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Old 08-26-2001, 10:52 PM   #1
Impreza Rider
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Question RS vs WRX braking distance

Who knows what the distances from 70-0 and 60-0 are for the 98-01 RS, and the 02 WRX?

Subaruparts has a kit to upgrade to the WRX rotors for $290, but I was wondering if I should just soend the extra money and get the 4pots. I'm most likely not going to the track, just auto-x and rally-x, but need something better than the stock brakes since I'm going turbo.

Let me know what you all think.

Jim

Knew I forgot something: https://ssl.autocenter.net/webparts/...ipers_524.html
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Old 08-26-2001, 11:20 PM   #2
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If its just the WRX rotors you are better off getting some slotted or X drilled. The WRX rotors are nothing to write home about.

I would say get the 4 pots with good rotors.

just my .02

p@
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Old 08-26-2001, 11:32 PM   #3
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The main reason for upgrading is because they're larger in diameter, not because they're off a WRX. Just want more braking power with less fade...for cheap
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Old 08-26-2001, 11:33 PM   #4
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if you go with the subaru brake kit just buy the (caliper support brackets part # 26225ae000) then get some good pads like the metal masters that primitive sells for aroud a100 for all 4 and some RS x-drilled rotors for the rear and some WRX x-drilled rotors for the front. You'll love it and it will cost about the same as the kit but with much better 60-0. I'm looking into WRX slotted & or drilled rotors. I have someone sending me some now I'm going to do a test fit and if they work I'll be selling them in a group buy I think 60-0 will be around 100ft. with that set up. 4 pots are good to.
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Old 08-27-2001, 11:42 AM   #5
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I would recommend the 4-Pots. Yes, it will cost more, but the braking performance will be SO MUCH better than even the WRX upgrade. So if you're going to spend the money, might as well spend a little more and get some REAL performance gains, especially with the turbo. Also, avoid cross drilled rotors, and I'm sure you know why.

I'll have to let you drive my car sometime to see just how well the 4-Pots work with Mintex pads and stainless lines.

Tim
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Old 08-27-2001, 12:03 PM   #6
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Might want to check into this...

4-Pots for sale.
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Old 08-27-2001, 12:05 PM   #7
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Yeah...I'll have to drive it. You going to be at Rev on Friday?
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Old 08-27-2001, 12:10 PM   #8
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But no 4-pot system exists that will clear snow wheels, right? I think that's an important thing to note. We all want (or should want) better braking. You can never have too much. But when you switch to snow wheels (16x6.5 Legacy GT wheels for me) it's nice to know you won't have to worry about the brakes clearing.

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Old 08-27-2001, 12:12 PM   #9
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The STi 4-Pots will clear the 16" RS wheels. Don't know about the Legacy wheels though.

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Old 08-27-2001, 01:05 PM   #10
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Yes, the RS wheels are 16x7. Current WRX and Legacy GT wheels are 16x6.5.

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Old 08-27-2001, 01:16 PM   #11
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Get a quality set of cross drilled or slotted (or both) rotors, braided lines, better fluid & better pads. No need to go upgrading to 4-pots. Check out the results I got:

http://www.i-club.com/forums/showthr...threadid=81074
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Old 08-27-2001, 01:24 PM   #12
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I agree that upgrading the pads and the lines is going to help braking, but think how much better you car would brake with the STi 4-Pots with the same pads and lines!
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Old 08-27-2001, 02:12 PM   #13
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that is what i am wondering.
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Old 08-27-2001, 02:33 PM   #14
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The biggest difference in the original question is going to be determined by the tires mounted, not the brakes.... At least for a single 70-0 test.

Any Impreza on real summer performance tires will outbrake the heck out of any other Impreza on lame RE92s for one 70-0.
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Old 08-27-2001, 02:36 PM   #15
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Think of this:

Your current braking system puts a certain amount of force to the existing 2-pot calipers. If you upgrade just the calipers to 4-pots, your braking system will still only be able to provide the same amount of pressure it was to the 2-pots, but now it will be used by 4 pistons instead of 2. In effect this will reduce the amount of force at each piston, but you will still have the same amount of overall force. In order to get the full benefits of the 4-pot calipers you would also have to upgrade the master cylinder & proportioning valve to ones from a 4-pot car. The only advantage of a 4-pot caliper would be that the pads & rotors would offer more surface area for grip than the 2-pots. That would be the difference that you could feel. Is it enough to justify the $800 it will cost? That's up to you.
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Old 08-27-2001, 03:04 PM   #16
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Quote:
In order to get the full benefits of the 4-pot calipers you would also have to upgrade the master cylinder & proportioning valve to ones from a 4-pot car.
Correct me if I'm wrong, but I believe the master cylinder is the same.

