discs, yet they're 2mm thinner. They are considerably smaller than the C6 Z06 discs (355x32mm), which have never proven to be terribly durable on those cars. Since the C7 Z51 is showing performance potential at a level more similar to the C6 Z06, I expect we're going to see some issues when it comes to heavy track use.

The design details and comparison to previous Corvettes above should give you a good idea of what to expect, but here's some further supporting evidence vs. other platforms:

The Lancer Evo VII-IX and '04+ WRX STI use a very similar front caliper to the C7, including an almost identical brake pad. The OEM disc size on those two platforms are 320x32 and 325x30. As with the C7, those cars weigh 3300-3400 lbs. Although they're both a bit heavier on the nose, those cars come with far less power than a C7 (280-320hp vs. 455hp). These cars also tend to max out on tire width somewhere around a 275 width, and have worse aerodynamics than a Stingray. We've seen many lightly modded Evo's and STI's on sticky tires and stock brakes run into problems on the track. Most of those cars are running sub 400hp.

We're already seeing reports of 500hp+ C7's. More power, wider tires, superior aero, faster cornering speeds, higher terminal speeds at the end of straights...these cars are not going to be easy on brakes by any stretch of the imagination.

The front Z51 units will be better than the base model brakes, but I'm not confident that they will be adequate for many heavy users. A highly optimized 345x30 racing disc could potentially do the trick, but the OEM discs have too many limitations inherent in their design to provide the type of performance I believe the most hardcore users will be happy with.

In the rear, I believe the OEM discs are the correct ballpark size for the platform, even under heavy use. There will be gains to be had on the rear discs in terms of weight and airflow, but they do seem to be an appropriate size.

Below from left to right...front C7 base model disc, front C7 Z51 disc, AP Racing 355x32mm heavy duty J Hook racing disc

Other features

Factory brake ducts
The leading portion of the front Z51 bumper brake ducts look highly functional, as do the brake ducts on the rear, which draw air from under the car. The trick however, particularly up front, is to properly channel the air from the termination point of the bumper duct into the center of the disc. To accomplish this, there is a series of plastic channels bolted to the front and rear uprights. While these channels appear to be a step in the correct direction, there is a large amount of air lost as it moves through those channels. Ultimately, the air is not being force fed into the center of the disc, which is what you want in an optimized setup. In other words, a lot of air is spilling/spraying off or out of those channels as it travels towards the disc.

Z51 Disc Cooling Rings
I have to comment on these little guys. I believe these are an afterthought. My theory is that late in testing the team was having front brake fade issues, and scrambled to come up with a solution...this is a result. From what I can tell, the idea is that these little aluminum rings will redirect air that would normally flow out of the discs, into the cooling vanes of the disc, reducing disc temperatures. Unfortunately, the pillar vanes won't flow or draw much air through them, even if air is force-fed directly into them. A directional vane design would have been a far better solution. Also, when you duct air to a disc, you want that air to ideally cool both sides of the disc equally. In this case, the inner disc face will receive more cooling air than the outer face. An imbalance of that sort can lead to deformation and cracking.

Conclusion and recommendations

If you plan to only ever drive your C7 on the street, you won't have any problems with the base or Z51 brakes. Most people would like the improved feel and heat capacity that upgraded pads and stainless steel brake lines would add to their car, but those upgrades are not a necessity for cruising or grocery chasing.

If you plan to track your car however, I'd recommend one of two things:

A) Buy a base model with the expectation that you're going to be buying a complete front big brake kit (we have something just like our C5/C6 kit in the works for the C7). Then add a better rear disc once they become available.

B) Buy a Z51 and plan to either upgrade the front discs to something that flows more air and has superior metallurgy, more vanes, etc., or buy a complete front big brake kit. The components of a BBK will have greater longevity, long-term running costs will be lower, and the performance gains will be significant. Throw the stock setup back on the car when you sell it, and it will look like brand new. In the meantime you'll save yourself a lot of headaches and wasted time and money.

Okay...I'll throw a third alternative C) in as well, which is to ignore the past, ignore physics, flog the OEM brakes on the track, see what happens, then talk to us later. I'm just sayin'... ;)

We're snapping some more pics of the bits and pieces and I'll post those up as I have them. We're off to SEMA next week so I won't be following up much here until we return. Thanks, and hopefully that is of use to some of you who are planning to track your new Stingray.

2014 Stingray OEM Brake Specs

Base C7
Front Caliper
Piston boresNA
Weight (unloaded)9.8 lbs.
Pad Shape- FMSI D1001FCP1334


Front Disc
Dimensions320x30mm
Weight20 lbs.


Rear Caliper
Piston bores4x30
Weight (unloaded)5 lbs.
Pad ShapeFRP3137


Rear Disc
Dimensions339x26mm
Weight19.4 lbs.


Z51
Front Caliper
Piston bores4x42mm
Weight (unloaded)9.8 lbs.
Pad Shape- FMSI D1001FCP1334


Front Dual-cast Disc
Dimensions345x30mm
Weight20.2 lbs.


Rear Caliper
Piston bores4x30
Weight (unloaded)5 lbs.
Pad ShapeFRP3137


Rear Dual-cast Disc
Dimensions339x26mm
Weight18.2 lbs.

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