For the past two years, I’ve been picking at my car trying to become more competitive in local SCCA Autocross events. My car is a 1989 GTUs and my current modifications put me into STS class. I had my first event of 2016, my co-driver and I came to the conclusion that the brake feel is “mushy” at best with very little feedback before the brakes lockup. Currently the fronts lock before the rears.

Where I’m at with my build:
-Stock brake booster and master cylinder. (replaced leaky MC in 2015)
-DOT 4 fluid
-EBC Yellow front pads. Bendix rear pads.
-All four calipers rebuilt/replaced in the last 3 years.
-Bridgstone RE71r tires – 215/45R17

I know that the S5 Turbo II has the larger double diaphragm booster and might be an improvement but for the most part the booster and MC that came on my GTUs are about as good as you can get for stock parts. I’ve also seen the 929 swap as well as this epic journey trying to improve brake pedal feel: https://www.rx7club.com/2nd-generati...t-deux-852021/

My question is, has anyone ever looked at the FD3S brake setup to see if it is an upgrade to the FC? Is the FD master cylinder still a 7/8” bore? I’m just wondering what other combination of RX7 parts I can swap in to improve brake feel. Or should I just build a MC brace and call it a day?
I do understand that anything I’m discussing here isn’t technically legal in my class, but I’m not building a nationally competitive car. My biggest concern is NOT flat spotting my tires! Feel free to chime in with any experience you may have. Thank you,

 

Get a 929 master, and 929 booster. Also make or buy a master cylinder brace. The FC firewall flexes quite a bit. The 929 master is larger than the FD. The Subaru master/ booster in the thread you referenced will work too and will give a shorter pedal than the 929.
Bottom line - you need a bigger master and different booster.

You also may want to try to change pad compounds to find a pad with finer control and feel. Run the same pad all around first, and if you can't get the lockup characteristics you want, try an adjustable prop valve for finer control. You need a more aggressive rear pad anyway to get the rear brakes to do more work. You are always going to be biased too far forward with the pad setup you are running.

Also get SS brake lines if you do not have them already.

 

My experience is with the Subaru 1-1/16 bore MC/booster setup, it gets my seal of approval (whatever that's worth ). That, an a brace made all the difference for me on my TII.

 

I've tried a couple of different combinations of pads. The Mazda OEM pads were some of my favorites for the front, but they didn't hold up for track days when you have a 20 minute session. Hence the switch to the EBC Yellow this winter. With those in the front and Mazda OEM in the rear, I was occasionally still locking up the rears. That's how I landed on the Bendix rear pads for better balance. First track day is in May, so I'll test this setup in a couple of weeks.

I forgot to mention that I already have SS brake lines.

The prop valve would be great except its technically not allowed in my class. I'd need to find a way to hide it. The 929 booster/MC would be easier to hide until another RX7 shows up at an event. LOL.

 

I think I can sneak a brace in there without anyone noticing. I'll have to take a look at the Subaru vs. 929 booster/MC swap... Figure out which one looks more stockish.

 

Are you running soft suspension? I ran HP+ Hawk pads on my car with the NA rotary and the stock MC, booster and prop valve and the rear locked before the front on the street, but on track the fronts locked just before the rears. The more rear brake you can get the better the car will feel. You just don't want premature brake lockup. Now that my car make more power, I run a staggered pad setup. DTC 60s in front and DTC 30s in the rear. The adjustable prop valve really helped dial it in though.

Your feel issue could be due to the bite in pads as well. Your fronts have a lot more initial bite than the rears.

 

I've went through a lot tuning the brakes on my SSM FC Turbo... I actually really like my current setup:

- 929 1" master cylinder
- 929 8"/9" dual brake booster
- master cylinder brace
- Ferodo DS2500 pads
- wilwood proportion valve dialed all the way rear

alternatives include:
- FD 15/16 master cylinder (bigger than your 7/8".)
- FD 8"/8" brake booster (larger than your single 9" and the S5 TII dual 7"/8".)
- miata proportioning valve. this swaps right in if I recall correctly and gives better brake balance and looks stock.

one of the best things I did to my car was put on the ferodo ds2500s. they're really great for autocross since they have a very flat torque vs. tempurature curve and give a lot of feel back even with my big hoosiers... I don't know if they'd be up for track duty though. you can buy them through stranoparts.com if you can't find them anywhere.