In any case, hundreds--maybe thousands--of Impreza owners worldwide have upgraded to the four pots. Seems to work pretty well.
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Old 08-27-2001, 03:17 PM   #17
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I kinda agree with Opie. Yes things like cross drilled rotors sell well, but so does bottled water. I have been reading about "big brakes", and from the real world tests that I have read, the numbers seem to show that bigger brakes don't slow the car any faster. What they do add is fade resistance. Before you go spending $800 try things like more heat resistant pads to deal with the heat rather than some big heavy brakes. In short, if you aren't feeling brake fade, don't worry about huge brakes. Just my thoughts, take them or leave them.

Later,
Seth E.
"Bring about a revolution in the way people think about brakes"
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Old 08-27-2001, 03:28 PM   #18
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I'm sure some of the braking system parts are the same worldwide, I've never researched it to find out for sure. IMHO I think that the improvement you would receive from adding a set of 4-pots might require to have other components upgraded to meet the 4-pots full potential. While making other ,relatively simple, upgrades to the current system may be cheaper and produce the same, if not better results.
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Old 08-27-2001, 03:53 PM   #19
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OK...true, the 4-Pots have lot better fade resistance, but I have to disagree about them having the same force as the stock 2-Pots. It's in the design of the 4-Pots that gives them greater clamping power. Yes, the master cylinder/braking force is the same, but it is now pushing 4 pistons, rather than 2.
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Old 08-27-2001, 05:37 PM   #20
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Quote:
Originally posted by mav1c
OK...true, the 4-Pots have lot better fade resistance, but I have to disagree about them having the same force as the stock 2-Pots. It's in the design of the 4-Pots that gives them greater clamping power. Yes, the master cylinder/braking force is the same, but it is now pushing 4 pistons, rather than 2.
And on top of that, don't forget to mention that the 4 pistons have more total surface area than the 2 pistons. Which translates to higher brake pad clamping forces for the same fluid pressure from the master cylinder.

Say the master cylinder produces x amount of psi of hydraulic pressure. Multiply that by the surface area of the 2-pot brake pistons (at all four wheels), and you'll get some nominal clamping force in lbs. Now apply the same x psi to 4-pots. If the 4-pot pistons have roughly twice the surface area, you've just doubled your clamping force! Since nothing is free, the tradeoff is that you'll need a longer pedal travel since you now have more pistons to move using the same master cylinder.

I've done an A-B comparision of stock RS brake rotors and cross-drilled N1 rotors, both on the street and on the track. (If anyone is interested, I wrote a little story in the NESIC forum.) The RS brakes won in every category, including braking ability, pedal feel, and fade resistance. (Sorry, the N1's sure look much nicer.) I think the N1's failed to impress because there are too many holes drilled into the rotor, removing valuable surface area with which the brake pads needed to work with. After all, braking force is directly related to the contact area of the brake pad and rotor, among other things.

Opie's improved braking is obviously due to the brake pads, although he believes that the rotors also help with fade resistance. I can't agree or disagree unless he performs braking tests comparing the cross-drilled Brembo's against the stock rotors.

-WaC
Wayne
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Old 08-28-2001, 08:57 AM   #21
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wac - You math is a little off

Quote:
Say the master cylinder produces x amount of psi of hydraulic pressure. Multiply that by the surface area of the 2-pot brake pistons (at all four wheels), and you'll get some nominal clamping force in lbs. Now apply the same x psi to 4-pots. If the 4-pot pistons have roughly twice the surface area, you've just doubled your clamping force!
If the master cylinder produces x amount of psi (Say for example the x = 100). You don't multiply that number, you would divide it, split it between the number of pistons. 2-pots would = 50psi per piston, 4-pots would = 25psi per piston. Whatever the pressure the master cylinder creates would not be increased by the number of pistons, it would be shared. Your overall clamping force would not change. The only advantage of 4-pots (as I said before) is the increased surface area.

