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Discussion Starter #1
I have a race-only Supra and wonder if anyone has done a boost-to-manual brake swap. I've been reading generic information about pedal and MC bore ratios but this site never failed to amaze me with good information specific to the Mk2.
 

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Why? I'm guessing that Peter needs the room.
A smaller MC piston will provide more pressure to the calipers.
Stock bore/pistons are 22-24 mm.19 mm MC may work. Adapt the MC to fit the firewall? Hmmm.
Start here: Can I Convert To Manual Brakes By Removing The Brake Booster?
ten: Some of theses:
Can you think of a Toyota that has 4 wheel MANUAL disk brakes?
Early Corvettes?
1964-1972 GM A-Body?

There is also a discussion of brake pedal linkage ratios.
4 wheel drum brake master cylinder will NOT work w/disk brakes.

Whoever is running Manual Wilwood disc brakes with problems read this! [Archive] - Pro-Touring.com
This will require a lot more research.
 

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Discussion Starter #4
The primary reason is consistent brake feel for racing. With the boost it's possible to lock the wheels during threshold braking even with larger sticky tires. The K24 develops plenty of vacuum, but the level of vacuum is dependent on engine rpm so the boost is inconsistent from one corner to the next.

Two secondary reasons:
1. The booster takes up space that I need to run an extension from the roll cage to the front strut tower.
2. I lose a few more pounds under the hood and eliminate another possible failure point.
 

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Sounds like you have enough reason to get rid of it. But if you find the braking too hard with it removed (have you tried racing with it just unplugged from vacumme for now?), there are vacumme regulators....


 

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Usually dedicated race cars select different sized master cylinders to offset the force lost from the vacuum booster.
Usually dual masters or triple masters.
They don't usually run the OEM one.
 

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Without boost from engine vacuum.The DRIVER will apply the pressure to the braking system. The HUGE left leg .
There will need to be a BIAS to the front and rear calipers. An adjustable per- portioning valve will be a must-have addition to this idea,guys. I have driven a big block with manual DRUM BRAKES. I needed to use both feet on an off ramp.
I used a small master cylinder bore and the biggest wheel cylinders that I could find. Tolerable,but scary,when the metallic shoes were cold.
Peter will do the same "oh,shit" thing.
 

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Discussion Starter #8
Pirate is correct, the braking effort will only be slightly higher than OE when I find the correct combination of master cylinder bore (smaller than OE, probably 7/8") and pedal ratio (6:1 seems like a common number). There is already a proportioning valve in the OE system (mounted to the left strut tower) and I already correct for rear bias by using cheap rear brake pads rather than racing pads.

The most important change will be the pedal ratio. With a booster it is about 3.5:1 - this is why people who unplug a vacuum line or remove a booster and use the OE MC find the brakes intolerable. Changing the ratio is as easy as drilling a new hole higher on the brake pedal and mounting the brake rod closer to the pivot. Of course that necessitates mounting the new MC a bit higher on the firewall.

I'll post the results as I find out what works... and what doesn't : )
 

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I don’t have a booster on the race car; 7 race cars now and no boosters. It is a real conpromise between getting good pedal feel (bigger master) with the required 1000psi generally required in the brake lines (smaller master or ratio change). At the end of the day it is more important to match your calipers to the master size and pedal ratio so without knowing which calipers you use any specs are useless. I have AP 4 piston fronts with 44mm pistons and they like a .75 master dia and I have a pedal box so rears which are large 2 piston use .625. I have about 5:1 on the brake pedal. Left foot braking was mentioned earlier, I have 2 dog boxes for the race car but still use the clutch to take the load off the dogs, I don’t recommend left foot braking in older race cars unless you have a big budget for gearbox rebuilds although I know many people do it.
 

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Discussion Starter #10
Thanks for the information, John. Are all 4 pistons of the AP calipers 44mm? That's big. I'm running the Nissan Z32 4-piston aluminum calipers in the front and OE brakes in the rear (which honestly don't add a lot once the front brakes are activated). Do you run dual master cylinders (front and rear) in your pedal box? I'll be running a single MC and plan to start with a 7/8" and a 6:1 pedal ratio, and keeping the OE proportioning valve.

In the endurance racing we do it is rare to need threshold braking so most is done in a straight line at less than 100%.
 

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Without boost from engine vacuum.The DRIVER will apply the pressure to the braking system. The HUGE left leg .
I think the left leg generally needs to work the clutch. Now you might have a racing clutch and so the left leg develops a lot of muscle compared to the right. You take out the brake master cylinder, that way the right leg gets a workout too and doesn't start looking scrawny by comparison when you're wearing shorts. ;)
 

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Thanks for the information, John. Are all 4 pistons of the AP calipers 44mm? That's big. I'm running the Nissan Z32 4-piston aluminum calipers in the front and OE brakes in the rear (which honestly don't add a lot once the front brakes are activated). Do you run dual master cylinders (front and rear) in your pedal box? I'll be running a single MC and plan to start with a 7/8" and a 6:1 pedal ratio, and keeping the OE proportioning valve.

In the endurance racing we do it is rare to need threshold braking so most is done in a straight line at less than 100%.
Yes all 4 pistons 44.1mm. And yes I run a pedal box with dual masters and Girling masters, sizes as noted in my post. I don’t like running proportioning valves as they do add some hysteresis to the system depending on design but understand with a single master it’s probably required. My balance bar is dash adjustable. And what do you mean you don’t brake 100%???? Race car!!!!
 

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Discussion Starter #13
When you're in the car for 2 hours at a time the mental focus to brake 100% in every corner is exhausting. Combine that with the constant traffic of 100 other cars on track and the fact that brake pads have to last up to 23 hours, and you get an idea why threshold braking is not the norm. The Z32/MPV front brakes are incredibly effective for endurance racing.

Does anyone know if the AE86 booster bolt pattern is the same as the Mk2 chassis? I can find AE86, Mk3 and Mk4 Supra delete plates but nothing for the Mk2. I removed the booster and checked the brake pedal, it should be fairly easy to drill a new clevis hole in the pedal to gain leverage and the Wilwood compact 7/8" MC should be a good starting point.
 
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