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I noticed a few years ago, that Baer had moved away from offering the C4 Corvette/Cobra style pad-guided PBR, and instead offered a pin-guided caliper similar to the C5 corvette with some of their kits. In searching, I read numerous sources state the pad area of the newer caliper was increased 30%, as well as the caliper being stiffer overall.
SO basically here's how to duplicate the basic baer kits using GM calipers.
As far as I know right now, the calipers that can be used here are C5, C6, XLR and 05-06 GTO calipers. Only the caliper portion can be used. The brake abutments (portion of the caliper that bolts to the spindle) cannot be used. I believe C5 and GTO calipers are the same, but the C6 calipers are slightly stiffer. Haven't confirmed though, and pricing of C6 calipers leaves the cheaper C5 and GTO calipers as a more likely alternative.
The portion that bolts to the spindle is different between the vette and GTO caliper. These abutements are what adapts the caliper to the spindle, and what we need to pay attention to here.
Standard Corvette brakes are 12.75" in diameter. So if you use the standard vette abutment, the caliper won't fit over the cobra 13" rotor. However, GM released a package called the Z51 package, which increased the front brake rotor diameter to 13.4". This will work....sorta.
Standard Base Corvette Caliper abutment (for 12.75" rotors)
Corvette Z51 Caliper abutment (for 13.4" rotors)
The additional offset of the Z51 brackets to get proper rotor spacing.
So what needs modification? Well, GM uses 14mm bolts to mount the calipers, while the Sn95 uses 12mm. This means you'll need to widen the holes in the spindle just a tiny bit to accomodate the larger 14mm bolts. Also, the bolt spacing is off...by 1mm
Z51 abutments usually retail for about $50 each at the local GM dealer. Since i didn't want to modify my spindle, i looked for a more bolt-on OEM solution.
So i went to Baer.
Baer part number CBK332325. Cost is about $130 each (plus shipping). The p/n stamped on the bracket itself is K332-216C1 This is a 100% direct bolt on solution. These are basically Z51 brackets, but drilled for 12mm bolts, and to the proper spacing for SN95.
They come zinc-coated, and you can see they feature the Sn95 style 12mm bolt hole compared to the 14mm GM bolt hole on the GM-specific brackets.
Basically, at this point, it's no different than any Cobra 13" brake swap. Slide on the 13" 94-04 Cobra specific rotor of your choice, then the Baer bracket, then pads, and hardware, and finally the caliper. I used my Sn95 SS brake hoses, but with a Corvette specific banjo bolt.
Piston sizes are 40.5mmx40.5mm, which is practically identical to the 99-04 Cobra/Mach/Bullitt caliper (which is 40x40mm). With rear Sn95 disks, i'm running a 1993 Cobra 1" bore MC, and braking feels great.
Things to pay attention to:
Brake bleeding: The position of the bleeder with respect to the center of the top piston does trap a tiny bi of air at the time. In order to bleed the calipers, i had to upbolt the top bolt on the caliper and rotate it vertically. I was using a gravity bleeder setup, so i did not press on the pedal to extend the pistons. Until i did this, i was having a hard time getting a good pedal
Tape weights. Clearance is tight, so a tire installer needs to position the weights as far forward as possible to clear the caliper. There's plenty of room for the caliper, but the weights get close.
Finally, the cobra 13" spare tire will not clear. Perhaps a spacer would work, but i haven't had a chance to explore that.
LOL I know it burns people but what 99.9% people don't know is the 96-04 Cobra calipers were first used on the early 90's Corvette. Baer also used the same caliper in their kits years back. PBR is the manufacture for most production vehicles and that's who produces these.
In my post reserved above, i'll show the big down side to Cobra PBR calipers. The pads only cover roughly the top 55-60% of the rotor surface. In street form, more than enough to do its job. Its when tracking does this become a problem. The abuse of the braking system leads to extreme heat and with the smaller pad, the less material the heat will absorb to. A larger pad will absorb more heat with less fading. The C5/C6 PBR caliper has a much bigger pad. I have heard from several track guys say that the C5/C6 caliper will out perform the 2000 Cobra R Brembo caliper with equal pads.
