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Author Topic: Z32 Rear disc brake upgrade - GTS  (Read 458 times)

Offline rwdfreak

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Z32 Rear disc brake upgrade - GTS
« on: Jun 17, 2019, 08:28:24 pm »
Hi all,

Last year durring a lapping event I finaly pushed the car hard enough to notice that my brake balance is all wrong due to the fact that I've got the Wilwood big brake kit in the front. It was particularly apparent with DTC-60 pads in the front and stock (PBR metalmaster) pads in the back. I then switched to HP-plus for a few laps and brake bias was way better, but after 15-20 min of punishment, the HP-plus in the front gave up and landed into brake fade world.

So I started looking for proper brake pads for the back and my research led me toward G-loc (Carbotech). They have a XP-10 XP-12 that easely matches the DTC range from hawk. The only issue is hat they are about 250USD a pair. Then one of my friend told me about the Z32 conversion he did on his MKII supra. I started looking into it and it looked promising (dual piston) and 11.7 Rear discs.

Then I gave it a bit of a deeper look.
1- The piston size is 38mm which is exactly the same size as the stock single piston. I made a few diagrams of forces and the way it works and unless I'm entirely wrong a single piston floating calipers generate the same calmping force as a double pistons fixed caliper (assuming no lost in the sliders and same piston diameter)

2- The Disc size may look impressive, but the celica one is not that bad. 10.4inchs which is only 1.25 inch smaller. So if you want to calculate the additionnal torque from a longer lever arm, you find that you gain only about 15% gain in torque (assuming same clamping force and friction material).

3-Pad size: From the info I found on EBC website, Z32 pads are about 50mm high by 73mm wide. Celica is 42mm x 104mm so pad surface i actualy bigger on the Celica.

4-Then comming down to cooling. Though the total disc area is about 25% bigger, however the donnut on which the pad is sweeping is only about 15%. This doesn't seem much.

5- Disc thickness is exactly the same at 18mm (3/4inch)

So unless I'm entirely wrong, it feels like this mod is not worth it because out of it you gain about 10-15% more torque, some bling factor and better pad choice (big market for Z32 pads out there).

Feels like I should stick to stock with the expensive pads unless you guys have other options.
V8 RA65L: Race/street beast
90 2wd pick-up truck: Sold
GS300 2001: Gone and won't miss it.

Offline Teranfirbt

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #1 on: Jun 18, 2019, 12:37:36 am »
I actually built a spreadsheet to run these calculations for when I redesigned the brakes on my MR2. Clearly the Celica is a different animal, so If you know your full weight, approx CG height, F/R weight bias, and disk/pad details, I can run all of that through my calculator and tell you how good a different setup would be :)
Btw, I only had to add 15% more rear brake to my MR2 to go from laughably front biased to properly biased, so don't discount it too much. On a front heavy car you may actually have issues adding that much to the rear.
1983 Celica GT-S 2RZ swap: Deered 2014 :(
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1986 MR2: What a beast!
2014 Elantra GT: The reliable one

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Offline Sigma Projects

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #2 on: Jun 18, 2019, 05:41:44 pm »
sounds quite interesting, what kind of adapter do you need to mount it all up?
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Offline rwdfreak

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #3 on: Jun 25, 2019, 07:00:30 am »
Well based on stock

Piston size
Front 57mm
Rear 38mm

This gives a rough 69-31 balance which is to be expected for a street car in order to avoid Joe Shmub to shit his pants first time he mashes the brake pedal and end up spinning around.

Wilwood kit from Raptorracing (same contact radius so same torque reaction arm).
Front 4 x 44.4mm
Rear 4 x 35mm

This yield a 62-38 balance.

Now comparing front upgrade with stock

Front stock 57^2/4 * π = 2550mm2
Front upgrade 2 x 44.4^2/4 * π = 3100mm2
Total increase 21%

Than there is the diameter change from 10.1 to 12.25 which is a 20% increase in torque.

Rear stock 38^2/4 *π=1134mm2
Balance is now 77-23 with same pads.

In order to improve I just found out that Supra NA calipers are 43mm this means that it would be 28% more force than stock. Going with the Z32 disc, that adds 15% more torque, that would yield to an increase of 43% over stock which would match the front.

