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Old 04-26-2001, 03:08 PM   #1
bsquare
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Post Big Brake Upgrade Kit in the Works

I'm putting together a brake upgrade kit for Imprezas (and other models, if they use the same front brakes as the Impreza). The kit will be composed of the following components:

2 Porsche Boxster Monobloc 4 piston Calipers with adapters to mate them to the Impreza
4 Porsche factory brake pads
2 Subaru STI 298mm rotors
1 set of stainless brake lines
All assorted bolts, washers, etc required to properly fit the kit to the car.

There will also be an option to exclude the rotors for those cars already equipped with them (such as the WRX, I believe).

The price target is $1150, or $995 without rotors (depending on the volume cost for the factory rotors, meaning changes in the cost of the rotors will change the price of the all inclusive kit, but not the price of the rotor-less kit).

It will take approximately 4 weeks to get the prototype system completed, and another 4 weeks to test and volume produce the kit (assuming all goes smoothly).

This is _not_ a group buy, nor do I have anything to sell at this time. I am trying to guage interest in such a kit. If you find this kit at the prices stated attractive, please send me a PM saying so. If you have additional questions, please ask them in this thread, _not_ in a PM, so that all readers can benefit from the answers.


Ben
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Old 04-26-2001, 03:55 PM   #2
WRXed
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bsquare,

Are the calipers from the Boxster S or from the base model?
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Old 04-26-2001, 03:58 PM   #3
bsquare
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The base model. The Boxster S calipers are for larger discs which would require 17" wheels. This kit is intended to fit stock 16" wheels.
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Old 04-26-2001, 04:10 PM   #4
Digital_Boy
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bsquare,

Would the Boxster calipers be compatible with pre-existing stainless brake lines?

And, would this be an improvement over a STi 4 piston brake caliper setup?
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Old 04-26-2001, 04:16 PM   #5
msv
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I have to caution you on going this route. The Subaru uses very large piston sizes in the front calipers. 43mm pisons for the OE and old STI 4-pots, and 40 & 46mm pistons for the new Brembo STI units. The Porsche Boxster calipers have much smaller pistons, which mean you will actually be LOSING braking power with this setup.

This also has the detrimental effect of shifting the overall braking balance rearward, creating an unstable condition which the ABS unit will have to compensate for to keep the rear brakes from locking up prematurely.

[This message has been edited by Mark Valskis (edited April 26, 2001).]
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Old 04-26-2001, 04:16 PM   #6
bsquare
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I don't know if existing stainless lines will work, but I will investigate once I have the calipers on hand.

This will improve upon the STI calipers in at least 2 ways:

They are lighter, not being cast iron, and they are Monoblocs, meaning they are a single, forged piece, greatly improving caliper rigidity (Porsche GT3 brakes are also Monobloc). They should also transfer far less heat into the hubs.

The pistons may be slightly larger, but I am still looking into that.
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Old 04-26-2001, 04:21 PM   #7
bsquare
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Mark,
The Boxster calipers do indeed have smaller pistons (30 and 28mm), but they are lighter and more rigid.

The surface area of the pads should play a larger role than the size of the pistons. Do you happen to know the surface area of the cast iron (not Brembo) STI 4 piston pads?


Ben
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Old 04-26-2001, 04:42 PM   #8
msv
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Ben,

Sorry, but the caliper stiffnes and pad area do not even come into play when calculating braking torque.

Braking Torque is defined:

(Caliper Piston Area)x(Effective Radius)x(Brake Pad Coefficient of Friction)x(Brake Line Pressure)

With 28 and 30mm pistons, you have LESS THAN HALF the piston area, which means you will have LESS THAN HALF the braking torque.

DO NOT DO THIS MODIFICATION, you will not come close to being able to stop the car properly, not to mention the problem with brake balance front to rear.

Trust me, I design high performance and racing brake systems for a living, I know of what I speak.

Mark
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Old 04-26-2001, 04:51 PM   #9
Jon Bogert
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Question

OK, makes sense. So what's the solution--change the master cylinder? Boxsters' have great brakes, and weigh about the same as a Scooby.

C'mon Mark, don't just pee in the Cheerios. If you design brake systems--suggest a solution.
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Old 04-26-2001, 05:04 PM   #10
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My recommendation:

Buy your brakes from a reputable source that specializes in them.

Brembo has a couple systems in the works, including supplying the new WRX-STI setups at a low cost. AP Racing has a nice 6-pot system with 330mm discs.

The brake system is a safety device, not only a perfomance upgrade. Don't screw with things that could kill you unless you know what you're doing.

Mark
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Old 04-26-2001, 05:20 PM   #11
bsquare
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Mark,
Could you provide a reference for your assertion that the old STi calipers have 43mm pistons? The one I have right next to me seems to have 36mm pistons.


Ben
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Old 04-26-2001, 05:23 PM   #12
bsquare
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Mark,
Could you also explain how the car stops properly with the stock 2 piston calipers?


Ben
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Old 04-26-2001, 05:26 PM   #13
msv
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The regular WRX and 2.5RS calipers have 43mm pistons, and that was provided by the Autospecialty brake manual that contains brake system information on virtually every car and truck. If I mistated the size of the older STI calipers, I appologize. The new STI calipers are definitely 40 and 46mm.

IF the old STI caliper piston sizes are 36mm, than the boxster calipers you were planning on using still result in a 35% reduction in braking torque.