Plus the Corvette caliper can take a 1.25" thick rotor vs the Cobra caliper's 1.1". And if people want the caliper to not say Corvette, the GTO caliper is blank along with remanufactured calipers. Im actually trading out my SSBC 4 pot calipers, because the pads are tiny, for the XLR blanks. I'm actually looking into a thicker off the shelf rotor too.
If you had a perfect differential, how would it behave under different driving conditions? To make this simple, lets look at three different situations. 1) Cornering at light throttle, 2) Cornering at high throttle, 3) Accelerating at high throttle.
Case #1. The differential needs to be able to allow the two different axles to turn at different speeds due to the different corner radii that the inner and outer tires are following. If the differential doesn't allow this, the inner tire is going to be forced to rotate slower than it wants to go (braking, reverse slip) and the outer tire is going to be forced to rotate faster than it wants to (acceleration, forward slip.
If the tires have high grip, they are going to protest this somehow. Either by making noise, hopping, breaking the differential or something. If the tires are low grip, they may make noise and will just wear fast. In both cases, the car is not going to want to turn. It will have massive understeer.
So the primary job of a differential is to allow the tires to turn at different speeds.
An open differential does this very well.
With a limited slip differential, there are clutches connecting the left and right axles together. For the axles to turn at different speeds the clutches must slip. The more friction and/or preload there is on the clutches, the greater the applied torque difference between the left and right axles must be before the clutches start slipping. In a corner, the front tires create a lateral force to accelerate the front of the car sideways. This force is resolved at the rear axles through the tire contact patch and becomes this applied torque between the rear axles. The greater the clutch lockup, the greater lateral force must occur at the front tires to overcome the clutch friction. This lateral force is subtracted from the lateral force that the front tires have that can be used to produce lateral acceleration. So the greater the clutch lockup in the differential, the more understeer the car has.
Case #2. The differential needs to still allow a speed difference between the inside and outside tires, but not as much as at low speeds since the rotational slip of both rear tires is going to be much closer to equal to each other than at low speeds. The differential also needs to bias torque. This means that the differential should send torque to each axle in direct proportion to the amount of vertical load on that axle. If the rear of the car has 1,000lbs of total weight on the ground, when under a hard corner, there may be 800lbs on the outside tire and 200lbs on the inside tire. This is because 300lbs of weight has transferred from the inside tire to the outside tire due to the lateral acceleration. In the situation with a perfect differential if there were 100lbs-ft or torque coming down the driveshaft, the differential would send 80lbs-ft to the outside tire and 20lbs-ft to the inside tire. If the driveshaft torque doubles, then the torque to each tire should double also. Under these conditions, as the driveshaft torque increases at some point both rear tires will reach their traction limit at the same time. This does two things. It guarantees that you get the maximum possible forward acceleration out of the pair of tires. And it makes the cars handling balance change minimally as the driveshaft torque is increased.
It is a very complex discussion to explain how well different differentials perform torque biasing and how good each one is at it. I'll only go into this a minimal amount.
In the cornering situation from above, if the differential has a maximum TBR (torque bias ratio) or 2:1, then it will deliver 66lbs-ft of torque to the outside tire and 33lbs-ft of torque to the inside tire. This will result in some amount of understeer, but more importantly as the driveshaft torque level is increased, the inside tire is going to start spinning first because it is going to receive too much torque for the amount of load that is on it. In this new case the ratio of torque to load is 0.0825 (66/800) for the outside tire and 0.165 (33/200) for the inside tire. The inside tire has double the torque for the weight that is on it. Once it starts spinning the cars acceleration barely increases with more driveshaft torque applied.
Inside tire spin is always caused by a differential that has a maximum TBR that is too low for the vertical load difference that is on the drive tires. With a LSD that uses clutches, the maximum TBR can be increased by using clutches with more friction or applying more preload. More friction increases the maximum TBR at all torque inputs, but more preload increases the maximum TBR percantage at low torque inputs. As the torque input goes up, the percentage of maximum TBR increase goes down. Increasing preload with stiffer springs mostly has an effect at low input torques. Don't bother to try to use this to affect handling or behavior at high throttle.
