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View Full Version : Question about A8 - 12.3" brake swap and the calipers



Corrado_Guy
10-09-2011, 07:44 PM
Has anyone compared the piston size on the B4 A5 caliper and compared this to the audi TT calipers? I bought a set of front calipers and carriers from the 225hp Audi TT from an e-Bay auction for a really good price a few years ago for my Corrado which is running 11.3" DE VR6 brakes. The caliper pistons were a fair bit bigger on the TT so when I did the swap I was going to use the calipers as well as the carriers but decided to use them on the Audi. I haven't taken the brakes off of the A4 to measure but I was wondering if there was a difference in size. If the caliper had a bigger piston it would give a better clamping pressure on the pads so it made me wonder if the caliper pistons are any bigger. If anyone has a caliper from a regular B5 kicking around please measure the size of the caliper piston and I will dig out my TT caliper and do the same. I wanted to use the better of the two calipers and I also wanted to put stainless steel flex hoses on the car and the two front calipers use different flex hoses.

Thanks for any input.

walky_talky20
10-09-2011, 07:52 PM
That's a very good question because this in particular caught me off guard. I got some TT calipers from a junkyard for my A8 brake upgrade. When they came in the condition of the calipers was fantastically better than the rusty crap on my car so I decided to paint them up along with the brackets and just swap the entire caliper too.

2 things:
- Yes the caliper piston size is different. The size is cast into the caliper in text. The TT's are 54mm. The A4 is 57mm. A little different, but in the wrong direction (I assumed) for better brake feel. My goal was a more sensitive pedal.
- Other thing: The brake line is not a direct bolt up affair. The A4 uses a flared line fitting. A metric "double flare" I believe. The the TT uses a banjo bolt connection at the caliper. So it is not interchangeable directly without changing the lines.

I was contemplating upgrading to braided stainless lines at which point I would purchase "TT" lines instead, so they would connect properly to the calipers. However, not being a brake expert I am unsure about how much the 3mm piston size difference would affect things. I ended up not swapping the calipers. But they are painted so nice and pretty waiting for me to make up my mind (or sell them to a TT driver who wants red calipers!).

Corrado_Guy
10-10-2011, 09:21 AM
That's good to know, the TT's were bigger than the VW 11.3" brakes so I was wondering how they would compare to the A4. As the A4 is a larger car it does make sense so I will use the A4 calipers and then get the stainless lines for the A4 brakes. Thanks for the input, that really helped and now I can spend more money on the car.

Justin@FCP
10-10-2011, 05:43 PM
The A8 Brake Upgrade is next on my 'to-do list' for both my B5.5 Passat, and B5 A4. I'm planning on staying with me OEM calipers, TT carriers, and OEM pads. Everything I've read about this upgrade, I've never come across anyone swapping their calipers out. I can't wait to get this upgrade though, I'd suggest it as one of the easiest, cheapest mods to start with. Being able to stop, and have wicked good stopping power is very important to me!!!!

coolgraymemo
10-10-2011, 05:47 PM
The A8 Brake Upgrade is next on my 'to-do list' for both my B5.5 Passat, and B5 A4. I'm planning on staying with me OEM calipers, TT carriers, and OEM pads. Everything I've read about this upgrade, I've never come across anyone swapping their calipers out. I can't wait to get this upgrade though, I'd suggest it as one of the easiest, cheapest mods to start with. Being able to stop, and have wicked good stopping power is very important to me!!!!

This is on my to-do-list too.

I will be doing it when I swap out my control arms (from...).

Corrado_Guy
10-10-2011, 06:39 PM
This is on my to-do-list too.

I will be doing it when I swap out my control arms (from...).