Pad size
Celica
Front 128 x 57
Rear 107 x 45

Z32
73 x 54

Supra NA
123 x 41

Hb-100
101 x 46

Rotor size
Celica
Front 10.1 (256)
Rear 10.4 (266)

Z32
Rear 11.7 (297)

Supra NA
Rear (307)

Big brake kit
12.19 (310)

Compound
Hp+=.55
Dtc30=.65
Dtc60=.7

Pbr = .45 (approximation based on oem vs hps hawk table)

Carbotech
Xp10 = .6
Xp12 = .65

Supra NA
Rear piston size 43mm

Assumptions
1UZ = +100lbs front
No spare = -25lbs rear

From rwdcelicadude post
LF: 724    RF: 713
LR: 668   RR: 666
total: 2771

LF: 26.1%   RF: 25.7%
LR: 24.1%  RR: 24.0%

Lf 774 rf 763
Lr 656 rr 653
Total 2846

LF: 27.2%   RF: 26.8%
LR: 23%  RR: 23%

So 54-46 balance give or take.

Link for z32 brake upgrade
https://www.celicasupra.com/forums/archive/index.php/t-42919.html
V8 RA65L: Race/street beast
90 2wd pick-up truck: Sold
GS300 2001: Gone and won't miss it.

Offline Teranfirbt

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #4 on: Jun 25, 2019, 12:18:48 pm »
Cool, I'll take a look this evening. Any guess at Cg height? It's a fairly sensitive parameter in this calculation, so the closer it is the better estimation I can get.
1983 Celica GT-S 2RZ swap: Deered 2014 :(
1986 Tercel 4WD 4AFE Swap: Going strong
1986 MR2: What a beast!
2014 Elantra GT: The reliable one

"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." ~ Albert Einstein

Offline rwdfreak

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #5 on: Jun 25, 2019, 09:40:53 pm »
Derek your guess is as good as mine. I am guessing above transmission (10inch thk + 8 to 10 from ground) or about top of wheel but below top of tire so 18 to 21 inch above ground sounds reasonable. Longitudinally I would say about 2/5 between the axles.

Also found some usefull info on brembo site will try to convert into something that can be uploaded later.

Seems that 370Z brake disc would be a beter match at 306mm x 16mm

V8 RA65L: Race/street beast
90 2wd pick-up truck: Sold
GS300 2001: Gone and won't miss it.

Offline Teranfirbt

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #6 on: Jun 25, 2019, 11:59:32 pm »
After running the numbers, you are correct, the Z32 calipers with 38mm pistons are not enough of an upgrade to matter. Depending on Cg height and the knee pressure and ratio of the proportioning valve, the 43mm Supra calipers will match up well with the 4 pots you have in front, although if your Cg is much above 20 inches or the prop valve doesn't knee over enough you'll start to have issues with rear lockup in the 1.2g's of braking range.
Another application that has 43mm pistons is an SN195 Mustang (it's what I ended up with on my MR2 in the rear). Super common, probably an easy part to find in the yards.
1983 Celica GT-S 2RZ swap: Deered 2014 :(
1986 Tercel 4WD 4AFE Swap: Going strong
1986 MR2: What a beast!
2014 Elantra GT: The reliable one

"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." ~ Albert Einstein

Offline rwdfreak

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #7 on: Jun 26, 2019, 08:19:46 pm »
Thank you Derek. Good to know that Mustang Caliper could work also. Pads area loos lie it's a lot smaller than the Supra pads..

I would tend to try it and use lower coefficient pads or install the adjustable proportionning valve I have laying around since 10 years.

V8 RA65L: Race/street beast
90 2wd pick-up truck: Sold
GS300 2001: Gone and won't miss it.

Offline rwdfreak

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Re: Z32 Rear disc brake upgrade - GTS
« Reply #8 on: Jun 27, 2019, 05:16:01 pm »
Just found out this thread
https://www.celicasupra.com/forums/showthread.php?t=3727

Apparently RX7 Front 4 pot doesn't require braket to fit with Z32 disc. They are 36mm pistons which would match Jym king BBK kit. Might be too big, but could then do split friction and adjustable prop valve.
V8 RA65L: Race/street beast
90 2wd pick-up truck: Sold
GS300 2001: Gone and won't miss it.