For handling, you want to use a differential with just a high enough TBR for the lateral weight transfer that your car experiences. The higher the cornering force your car can generate, the higher the TBR needed to avoid inside wheel spin. The downside to a high TBR with an LSD differential is that this is achieved through more clutch friction. More clutch friction increases the amount of understeer the car has.
Differentials which have a high TBR and no clutches (Torsen, Quaiffe, TrueTrac, etc) do not have this tradeoff, since they are no clutches to artificially increase the TBR, and at the same time cause more understeer.
Case #3a. This is an LSD in a car with a solid axle. When a rear wheel drive car with a solid axle accelerates, the driveshaft torque is reacted through the rear suspension. This causes the vertical load on the RR tire to be decreased and the load on the LR tire to be increased by the same amount. The RR corner of the car drops closer to the ground because the engine is trying to rotate the chassis in the opposite direction of the driveshaft. This load difference across the rear tires makes the differential behavior exactly the same as in case #2. Optimally the differential needs to bias torque in proportion to the amount of load on each tire. An open differential can't bias torque. It always sends 50% of its torque to each axle. So if the car has an open differential and accelerates even moderately hard, the RR tire spins first because it has too much torque for the amount of load on it.
With an LSD differential that can torque bias, the clutch friction can be adjusted so that for a given lateral load difference between the tires, both tires have the same ratio of torque to load, so they spin at the same time. As the car accelerates harder, there is more lateral load difference between the tires and therefore a higher TBR is needed, which necessitates more clutch friction.
If the rear suspension of the car is statically weight jacked so that the rear tire loads are equal at peak acceleration or the rear suspension is designed to null the driveshaft torque reaction (offset 3-link), then the car can have an open differential and it will never spin the inside tire. We have some sponsored drag cars that are in the 9 and 10 range and I've always wanted to demonstrate this at the track with a video afterwards showing the gear cover being pulled. Not enough time for everything we want to get done.
Case #3b. This is the case of a car with an open differential and an IRS. Since the driveshaft torque of an IRS car is not reacted through the rear suspension, there is no load difference on the rear tires under acceleration. Since there is no load difference, there is no reason to have an LSD to bias torque. An open differential which always sends the same torque to each axle works fine. Many European cars in the 300-400hp range, which all have an IRS, use open differentials without any problems of inside tire spin.
So what does all of this have to do with this situation?
Lubricated steel plates have very, very little friction on each other. There are viscous coupled differentials which are LSD, that use spaced steel plates. They are set up with a particular gap between the plates and use a fluid that has high resistance to shearing. The shearing that occurs with a speed difference between the plates creates a drag force between the two axles so that they try to turn at the same speed. A viscous differential can't really lock up. It only has a limit to the speed difference. With the friction material sanded off, the "friction" plates and the slipper plates, there isn't going to be a gap between them. They will just be a solid stack, so it won't behave as a viscous differential.
All of the following things will cause the rear tire loads on drag launch to be more equal to one another on a car with a solid axle. No front swaybar, stiff rear swaybar, soft front springs, airbag in RR spring, UCA configuration that results in binding. If enough of these things are done to the car and the car doesn't accelerate too hard, then it won't matter if the differential is an LSD or completely open. The RR tire will not spin on acceleration.
If the steel on steel LSD has very low friction (which it does), then it also has a low TBR and is much more likely to spin the RR tire in any given situation compared to a typical LSD with a higher TBR. The low friction does allow the car to drive smoothly around corners, since their is little friction to create understeer.
If the steel on steel LSD has high friction (which it doesn't, but I'm pretending to question here to prove a point), then it has a high TBR and will never spin the RR tire. It will also make the car understeer a lot in corners, which will have some or all of the effects mentioned above. In addition it might weld itself together if driven around corners briskly enough.
One other major issue with post #18. If this works as well as outlined, there is no chance that every major auto manufacturer in the world would be wasting money on friction lining in LSDs. They fight over less than a tenth of a cent on parts price. If they could eliminate all of the friction material from all of these clutches, they would do it in an instant.