You and I both, this is the reason I was asking the question because it has to come apart anyway and doing the brakes isn't all that much more. I was also going to replace the rubber lines with stainless lines, much better modulation on the braking side and not many people flush the brake fluid so most cars are usually due. I bought EBC yellow stuff pads for the front and rears for the Corrado but the part numbers are the same for the A4 so I will just throw them on this car. The control arms on mine are not making noise but the car feels sloppy so I will throw those in with better tie-rod ends.

coolgraymemo
10-10-2011, 06:59 PM
You and I both, this is the reason I was asking the question because it has to come apart anyway and doing the brakes isn't all that much more. I was also going to replace the rubber lines with stainless lines, much better modulation on the braking side and not many people flush the brake fluid so most cars are usually due. I bought EBC yellow stuff pads for the front and rears for the Corrado but the part numbers are the same for the A4 so I will just throw them on this car. The control arms on mine are not making noise but the car feels sloppy so I will throw those in with better tie-rod ends.

LOL. I have one bad control arm making a slight noise. I just want to get it all done and not have to worry about it later.

The noise doesn't bother me, but I'm getting tired of passengers asking "what's that?" or "did you fart?"

walky_talky20
10-10-2011, 07:31 PM
This upgrade was night and day for me. My stock brakes were getting very poor, especially when the rotors got wet. Almost scary. Engaging the ABS was impossible even on wet roads. With the A8's in I can engage the ABS on dry pavement without issue! The first day or 2 I had to relearn how to use the brake pedal. The pedal is much more sensitive but still a tad squishy. Probably from my lousy rubber lines (very worn). Will be going to stainless lines and a "stiffener" bushing kit in the future.

Seerlah
10-10-2011, 07:54 PM
The pedal is much more sensitive but still a tad squishy. Probably from my lousy rubber lines (very worn).

Could your brake fluid be a contributing factor? I know I felt a difference when I flushed mine the first time around with Motul 5.1. Not a huge difference, but a difference. The huge difference was the second flush (same Motul 5.1) when I did the SS lines and added Hawk HPS pads on the rear to match the fronts (A8/TT). Not much room for brake travel, but I like it that way. It goes along sort of complimenting the high engagement point of the clutch pedal.

Corrado_Guy
10-11-2011, 05:08 PM
Could your brake fluid be a contributing factor? I know I felt a difference when I flushed mine the first time around with Motul 5.1. Not a huge difference, but a difference. The huge difference was the second flush (same Motul 5.1) when I did the SS lines and added Hawk HPS pads on the rear to match the fronts (A8/TT). Not much room for brake travel, but I like it that way. It goes along sort of complimenting the high engagement point of the clutch pedal.

Why do you have a high release point on your clutch? Is it by chance a South Bend clutch? I have a South Bend stage 3 clutch and flywheel and the last inch of travel is where it actually engages.

walky_talky20
10-11-2011, 05:55 PM
Fluid is new Ate Typ 200, and bled the entire system, even cycled the ABS pump. All the air is out. The brakes feel great, but just have some "squish" to them like when sitting at a traffic light with my foot on the brake I'm able to "squish" it down a bit. I'd like it more stiff. I think stainless lines and especially the upgraded stiffener caliper slider bushings will do the trick.

Thanks for asking, though! Always good to have more input.

I am really considering swapping in the TT calipers. I need to find out for sure, but I think the slightly smaller piston size will help to put the front/rear brake bias back to stock. I just need to get the right lines to hook them up.

Corrado_Guy
10-11-2011, 06:58 PM
The smaller piston means you will apply less pressure to the pads making them less effective rather than more. It will help with the brake bias if you think the front is over braking as it currently sits which is usually not the case. The pistons on my Corrado 11" brakes were smaller than the pistons on the 11.3" upgrade kit and the TT caliper pistons are bigger than the 11.3" brakes yet the heaviest car has the biggest calipers. I think you will see better panic braking with the A4 calipers as most of the stopping is done through the front after weight transfer and all that.

walky_talky20
10-11-2011, 07:07 PM
^Yes, probably all true. But how much different could it be if it's just 3mm? That's what I want to know. Probably more than I would think, multiplicative forces and what-not.