 

+1 we have a similar setup aside from pads, and I have larger front rotors. I am starting to exceed the capability of the OE calipers though.

My prop valve is not dialed all the way to the rear, I think I would die. My car starts to wheel hop in the rear when I have too much rear brake. I finally have it set where I really like it.

 

I'm running coilovers with 450lb front springs and 325lb rears. My car has a stock interior and weighs 2860 with me in it. I've got the corner balance with me in the car 49.8%/50.2% and the front to rear is 51% front. Obviously left side is heavy. sorry, I don't have my notes in front of me for exact numbers. On the street from a 50mph panic stop, my rear brakes only locked up first when I had the EBC yellow or the OEM Mazda. Never with the Bendix pads. On course on Saturday, the fronts locked but maybe I'm leaving something on the table by not running the better pads in the rear? Hummm. I'll have to keep trying some different combinations in the rear.

I agree the EBC Yellows have more bite, but they never really get up to temperature on a 60 second autocross. With them being so cold, they don't feel like they are overpowered.

 

You perked my ears with the alternative when you said it "looks stock". For STS, I'd prefer to fly under the radar with my mods and stay away from obvious swaps... Does the FD stuff bolt right in? Any idea which Miata prop valve to use? There are a lot to pick from: https://www.flyinmiata.com/tech/stock_bpv.php

 

No worries. You have a decent setup, you probably aren't getting excessive nosedive or anything weird.. that was really where I was going.

It sounds like you need a pad with better feel at lower temps, but I'd also give it a whirl with the same pad all around. For me rear lockup on the street= perfect on the track.

 

according to this thread the specs on the FC valve are in the FSM:
https://www.rx7club.com/race-car-tec...-valve-945620/

you can look here:
RotaryHeads.com - 2nd Gen RX-7 PDF Technical Manuals FC3S

considering the 929 stuff bolts to the FD and the FC it should be. but you might have to bend the lines a bit different, or get some flare to banjo adapters.

you also have non-abs, so you'll need a "Y" fitting to plumb the 2 front brake lines into 1 BMC hole.

 

I'd definitely get your brake pad situation fixed before you mess with the proportioning valve... you can't use track pads for autocross, they just never get up to temperature and will feel terrible.

 

From Saturday: (I know I left tons of time on the course...I was miles away from the cones! It was a long winter...)

 

Got it. The EBC pads will come out and only be used for track day events. I think I just wanted to try them. On the shelf I've got for the front: Hawk HPS, Mazda OEM, and a set of Hawk HP+ that are about 50% gone. For the rear I've got Hawk HPS, Mazda OEM, and Bendix. I'll probably try the Hawk HPS in all 4 corners for the next event. I know this locks the rears on the street, but might be perfect for autocross... have to try it.

 

Say NO to HPS. They are the worst pads ever. Throw them
In the trash.

Put those HP+ back on. Those are pretty good, great feel and a lot of initial bite.

 

i know this probably isn't what we're really talking about but i found on my current FC that the adjustment between the master and the booster was way off. clearance is 0-something like 0.010" and mine was like 0.030". doesn't sound like much but 0.030" x the 6x1 pedal ratio....

 

You probably had a pretty noticeable deadband in pedal travel!

 

EBC yellows are some of the worst track day pads I have ever used. Hawk DTC pads are much better. I am now using Carbotech XP10/12 (on Lotus, not rx7), which I love, but takes a little bit to get up to temp, so not good for auto-x.

I would try some better pad compounds before changing much else.

Pro-tip: go to your local welding store (or mcmaster-carr) and buy some temp crayons. You can paint the pads with these and see how hot they are getting. This will tell you what compound to switch to.

 

FC's have a crappy pedal feel a lot of the time, and its been my experience that you can change everything and bleed it forever, and its still there. so the next step is the adjustments

 

-Dual MBC's on a OMP adjustable pedal assembly bolted to the floor with 1"*1"*0,1"steel profiles under the floor connecting it to the frame rails.
-Evo 9 brakes up front with 0.7 mbc and yellowstuff pads

-FD Type rz (big rear rotor) rear brakes with yellowstuff pads and 0.625 mbc
-Steel braided hoses everywhere and new hardlines
-No abs
Feels great so far especially compaired to stock but i havent tested it on track yet,
But tbh my volvo v70 feels better

Edit: i dont know whats legal in your class or not

 

Remove the yellowstuff pads, go with something else. The feel will get a lot better.