there's a fitting on the rear brake lines that seems to pinch the lines. can that be changed/modified to make the rear work more?

 

Rotor size doesn't affect clamping force.

 

The rotors are <2% larger. The piston area on the caliper is ~15% smaller.

Piston force is determined by area, not diameter. Even a 1mm change at a large diameter can make a large difference in area. For example the 2 pot CP5317 is only 1.3mm larger than the stock caliper but shifts bias to the rear 6%.

Force changes from rotor size are calculated by diameter because it's considered a linear lever so diameter changes result in a linear effect on that braking lever.

As long as the ABS system can cope with the bias change it's up to the driver to decide which way they want the bias to shift.


No. Hydraulics can only be affected by the input and output. In the case of the braking system that's the master cylinder and caliper respectively. Putting a restrictor doesn't make it work less, just slower to respond, like we have in the slave cylinder for the clutch. If you did something like that for your brakes you'd get a wildly inconsistent brake pedal.


Piston area, rotor diameter and coefficient of friction all affect braking force and consequently bias.

 

im confused - again, isn't this a bad thing then with the 8350s? a downgrade vs the 9s, and certainly vs the 10s that I'm coming from?

 

Not a downgrade, just a bias change. If the OEM setup has too much front bias for your configuration, reducing the front brake torque (relative to the rears) would be expected to improve braking.

On the other hand, if the rears tires are already at their limit under braking (front-heavy Evo combined with forward load transfer under braking) then too much rear bias could overwhelm rear grip before front grip. This doesn't appear to be a problem, though.

The advantages of an aftermarket brake kit are some combination of: Better cooling through the rotors, better caliper construction leading to longer time between rebuilds, stiffer calipers, reduced heat transfer from pad to caliper due to piston design, and so on. A large rotor kit would allow for more thermal capacity and faster heat rejection.

Usually the braking torque isn't changed (much), and that's by design. You could design a brake kit with higher brake torque front/rear for the Evo, but it wouldn't help you stop better. It would just make the brake system more touchy, at which point you'd probably want to change the master cylinder diameter to cancel out the change and get back to stock-like pedal stroke/effort.

 

a while back I used a grippier compound for the rear and I really liked the initial bite happening earlier from the rears. a quick jab induces the rears to slip a bit without power. This is just for autox though. I still have mismatching pads but the fronts have the grippier compound so I brake before the turn again

 

Trying to test a shroud for the rotor vents. I think I might go with a 4" wide/full circle (this test is with a C-shaped)


Now to deflect air to the hub somehow

 

ok i see. i guess that's not what I was looking for, was looking for "better" braking - I guess this will feel much weaker than the Evo 10 setup I am currently on?

 

LoL where are you getting that from? The 8350 moves rear bias only 1% as well which is totally nu-noticeable, well documented. No way on earth would they sell a kit that "ruins" it that much, plus I have 3 years with the kit and used the same rear pads a whole season. I woulda been locking the hell out of the rear brakes if that was fact.

There are multiple 8350 kits, so be sure you're looking at the right pistons.

EDIT: From the initial build thread - "With the AP system we’re shifting 1-2% brake bias from the front to the rear of the car." Just sayin, but if the math is confirmed by Jeff. My experience and data still shows more front bite rather then "less".

 

I got it by doing actually doing the math...

8350s use pistons sized at 38.1mm and 41.3mm. Stock calipers use pistons sized at 40mm and 46mm.

Piston area on the 8350 is 4957mm^2. Piston area on stock caliper is 5834mm^2.

5834 - 4957 = 877
877 / 5834 = 0.1503 = ~15% smaller piston area.

8350 rotor diameter is 325mm. Stock rotor diameter is 320mm.

320 - 325 = 5
5 / 320 = 0.0156 = ~1.56% larger rotor diameter

-15% + 1.56 = -13.44%

The Essex 8350 kit moves bias to the rear by over 13%.

Unless Essex has their own bespoke 8350 caliper for JUST the evo 8/9 that has different piston sizes than the 8350 both they and AP list on their sites, which makes no sense from a manufacturing standpoint, that's what the math comes out to. I have searched and I see no mention of 8350s being offered with any other size pistons. if you know something I don't then please feel free to correct me.

By no means am I saying the 8350 kit doesn't work, plenty of people use it with great success. So please don't go putting words in my mouth that I'm saying their brake kit is ruining anything. I'm just simply laying out the math behind it.

 

Agreed, but I think you're backward (probably miss typed as you were putting down the data). Less area = less force = bias goes to the other end. So kit would move bias forward ~13%.

 

CP8350 front is smaller than stock front. Less area in front = bias goes to the rear.


In other news, I had some time to dig more. Since Essex is touting a 1% change I'm going to take a stab and say they're using CP8352 calipers, not CP8350 calipers. The 8352 uses 41.3mm and 44.5mm sized pistons. Going thru all the same math as before it works out to a 0.8% bias towards the front (1.56% rotor diameter increase, 0.8% piston area decrease).



So with that sorted, now let's get back to what solution that will be best for getting a more robust rear brake setup. I'm still heavily leaning towards the CP5317 2 pots. I just found out they use the same pad shape as the factory rear caliper so we'd have plenty of easy to obtain pad options too. It'll shift bias 6% to the rear but I think that's fine. My existing setup is already biased 10% to the rear and I noticed no noticeable downsides vs the stock brakes.

Alternatively there is also the CP5316 which is the same caliper but with 38.1mm pistons instead, which would bump bias towards the front by 9%. Unfortunately pretty much all aftermarket calipers use standard sized pistons and our stock 40mm rear fall right between 1.5" (38.1mm) and 1.625" (41.3mm) so if we stick with a 2 pot rear we've got to decide which way we want bias to go. If we can find a caliper using 1 9/16" (1.5625" = 39.7mm) we'd be golden but I don't think that exists

Edit: Disregard the CP5316/7, they use aluminum pistons and dust seals like factory. They may be a tad stiffer but I don't think it's really what we're looking for as a racing capable replacement.

 

Oops, I thought you guys were talking about rear brakes.

 

As far as I recall in brake kit manufacturing it's not an "off the shelf part" in the big names. They always talk about matching piston sizes to balance bias or there wouldn't be any point of a place like Essex. They wouldn't put the exact same caliper in a 991.2 and an Evo, completely different ABS, balance, weight, electronics, the works and expect a positive result. THAT wouldn't make sense. But I'll verify.

 

Thats like saying Ohlins takes fully into account geometry and weight to design their rates and valving around. But then we get crap like backwards rates in our case where we just luck out reversing them works, or the mustang where it was massively off. Most these places are cranking out kits to have a wide range of applications and not spending time with the platform to really understand it.

They may do that with some platforms but the majority of kits are a one and done design. I trust companies that specialize in a platform far more than ones that support wide ranges of them. Thats not to say they don't have epic products but in this game its worth digging deeper.