ABS & Proportioning Valve Question

[Xusia]

Hi Smart People!

What's the best location for the proportioning valve: Before the ABS module (meaning a single valve), or after the ABS module (meaning 2 valves), and why? My assumption here is that the valve is being plumbed into the FRONT brakes.

Also - and this is not about ABS - anyone with real world experience on brake bias in an 818 yet? I've been following the build threads (especially Erik's - SOOOO jealous, dude) and from what I recall, no one has actually felt the need to use the proportioning valve to reduce braking force to the front brakes. Is my memory correct? Or has anyone felt the need to adjust the bias?

[Bob_n_Cincy]

Hi Smart People!

What's the best location for the proportioning valve: Before the ABS module (meaning a single valve), or after the ABS module (meaning 2 valves), and why? My assumption here is that the valve is being plumbed into the FRONT brakes.

Also - and this is not about ABS - anyone with real world experience on brake bias in an 818 yet? I've been following the build threads (especially Erik's - SOOOO jealous, dude) and from what I recall, no one has actually felt the need to use the proportioning valve to reduce braking force to the front brakes. Is my memory correct? Or has anyone felt the need to adjust the bias?

You can't put it before the abs.
One side of the MC goes to FR and RL.
The other side goes to FL and RR.
Bob

[Xusia]

I recall reading something about that. Are we sure that's how it's actually plumbed though? It makes sense, I'm just trying to get to facts and not work from assumptions.

[Pearldrummer7]

Also - and this is not about ABS - anyone with real world experience on brake bias in an 818 yet? I've been following the build threads (especially Erik's - SOOOO jealous, dude) and from what I recall, no one has actually felt the need to use the proportioning valve to reduce braking force to the front brakes. Is my memory correct? Or has anyone felt the need to adjust the bias?


Hey Xusia,

My friend interned at FFR for a year as an engineer and spent quite a bit of time with both 818's they run. He said they didn't touch the value at all, and the brakes feel great.


Hope this helps a little!
Frank

[AZPete]

I probably shouldn't be replying to this because it was addressed to "smart people" :D, but I asked the same question a while ago in a thread called ABS Plumbing. http://thefactoryfiveforum.com/showthread.php?11724-ABS-plumbing

Yes, it's complex! I added two prop valves after the ABS module, as you'll see, and the system is complete and bled. But now I'm entangled in wiring so I haven't yet driven and tested the ABS. The jury's still out.

[Bob_n_Cincy]

I recall reading something about that. Are we sure that's how it's actually plumbed though? It makes sense, I'm just trying to get to facts and not work from assumptions.
Hey xusia,
this is a Subaru training document. I don't know if it's true for every Subaru.
Bob
http://thefactoryfiveforum.com/attachment.php?attachmentid=26688&d=1393873121

[Scargo]

What Bob said... I don't think adding proportioning valves and maintaining ABS is mutually inclusive. And, I am a smart guy.:cool: Bob, too!

[Goldwing]

As to whether to mount the prop valve before or after the ABS? Definitely after. The idea was to reduce pressure to the front brakes in FFR's non-ABS setup due to so much weight being moved to the rear of the car compared to the WRX setup. The front had too much pressure and locked up before the rears in an unbalanced setup. The prop valve's intention was to balance the brake system setup so lockup occurs simultaneously front and rear in heavy braking. The final adjustment was up to us after we got rolling. To use ABS, you have to plumb the valve after the ABS unit as mounting it before ABS would reduce pressure diagonally after the ABS. You need to find a second prop valve (just use the part number on the package, it was easy to find) and put one in each front brake line, again, after the ABS unit.

[Goldwing]

As far as how the ABS is plumbed (diagonally), to verify and see for yourself, squirt brake cleaner in one of the ABS in ports and watch which two it comes out of. Sometimes I find this sort of thing helps me see it better.

[Xusia]

Hey xusia,
this is a Subaru training document. I don't know if it's true for every Subaru.
Bob
http://thefactoryfiveforum.com/attachment.php?attachmentid=26688&d=1393873121

That's definitive as far as I'm concerned. I don't need it to be proven to me. Like I said, I was just trying to make sure I'm working from facts and not assumptions.

Thanks to all you smart people who replied. Oh... You too, Pete. <-- Just kidding!!!!

