# RX7 brakes on a MKV?



## GodSquadMandrake (Feb 17, 2004)

Here's the facts:

The MKV's brakes are an embarrassment to the fatherland.
Porsche Cayenne brakes rock, but everybody knows this and drove the price out the roof. 
RX7's come with 4 piston aluminum front calipers and you can find a set for CHEAP ($100). Here's a set on ebay right now for $80

Now the question is... will the RX7 brakes be big enough? The MKV probably has twice the weight on the front wheels and the rotor size on the RX7 is actually smaller, meant for a 15" wheel. The pads are actually probably about equal in size comparing the two, I guess the only difference is the RX7 has 3 more pistons in each caliper, but they are all small pistons. I could fab up an adapter bracket to use the stock rotors pretty easily.

This is for track use obviously. The problem I am getting now is that when I dive into a corner, the brakes are just total hammered dog crap. I've put the tyrolsport caliper bushings on, Hawk HPS pads, braided SS lines, and Motul RBF 600 fluid in. I know I could switch to HP+ pads, slotted and x-drilled rotors and maybe add a cooling duct but I don't think it would be enough. I just need more power in the brakes period. The fade isn't too much of a problem and I can manage it, but I need that strong consistent braking to give me confidence in my braking zones. Right now I either brake early and stay off the threshold, or I try threshold braking and just mess it up. To really threshold brake with this car I practically have to stand on the pedal but with that much pedal effort it's really difficult to modulate the pressure with the kind of precision that is required. It's pretty much like run up, slam the brakes and let the ABS do the work. But the pedal is so far to the floor heel-toe is impossible so my shifting is screwed. 

I just really miss the brakes from my FC RX7. They were powerful and consistent. The pedal was extremely solid and I could easily push it right to the threshold while staying in complete control, heel-toe downshift, release the brakes at exactly the right moment and drive on. 

Please somebody help me with brakes here. I don't really know the science, math or mechanics involved well enough to figure this out on my own. I just get jealous when I see all these other turbo 4 cylinder cars running around with brembo's and other multi-piston calipers. Even the new Cobalt SS has Brembo brakes from the factory!


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## GTijoejoe (Oct 6, 2001)

Here is some general info so you can get an understanding for the 'power' of a brake system.

Every brake system is limited by the amount of traction you get from your tires to make brake torque. If you want the most torque out of your system you need to invest in the correct tire compound and size for your application, bigger is not always better, you gotta heat those tires up to get that sticky icky friction.... I'm sure you probably know this since you are already performing some track duty. 

Secondly the 'power' is made up from a combination of things. These attributes can be formulated differently, mixed and matched to get the type of torque for a specific decel level. 
-Working backwards we can start with the calipers. The overall piston area is the main factor of a caliper to give the highest clamping forces (for a given brake line pressure), fixed verses floating calipers give you other benefits, but if the piston area is the same, they will develop the same clamp loads. 
-Next is the disks. Fundamentaly a larger disk will increase the effective radius where the caliper is clamping. Brake torque is = clamp load * Radius. So of course increasing the radius will increase the brake torque. For racing applications, the only reason to gain larger disks is not to increase the brake torque, you can just increase the piston area on the caliper for that purpose. The size of the disk is really to gain thermal mass to deal with the brake heat to counter fad resistance. To easily understand this it is logical that large disks weight more and have more inertia, of course for racing applications you want less weight and less inertia.
-Next is your pad friction. Correct pad application is probably the most important factor of your brake system. Friction compounds can be very complicated, there is no such thing as a single friction coefficient (mu) of your pads. Generally the number given dictating a specific pad is the highest mu that the pad can achieve. With this coefficient is also a temperature scale, you get different mu's at different temperatures, this is the largest difference between race/track and street pads. Street pads aim to get good consistance mu level's at low temperatures. This is ideal for cold weather and also for those first couple stops before they can heat up. For track duty, it is ideal to pick the friction to have the highest and more stable mu for your operating temperatures, this will give you consistant braking for track duty. Of course the larger the mu, the more brake torque that can be achieved. The caliper gives an axial clamp force, to make a radial force for brake torque (Torque=Radial Force*radius) the mu acts such as this: Radial Force = Clamp force *mu

