Minimum level brake hardware for adequate stopping power?

[CNC_Geek]

I fully expected to use the OEM brake booster on my street (no track) 818. Perhaps the first generation chassis allowed room for the booster but mine (#340) does not. This issue became my first introduction to the build forum. While searching for information on the brake system I learned that most people were afraid to drive their car with the OEM brake hardware. Recommendations involved better pads, H6 larger discs with caliper spacers, and smaller master cylinder diameter. I have done all of these and I can not lock up the front wheels much less the rears. This setup is just barley adequate for the street. My rear calipers are single piston and the front are 2 piston. I'm really frustrated with trying to make this work on a budget yet $2,000+ for a complete new system is just not in my budget.

What can you recommend in the way of proven (works on your car) brake hardware? Particularly for the rear.

[Hindsight]

It seems highly sensitive on pads. The OEM brakes were adequate for a 3300lb car so there is no reason they shouldn't work on a 2000lb car.

Tamra and Andrew are running EBC yellowstuff pads and she can easily lock them up. That's with regular master, no booster, regular OEM brakes.

I have the same issue as you, though I have what I thought were good aftermarket performance pads and I also have put only 3 miles on the car since it isn't street legal yet so the pads aren't bedded in. It could get better after bedding but if not, I plan on switching to the EBC Yellowstuff pads per Tamra's suggestion.

Are you sure your proportion valve is setup correctly? If you adjust it all the way OUT, that means it's providing maximum pressure blocking which is not what you want. Start with it turned all the way in.

[Coolspot]

No room for booster on new chassis? What have they changed?

[RM1SepEx]

your pivot location wasn't changed enough and you don't have the correct leverage, verify that you redrilled the brake pedal and relocated the pin properly. I can smoke my tires up fairly easily. Now they are manual brakes and take more effort than my E30 or Miatas but they are very good. I do have Hawk HPS pads and stock 05 WRX hardware.

[Hindsight]

I have the same pads Dan..... were they great immediately for you or did they take some time to bed?

[CNC_Geek]

your pivot location wasn't changed enough and you don't have the correct leverage, verify that you redrilled the brake pedal and relocated the pin properly. I can smoke my tires up fairly easily. Now they are manual brakes and take more effort than my E30 or Miatas but they are very good. I do have Hawk HPS pads and stock 05 WRX hardware.

I just installed the HPS 5.0. Of those who have responded do you have single piston or 2 piston calipers in the rear? My car is not fully legal yet. I have a temporary permit and hope to drive it to the muffler shop tomorrow. I have stomped on the brakes several times from 30-40MPH which is what I read for bedding in the pads. How long should it take before I can expect full capability?

As for the pin relocation The recommended distance was .5" and I moved mine .6" because I could see the effect it would have. I'll recheck the por valve and the actual pin location in use. I hope I just missed something.

For those who are knowledgeable about brake hydraulics. Do 2 piston calipers apply twice the force of a single piston caliper? 4piston=4X?

[Bob_n_Cincy]

For those who are knowledgeable about brake hydraulics. Do 2 piston calipers apply twice the force of a single piston caliper? 4piston=4X?

The simple answer to your question is NO.
Below is a chart out of 2006 Factory service manual.



Lets start with an example.
master cylinder pressure of 500 psi typical stop. (bob's WAG) Panic stop could be 2 or 3 times this.

Starting with the front 15 in type. This has 2 big pistons on the inside of the disk only.
Each piston has a diameter of 1.685". the square inch of each piston is (d/2)^2 x pi = 2.230 sqare-inches.
So the piston is pushing the inside caliper piston with 500p/si * 2.230si = 1115 pounds.
The housing is also being pushed inward by the fluid in the piston housing applying the same 1115 pounds to the out side pad.
So we will call the squeezing for of this one piston as 1115 pounds.
This caliper has 2 pistons for a total of 2230 pounds.

Now lets look at the 4 pot wrx front brake.
Each piston is 1.591" which is 1.988 square inches.
So each caliper piston with 500p/si * 1.988si = 994 pounds
So each pair (1 inside and one outside) squeeze with 994 pounds
The are 2 pair so total squeezing force is 1988 pounds.

As you see above the squeezing force of the WRX 4 pot is less than the 2 pot cheapo calipers by 10%.
The WRX makes up for this by having 10% bigger rotors.

General comment. Either of the above brake systems in a OEM boosted application can lock up the wheels. Stopping power is limited by tire-road friction.

As you see in the chart above the cheapo floating caliper and the wrx 2 pot rear caliper both have the same 1.5" piston.
So the clamping force for the rears is 884 pounds.

Let me know if the answers your questions.

The first brake setup on my car was FFR spec with no proportional valve. The front wheel would lock up very easily.

Since then I have put front cheapo calipers on the rear with special STI rotors. I am comfortable heading into turn one at 100+ MPH.
Bob

[RM1SepEx]

I have the same pads Dan..... were they great immediately for you or did they take some time to bed?

don't have any significant time on them but once bled properly they work fine. I do normally drive one of 9 different vehicles so I am used to quick adjustments to the differences as well as racing a shifter kart which is a whole different discussion. Someone else posted a while back that they had issues and found that they had not relocated the pivot pin enough. Changing the leverage ratio makes a big difference, you could go farther than FFR rec and gain some more leverage increase

As CNC Geek posted didn't read them all before posting

[Hindsight]

I relocated the pin exactly where FFR stated. I'd definitely move the pin higher, but doing so would result in the pushrod being pitched up at an angle which would concern me a bit.

[CNC_Geek]

I relocated the pin exactly where FFR stated. I'd definitely move the pin higher, but doing so would result in the pushrod being pitched up at an angle which would concern me a bit.

I know exactly where I drilled that new hole. It's been about 11 months since I assembled the pedal box. And I know I drilled that hole - I must have used it! But a flashlight and 30 seconds told me I had assembled the pushrod in the OEM hole. I am upset and relieved all at the same time. After correcting this oversight my car stops as expected and the pro valve is adjusted correctly. Thanks for the reminder. Once I heard that others were successful without installing expensive hardware I was forced to reconsider each aspect of my installation.

[Hindsight]

whew, glad you got it working!

[RM1SepEx]

I know exactly where I drilled that new hole. It's been about 11 months since I assembled the pedal box. And I know I drilled that hole - I must have used it! But a flashlight and 30 seconds told me I had assembled the pushrod in the OEM hole. I am upset and relieved all at the same time. After correcting this oversight my car stops as expected and the pro valve is adjusted correctly. Thanks for the reminder. Once I heard that others were successful without installing expensive hardware I was forced to reconsider each aspect of my installation.

Let's hope that this is the only thing you mess up, and it is a simple fix. I can't tell you how many things I've had to re-check and redo on mine!

[Pearldrummer7]

I did this exact same thing. Was so confused why it felt terrible. Found that I drilled the hole and neglected to use it! Glad you caught this mistake and glad it's an easy fix!