[Scargo]

... To use ABS, you have to plumb the valve after the ABS unit as mounting it before ABS would reduce pressure diagonally after the ABS. You need to find a second prop valve (just use the part number on the package, it was easy to find) and put one in each front brake line, again, after the ABS unit.
I'm not saying you are wrong, but I certainly wonder how this will work with "the brain". You may get the desired effect until you need ABS and then it may or may not modulate correctly. If ABS is a simple relationship of rotating speeds of the four wheels, and one that doesn't show to be rotating, then it might be fine. Who has real-world experience with this?

A racer buddy said to pull the fuse (or was it the relay?) for the ABS when driving full-tilt on the track (in my '08 STi). That really has nothing to do with bias that we're discussing, but my experience with ABS in my '08 track car is that ABS is damn good on the track. I suspect the ABS computer settings may have changed or algorithms massaged over the years. An '02 WRX may not work as well as my '08 STi...

With my Stoptech Trophy BBK on the front and Girodisc 2-piece rotors in the rear, I was able to threshold brake right up to me feeling lockup before there was any ABS intervention. I didn't feel like I was realistically going to squeeze any more out of the brakes than they had by the time that ABS was felt... which was very seldom! There were several instances when my Hawk HT10 pads gave out from heat before ABS kicked in.
So, where was I?

[Xusia]

Based on the information in the Subaru manual, I think Goldwing's interpretation is correct. The ABS module has 2 inputs and 4 outputs. If you pull the fuse, you still get braking from all 4 wheels. This means the system is passing the existing pressure from the master cylinder through the 2 inputs to all 4 outputs. Therefore, there must be some kind of splitting going on inside. It doesn't make sense to have the splitting one way (i.e. Input 1 goes out to FR & RL) when ABS is engaged, and another way (i.e. Input 1 goes to FR & FL) when ABS is not engaged. Why do that? And the ABS circuit isn't the only thing that could fail (one of the outputs from the master, as weil as any of the inputs or outputs from the ABS unit could fail). If they went to trouble to address failures of the ABS system to ensure balanced braking, it stands to reason they would simply plumb it that way and get the same benefit for other failures.

[Erik W. Treves]

Mine is still disconnected(ABS) and during the last autox event I left my prop valve wide open.... I was about 75% of flat out and it still seemed about right...I didn't even think about wanting to change anything,.... but that's me... Don't think I will be messing with the ABS setup in my next car.

[Scargo]

Stoptech has an article about proportioning valves and ABS (http://www.stoptech.com/technical-support/technical-white-papers/proportioning-valves) There's some useful information.

An excerpt:
We’ll start here with three of the most basic rules regarding proportioning valve installation and selection.
1. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed if the factory unit is still in place. Proportioning valves in series with one another can do nasty, unpredictable things!
2. If you have the deeply-rooted need to install your own adjustable proportioning valve, be advised that they should NEVER be installed in-line to the front brakes. The effect would be to make your vehicle rear-biased before you could say “terminal oversteer.” Front brake line pressure should always be left alone – only the rear pressures should be considered for proportioning.
3. In all cases, the basic brake system balance needs to be close to optimized to start with. This is the only way that a proportioning valve can be effectively utilized. You should never assume that simply adding a proportioning valve will address all rear-bias conditions, as even the best proportioning valves must be well-matched to the target vehicle.

While a little of what's below the excerpted portion in the article is confusing to me I think I absorbed most of it. I don't understand how "Electronic Proportioning" differs from, or relates to, ABS.

Can't remember whether the WRX/STi have the sensor for rear lift or not.

[Goldwing]

This is a good discussion guys, keep it up.