Now that you got the corner figured out we'll go to parts that arn't quite comprehensive by most.
-Master cylinder: Increasing your master cylinder piston diameter is actually not an 'upgrade'. The reason for larger diameter pistons is to increase fluid volume for a given stroke. Large caliper pistons require more fluid volume, if you have a very small master cylinder, cylinder stroke will increase.
The smaller your master cylinder is, the higher the brake fluid pressure will be for the same input force. 
-Booster: Brake boosters can be very complicated, larger does not always mean its an 'upgrade' to your system. Generally the larger diameter or the more diaphrams will give you a larger boost assist, but internally to the booster is also a servo ratio. The servo ratio will control the amount of boost assist, think of the servo ratio just like a lever ratio on pliars. Also internally to the booster is a valve body which will control the jump in point of the boost assist. Because the jump in point is controlled by input stroke of the pedal, it will dictate how much and when the boost assist will come on in the system. Changing boosters from various cars is not always a wise choice without any experience, it can actually force your brakes to drag.....the booster is a mystery to most.
-Lastly is your pedal: The pedal controls the input ratio and stroke. Small ratio will require more effort by the user, so logically you think you want a large ratio to give high brake pressure with minimal effort. Unfortunately brake systems are always a compremise, if you increase the pedal ratio, then you also increase the pedal stroke. Think of your shifter, you can decrease the throw by cutting it down smaller, it does increase the force required to shift into gear, but the resistance from the transmission is very small so its almost never noticed.

OEM's balance the pedal ratio, mastercylinder size, and booster tuning to give your brake system the desired pedal feeling (force and stroke) and performance that is targeted for that vehicle.
I gave you all this info to educate you on what you are actually looking for to increase your performance and what you want to look for when choosing different calipers and such. Getting calipers off different vehicles with different piston area's will change your vehicles FR and RR Brake bias, improper bias can decrease brake performance...if you increase the FR bias by 15%, you need to increase the RR by 15% to maintain the bias. A good example is on BBK's, many times they increase the brake disk, but also at the same time will decrease the piston area or friction mu to try and maintain the same bias. :thumbup:


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## GodSquadMandrake (Feb 17, 2004)

Thanks for the lesson GTijoejoe. That's very useful information I was looking for but couldn't find on google. That really helps me in choosing how to upgrade these brakes. 

I guess now what I need to do is find out the size of the pistons on the calipers I am considering to install to know if they will give me additional clamping force. I know the Porsche Cayenne calipers have a much large piston area and they can be installed without actually increasing the size of the rotors. This to me sounded like the ideal upgrade in combination with a higher temperature pad like the HP+ from Hawk and a better cooling system for the rotors. 
To make the RX7 brakes give the same torque as the Porsche units I would need to increase the radius of the rotors but I've always known this isn't necessarily the best way to go. I remember an episode of Top Gear where James May put ridiculous brakes on a Renault van and he actually made it slower. Also there was an issue of Sport Compact Car where they put bigger brakes on some car and they lost noticeable power on the dyno. 

As for the braking bias, it's really convenient on the MKV. It's actually all controlled through the ABS system electronically and there is no mechanical proportioning to adjust. There's actually only one or two lines coming out of the master cylinder instead of the usual 4. I'm not sure how this wizardry works but I've been told you don't even have to worry about it, it's all taken care of by the computers. I know that doesn't sound right to me either, but when you drive it you can feel how it dynamically adjusts the ratio. For slow stops it uses almost %100 rear brakes. Then depending on how fast and hard you push the pedal you start to get a lot more power to the front brakes. Just given that experience I would believe that it could handle changing the calipers without adjustments.

So what you're saying about the master cylinder is that if I increase the piston area of the calipers, the stroke of my pedal will increase to achieve the same brake line pressure? I think right now I have too much stroke in the pedal already but that has something to do with how much I am over-working the stock brakes. I was considering upgrading to a larger master cylinder, but after hearing what you said that might not be a good idea. So if I am understanding this right, a larger master cyliner will require a shorter stroke but greater pedal effort? 