As far as my upgrades, they were all done individually. My original problem was brake fade after/during auto-x runs.

First attemt to correct the problem was to flush and refill the brake fluid with the Castrol GT-LMA - No improvement.

Next I replaced the stock rotors with the cross-drilled, cast Brembo's - I have not had a brake fade problem since these were installed.

The Goodridge lines and KFP pads were added after the fade problem had been rectified. They were added specifically for better pedal feel and a shortened braking distance.

I have never seen or tried the N1 rotors so I cannot comment on there performance or quality. But I can say without doubt that the Brembo's are an improvement over the stock rotors.
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Old 08-28-2001, 09:19 AM   #22
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OK...I still think wac is right. If your logic is correct, why not just run a 1-pot brake?

So why do all the upgrade kits have more pistons? Why not just get a Master Cylinder/Brake Booster that produces more power?

And I think cross drilled brakes are just for looks. You actually decrease braking power because you lose surface area. And they are more prone to warping and cracking.

There was a really good article in Grassroots Motorsports about this...I'll have to see if it's online.
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Old 08-28-2001, 10:11 AM   #23
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"upgraded" brakes tend to have more pistons to allow the caliper to apply force more equally across the pad area. One-pot brakes put most of the braking force in the center of one pad, whereas four-pot brakes equalize the pressure across most of the pad area.

Bigger brakes, aside from adding fade resistance, also tend to be bigger in diameter (rotors) and will add certain leverage advantages compared to stock. This should be noticable during a brake upgrade.

But you can't really argue with Opie's numbers in the linked thread. As a package, it is impressive.
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Old 08-28-2001, 01:02 PM   #24
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Quote:
OK...I still think wac is right. If your logic is correct, why not just run a 1-pot brake?
I still maintain that wac's statement about the greater contact area is correct & will result in some braking improvement. But there is still the problem of not getting the optimum pressure to the calipers to provide the best results.

The more pistons (pots) you have for each brake pad increases the contact area of the pad. The greater the contact area, the more friction will be created to slow the car. In order to make the most of this increased friction, the pistons must have enough pressure (from the master cylinder) to press the pads onto the rotors. If you increase the caliper (say from a 2-pot to a 4-pot) without adding pressure (which is created by the master cylinder and regulated/disperesed by the proportioning valve) you will have only slightly improved your braking because of the increased contact area. To get the most out of a 4-pot caliper upgrade, you should upgrade the master cylinder or proportioning valve (or both) to units that are designed to provide the recomended pressure to the calipers.

This may not be very cost effective.
(Although the 4-pots do look great )
The more cost-effective method to upgrading your brakes would be to address the deficencies in your current system and improve on them. (Through pads, rotors, fluid & lines).

A set of 4-pot calipers alone cost $700 (new, priced at Troncalli Subaru), not including addressing the other components I mentioned.

My setup cost just over $700 but addressed everything attached to the braking system. And the results were very positive. Plus I needed to retain the stock size rotor and caliper to remain STS (or GS for that matter) legal.
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Old 08-28-2001, 01:10 PM   #25
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Opie: I may not be a brake expert, but I understand physics just fine, thank you very much.

A little high school physics is in order:
Hydraulic pressure is measured in psi, or lbs/sq.in.
Piston area is measured in sq.in.
Multiply the two, and you get lbs of force.
Pressure = Force / Area
or
Force = Pressure * Area
Inside the brake piston and caliper, hydraulic pressure is pressing out equally on everything, including the brake cylinder inside the caliper and the brake piston. But since the brake piston is the only moving part, the hydraulic fluid can only perform useful work on the area occupied by the piston. And the above equations work just fine. Hydraulic pressure (also brake pedal force) does not need to change to get the additional braking force on the brake pads. This is the key to increased braking power from 4-pots. The penalty you pay is longer stroke at the master cylinder (and your brake pedal).

If we do it your way, we would get lbs/(sq.in.)^2, a unit that makes no physical sense.

Julian: your argument does not really matter, since the braking force from the pistons are distributed by the metal backing on the brake pads themselves. There are limitations, of course, if the metal backing starts to deform, but I don't think that consideration is as important as the surface area of the pistons in this case. Opie's setup is certainly very impressive, even if his math is wrong.

-WaC
Wayne

Edit: same pedal force.

Last edited by wac; 08-28-2001 at 01:16 PM.
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