The stop tech article definitely gives me pause. FFR's goal of using the prop valve was to address point 3 front bias after the rearward weight shift, such that the front/rear bias is balanced. Even so, like Eric and I think Wayne, I installed the prop valves wide open (just a pass through) to adjust down as needed. My chief concern was whether any harmonic condition could develop where the two units (ABS and prop valve) might fight each other. I doubted whether this was likely, but can't help but wonder if that concept is part of what the article was leery about (remember there is a factory prop valve in the wrx to the rear that works fine with the ABS.). Though chiefly, they were concerned about having two prop valves in series, or having factory set prop valves to the rear then adding an adjustable valve to the front which effectively reduces the pressure to the entire system which is counterproductive and not our setup in the 818. The article's primary point in 1 & 2 (if I read it right) was it was better to remove the factory set prop valve and replace it with an adjustable one so that your only reducing the pressure to one axle. Might as well keep the ability to have full pressure somewhere, right? Unless you just like to push hard on the middle pedal, lol. SOMEONE PLEASE CORRECT ME IF IM WRONG, but I believe the factory set prop valve to the rear is independent of the ABS unit and removed on donor disassembly ( I bought a donor kit, so I admit some ignorance here). All that said, if ABS proved to be a problem, in a de-powered state, the ABS unit merely acts like a T valve, splitting the primary line to two corners, and the secondary line to the other two corners with no connection between the primary and secondary. If one system fails, we would end up with braking on each side, one front and one rear like it seems most car manufacturers do. Pull the fuse, and the ABS unit ceases to exist, hydraulically, with brake pressure just passing through the over-sized brick of a splitter. Add a G sensor and we should have some EBD function as well. There will be no thrust vectoring stability control in the 818. We will only have the brake functions of the stability control available. I view ABS as safer, and thus I'm committed to get it to work. The first ABS builders will hopefully save subsequent builders some trouble. I'm only just now hooking the lines up. It'll be a while before I can offer real world help.

If one were eager to avoid prop valves altogether, a bigger brake upgrade on the rear wheels only might address this. The front brakes are a healthy size, while the rears look tiny. Though it could just as easily go too far and still require adjustable valves in the rear, though after reading the article, perhaps preferred? I live in the snow belt, which ought to be a good place to experiment with this, lol. There is another thread in the brake section discussing a poor man's upgrade using the H6 legacy rears I believe. Ok, thoughts on that anyone? Calling the smart people out again. :)

[Goldwing]

Keep in mind with the ABS opinion above, this will be a street car in my case, and it is just an opinion. I don't have any track experience and understand that opinions here can vary, especially when talking track cars.

[wleehendrick]

The stop tech article definitely gives me pause. FFR's goal of using the prop valve was to address point 3 front bias after the rearward weight shift, such that the front/rear bias is balanced. Even so, like Eric and I think Wayne, I installed the prop valves wide open (just a pass through) to adjust down as needed.

I thought the same thing when I read that... that's why you never say NEVER... the Stoptech article clearly wasn't intended to apply to the 818. Basic physics says you need to shift brake bias to the rear, and with the OEM brake H/W, a valve on the front is the way to do it. Just make sure to start wide open and slowly dial it down, otherwise you may be swapping ends unexpectedly!


If one were eager to avoid prop valves altogether, a bigger brake upgrade on the rear wheels only might address this. The front brakes are a healthy size, while the rears look tiny.

Yes, that would be ideal... the relative brake size is a reflection of the Subbie's weight distribution!


I live in the snow belt, which ought to be a good place to experiment with this, lol.

Sounds fun, but doing so in very limited traction conditions will adjust for the static balance. However, in dry conditions, weight transfer will lighten the rear end, so I'd make sure to back off the setting dialed in from testing in the slick, otherwise you could be prone to lock up the rears under heavy braking in the dry.

[Goldwing]

"Sounds fun, but doing so in very limited traction conditions will adjust for the static balance. However, in dry conditions, weight transfer will lighten the rear end, so I'd make sure to back off the setting dialed in from testing in the slick, otherwise you could be prone to lock up the rears under heavy braking in the dry."

Yes, good point! :)

[Xusia]

Oh, this is fun! Let's look at the facts and make some deductions (based on my 2003 WRX donor)...

Fact 1: There is a factory proportioning valve installed inline after the ABS unit going to the rear brakes.
Fact 2: The rear brakes are smaller than the front brakes.

Both of these provide a front brake bias. Together, the effect should be compounded.

Fact 3: In the 818 we do not use a proportioning valve on the rear brakes.

So already we have an increase in rear braking force, but we don't know by how much.

Fact 4: The WRX has approximately a 60/40 front/rear weight distribution.
Fact 5: The 818 has approximately a 42/58 front/rear weight distribution.

A significant percentage of the weight distribution is moved rearward in the 818. The prevailing wisdom is this means we need more braking force in the rear. But because aren't using a proportioning valve on the rear brakes, we have an increase in rear braking force - we just don't know how much of an increase.