My guess is that the LSD was assembled without a preload spring to allow the car to turn corners easily at low throttle, but was shimmed tight. This would make it lock up some at high throttle inputs. The car really didn't spin the RR tire due to chassis tuning that equalized the rear tire loads plus tires that just have a lot of grip for the power of the car. According to MM&FF the car's best ET was an 11.1 with a 60' time of 1.45 seconds. That can be done with slicks, some chassis tuning and an open differential.
Other than the obvious actual Bullitt and SVO intakes (the SVO being less available and not much knowledge in the forums about it), a few options i have tried and successfully fit to them, are the 03-04 intake (pictured on my SVO intake) and the 96-98 Cobra intake (sorry i didnt get pictires with the JLT I had for it). They both fit fine but I beleived the 03-04 intake fit better. I had the JLT on this car but wanted to get the filter out of the fenderwell for wet track days. the only draw back is you will have to rearrange the PCV lines.
Last edited by school boy; 06-05-2018 at 12:18 PM.
Aftermarket throttle bodies allegedly have the throttle valve (butterfly) properly set when they leave the factory, however I have seen a number of posts on the forums from people who had idle problems after installing an aftermarket TB and either did not know what to do, or decided to muck about with the throttle valve stop screw (aka "idle screw", which it really isn't).The PCM can tolerate and correct for a lot of things that are not entirely perfect, however a lot of what it does is based upon an assumption of certain base settings. The base throttle body idle stop screw setting is one of those things. Some have been lucky and solved their problems, other have gotten things so messed up that they sent the new TB back and reinstalled the stock unit. Note: Checking and if necessary adjusting this setting is something that should be done on older throttle bodies if idle issues are encountered. Over time and miles the tip of the stop screw and the tab on the throttle plate bell crank will wear. As little as 0.002" wear will allow the plate to close further than needed to maintain the air flow expected by the PCM.Here is the procedure for resetting the throttle valve back to it's proper initial position.It is important that you first disconnect the battery for 5-7 minutes (this can be while you are performing the steps below) to clear the PCM's KAM (Keep Alive Memory). This is where all learned parameters, such as idle air and fuel trim values are stored. Clearing the KAM will force the PCM to relearn these values.Loosen the adjustment screw locknut, and turn the throttle stop screw anti-clockwise (loosen it) until you can see the gap between it and the throttle linkage lever;Using a feeler gauge adjust the screw until there is 0.010" clearance between the screw and the throttle lever; Turn the screw clockwise (tighten it) one full turn;Start the engine to check idle speed, if too high turn the screw anti-clockwise in small increments (1/16 turn) until the idle speed is as you like; if too low then turn the screw clockwise in small increments;Hold the stop-screw screw in position and tighten the locknut;If there is any binding of the throttle plate at tip-in then loosen the locknut and turn the adjusting screw clockwise very slightly (no more than 1/16th turn) to eliminate the binding--retighten the lock nut of course.Now let the engine idle for 15 minutes so that the PCM can learn the new idle air trim values. If the idle is still not as you would like (it should be, however it is a possibility it may not be) repeat step 4. If the battery disconect to clear the KAM, and the 15 minutes of idling. Should quite a bit of adjustment be needed then once again disconect the battery to clear the KAM, and let the engine idle for 15 minutes. If it is still not right make sure there are no vacuum leaks and that the Idle Air Control (IAC) valve is working properly.
Why Are We Doing This?
The PCM expects a specific airflow through the TB at "closed" throttle, that being just enough to keep the engine barely idling, so that it can then use the IAC to adjust the idle up or down as needed. The adjustment described above holds the throttle plate open just a bit to allow that small air bleed. In normal hot engine operation--no added load from the fans, generator, p/s, AC etc.--the IAC is held 35% to 40% open by a variable duty cycle square wave supplied by the PCM. As the engine load increases the PCM would extended the duty cycle of this signal so as to keep the IAC valve more open, allowing more air into the engine. The MAF sees this additional air, reports it to the PCM, and the PCM adds more fuel thereby increasing the idle speed. http://www.paladinmicro.com/pmicro.p...itAdjust01.htm
12-17-2017, 04:45 PM
C5/C6 Calipers in place of Cobra calipers