Mark
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Old 04-26-2001, 05:30 PM   #14
bsquare
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Mark,
So the stock 2 piston calipers should have essentially the same torque as the Boxster calipers, assuming 43mm pistons. I remain unclear on how this represents an unsafe level of torque.


Ben
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Old 04-26-2001, 05:39 PM   #15
msv
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Effective piston area only comes from ONE SIDE of the caliper.

For a four piston caliper, with two pistons on each side, the effective piston area is determined from the pistons on one side.

Since the stock caliper has two pistons, both on the same side, both add toward its effective area.

Mark
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Old 04-26-2001, 05:47 PM   #16
bsquare
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So, is Porsche just populated by stupid engineers who can't figure out how to properly size brakes for their cars? The Boxster has a similar weight to the Impreza RS, why such small brakes?
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Old 04-26-2001, 06:11 PM   #17
msv
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No need to get huffy.

First Brembo designs and builds Porsche's brakes for them, as well as the whole corner of the car, upright, hub, etc.

The other areas of the brake system that come into play are the pedal's mechanical advantage, the assist provided by the brake booster, and the bore size of the master cylinder. Add to this the wheelbase of the vehicle and the center of gravity height.

Look, do whatever you want. Be careful, and don't be upset when you end up wasting a lot of money.

If you don't trust me, call Brembo, ask for the guy that designs all of the brake systems, and I guarantee that you'll get a knowledgeable gentleman by the name of Mark Valskis to talk to.

Mark Valskis
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Old 04-26-2001, 06:16 PM   #18
bsquare
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I am not huffy, I am curious. If the primary feature is torque, and the torque is primarily determined by the piston diameter, then what could Porsche do to overcome the inherent limitations of such small pistons?

Ben
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Old 04-26-2001, 06:30 PM   #19
bsquare
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If I may be so bold as to cull the information from all your posts together:

All other things being equal (all other things, in this case, meaning the master cylinder, the pedal advantage, the brake lines, etc), smaller pistons will give lower torque.

So, could a new, larger bore master cylinder be used to increase the torque in the existing calipers? What about a brake pedal with a relocated pivot point?
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Old 04-26-2001, 06:37 PM   #20
msv
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Ben, sorry for the huffy comment didn't mean to jump to conclusions ;-)

Additional line pressure would be needed to compensate for the small piston area. This can be provided for in a number of ways.

Increase the mechanical advantage of the brake pedal itself, increase the amount of assist provided by the brake booster, or the use of a SMALLER master cylinder bore size (remember, pressure = Force/Area)

In a complete braking system all of these items, as well as the disc size, caliper piston area, pad material choice, vehicle weight, weight distribution, CG height, wheelbase, and many others must be taken into account.

Designing a complete braking system from scratch is a complex problem requireing reams of information in order to perform many calculations.

Trying to design an upgrade is a simpler problem because you already have a starting point, the stock system. However, you have to take into account the parameters of the stock system for THAT vehicle, since there are so many other variable that come into play.

All that said, I'm pretty sure that the boxster calipers your looking at are rear units, and that's the reason for the extremely small piston area. Also consider that you need to fit these things into wheels, and our cars don't have a lot of room for caliper overhang (i.e. distance to the back side of the wheel spokes).

Mark...signing off now, so don't expect any more quick responses ;-)

[This message has been edited by Mark Valskis (edited April 27, 2001).]
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Old 04-26-2001, 06:39 PM   #21
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Thumbs up

wow! great info. keep it coming.
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Old 04-26-2001, 07:24 PM   #22
CP
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There is a similar kit for the Audi A4. Goto www.bira.org , then to "information," then to "designs," and possibly contact them for more information. Brake Improvement Research Association, I think. This is the Stage 1 kit...Boxster calipers, stock 312x25mm A8 rotors, custom designed caliper mounting brackets, ss lines. Stage 2 is Boxster S/Carrera calipers, stock 321x30mm S4 rotors, lines, and the brackets. Goes to stage 6, twin turbo calipers, stock GT2 322x32mm rotors, lines, and brackets.

These are very well researched systems, and alot of thought went into them, with continued development. It is a bunch of enthusiasts too...not a company. The only thing BIRA has engineered are the brackets...about 2.5 years ago. The system has proven to be very effective on the track. They also have a list of specific vendors for the stock components that offer considerable savings over what the normal consumer would pay. I believe it is $20 to join, and you get the price/vendor list. A few of my buddies have these kits and they are very impressive.

This may be worth a call to Greg Amy to set up something similar for Subarus, but will take some work, effort, and technical research.

-Cy
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Old 04-26-2001, 07:45 PM   #23
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I want to second over_rev's comment. My knowledge and understanding of brake systems was tripled after reading this thread. Thanks, guys!
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Old 04-26-2001, 08:18 PM   #24
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I highly recommend getting a good brake book, as the concepts that Mark explained here today are published in a few books, and could have saved him some his time explaining basic brake concepts. A very good source of such books would be the Classic Motorbooks catalog , which is free, and can be ordered on-line. The catalogue has more books than their on-line store, and is well organized in a user-friendly manner (un-like their new website). They have been around many years. As a specialty book store, have a better selection of books then any general bookstore, including amazon.com.

HTH, Ken
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Old 04-26-2001, 09:37 PM   #25
OnTheGas
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Besides the brake system design 101 lesson which Mark provided, it was interesting that he mentioned...
Quote:
Brembo has a couple systems in the works, including supplying the new WRX-STI setups at a low cost. AP Racing has a nice 6-pot system with 330mm discs.
It'll be interesting to see what Brembo comes up with...
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