The brake booster sounds like something worth messing with, but I'm not sure how to adjust things like the jump point. If I installed the Audi TT master cylinder and brake booster together would that give it a good OEM match for the servo ratio and jump point? 

I'll get doing my homework and see if I can figure out the piston size on those RX7 calipers. I know I've seen it for the Porsche units so I can figure that one out pretty easy.


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## GodSquadMandrake (Feb 17, 2004)

So I sat down at the computer with All Data and did the research and math. Here are the results...

The RX7 brakes would actually be a downgrade! I was surprised too. But even with 4 pistons they have less surface area and less clamping force. I would have to go with huge rotors to get more brake torque and even then it wouldn't be worth it. 

The Porsche Cayenne 6 piston calipers are monsters as expected. Just from looking at the math alone these brakes should be enough to launch passengers through the windshield. They have right around twice the clamping force but that doesn't show the real advantage of having the force spread so evenly with 6 pistons. These brakes would be way more than enough. They would probably be enough to lock the wheels up at 200 MPH if I could ever hit that speed. But one interesting thing about them is they come in two size. One has a 330mm rotor, and one is 350mm. The 330mm version is actually more powerful than the 350. It has bigger pistons on the leading edge and in the middle. This would be the way to go because the 330mm caliper would fit under a 17" wheel and the 350 would probably require an 18" wheel. Plus the weight difference on the rotor.

The R32 brakes actually aren't a bad upgrade just from looking at the math. They should be at least %20 better. These would probably be sufficient but they do suffer from being a single piston caliper. They would stop the car nicely but the pedal feel wouldn't be like the Porsche/Brembo units. 

So I think the Porsche Cayenne/VW Touareg/Brembo 6 pot brakes are the best option. They are actually cheaper than the R32 brakes and have much better performance. Even going with the 330mm rotor you could have it cross drilled and the weight difference from stock wouldn't be that much. 

Once again thanks for the tips on how to approach the math/tuning side of this GTijoejoe.


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## GTijoejoe (Oct 6, 2001)

GodSquadMandrake said:


> Thanks for the lesson GTijoejoe. That's very useful information I was looking for but couldn't find on google. That really helps me in choosing how to upgrade these brakes.
> 
> I guess now what I need to do is find out the size of the pistons on the calipers I am considering to install to know if they will give me additional clamping force. I know the Porsche Cayenne calipers have a much large piston area and they can be installed without actually increasing the size of the rotors. This to me sounded like the ideal upgrade in combination with a higher temperature pad like the HP+ from Hawk and a better cooling system for the rotors.
> To make the RX7 brakes give the same torque as the Porsche units I would need to increase the radius of the rotors but I've always known this isn't necessarily the best way to go. I remember an episode of Top Gear where James May put ridiculous brakes on a Renault van and he actually made it slower. Also there was an issue of Sport Compact Car where they put bigger brakes on some car and they lost noticeable power on the dyno.
> ...


See above


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## GodSquadMandrake (Feb 17, 2004)

GTijoejoe said:


> See above


What is the modulator block? Does that mean I can make adjustments to make brake ratio? Or would I have to install a valve?

Yeah when I calculated the math, I just doubled the area for the sliding caliper, but calculated both sides on the double sided calipers. Is that an accurate way to do it too?


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## GTijoejoe (Oct 6, 2001)

GodSquadMandrake said:


> What is the modulator block? Does that mean I can make adjustments to make brake ratio? Or would I have to install a valve?
> 
> Yeah when I calculated the math, I just doubled the area for the sliding caliper, but calculated both sides on the double sided calipers. Is that an accurate way to do it too?


Modulator block is the device that controls ABS, VSA and other functions... I don't normally refer to it as an ABS pump because of course it has more purposes than just ABS (now-adays). You can not adjust the bias, nor would I suggest it. 

You did the math correctly, it would be the same difference:thumbup:


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