Based on the feedback from Erik, I'm going to guess that it's close. Close enough for the street anyway. Especially when you consider that under braking, any weight forward of the rear axle is going to transfer somewhat (the closer the weight is the center of the car, the more significant the transfer) to the front axles, effectively changing the weight bias of the car at that moment.

Based on all that, I think I'm going to omit the proportioning valves (I only have one, so I would need another one). Before anyone rails on me, understand my 818 will have a more neutral weight distribution because I'm putting the gas tank up front, so it's likely the braking balance will be a bit better than Erik's. Also, I'm not racing or tracking the car, so I feel confident ABS can deal with whatever unbalanced brake bias may still exist.

[Bob_n_Cincy]

Hey Xusia.
You can add to the list above that the 818 will have a much lower center of gravity. So much less weight shift to the front wheels during heavy braking. Might just take the 818 to 50/50. Great for turn-in. Is there a way to put an extra set of front calipers in the back to make all 4 brakes the same size?

I had a car or truck back in the 80's that only had antilock on the rear wheels. They didn't want the back locking up and switching ends during panic stops.


http://www.youtube.com/watch?v=6AQ7FC0BwE8

Bob

[AZPete]

Yes, this is a fun topic to discuss now and when some ABS systems are on the road we will know more. To add complexity, some of the newer Subarus don't have a prop valve according to the Factory Service Manual: (ABSCM = ABS control module)

E: EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)
The EBD system utilizes the function of the conventional ABS. This system prevents premature locking of the rear wheels by using electronic control instead of a proportioning valve. The system provides a feature to optimize the brake force distribution to the front and rear wheels according to change in loading conditions or displacement of the center of gravity during deceleration.

The ABSCM calculates the optimum brake force distribution from the difference in speed between the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the driving condition.

The proportioning valve is disused on EBD equipped vehicles.

Will the EBD system "optimize the brake force distribution to the front and rear wheels" in my 818? Only time and testing will tell. I put 2 prop valves on the 2 fronts in case the EBD doesn't.

[Xusia]

Hey Xusia.
You can add to the list above that the 818 will have a much lower center of gravity. So much less weight shift to the front wheels during heavy braking. Might just take the 818 to 50/50. Great for turn-in. Is there a way to put an extra set of front calipers in the back to make all 4 brakes the same size?

Yeah, I forgot about the lower center of gravity, and your're right; that will result in less weight transfer.

As for the rear brakes, my gut says they wouldn't need much of an upgrade for street cars, if at all. I'm basing this on 2 things: 1)Feedback from Erik (and I think someone else said the same); and 2)Forward brake bias is generally safer and more desirable than rear. It would be silly to upgrade the rear brakes to the point where you needed a proportioning valve on them. Perhaps a more aggressive pad compound would suffice? (<-- Remember, I'm not racing!)

[Goldwing]

The H6 rear upgrade might be a good option for those tracking their cars. The pics I've seen showed a one sided caliper, so may not work with my 2 pot rears (07 donor). One advantage would be a higher thermal capacity in the rear. Which brings up a possible concern on the track, but not likely a problem on the street: unbalanced thermal capacity. The rears might heat up a lot, while the fronts stay pretty cool (especially with adjusted downward front brake force) moving the bias forward during the unbalance. That upgrade might just balance braking force and heat capacity. It involves a 1" larger rotor with the same caliper. Playing around with such a setup would be needed to determine if any prop valves would be needed with that setup. Just thinking out loud, so to speak.

[tmoretta]

And - don't forget to mount and wire in the "G" sensor in the 818. My '02 WRX donor comes with a G sensor as part of the ABS system. I am thinking that this is another necessary component that must be included in an ABS 818.

[Bob_n_Cincy]

I have and ABS question.
I switched from an 2004 auto to manual transmission. There was 1 wire from the transmission controller (TCM) that goes to the ABS unit.
Most have said the abs will not work without this wire. It is a blue.white wire and goes to pin 3 & 31 on the ABS. Does anyone know the signal on this wire?
Thanks
Bob

[Goldwing]

Bob, I noticed a possible solution in the thread below, though it means carrying around a little extra or doing some testing. Look for the post like the picture, and read on from there. The long and short of it is to keep the auto tcu and run the gear selector switch off of one of the clutch switches. Once the clutch is out, the tcu reads in gear and sends whatever signal the ABS unit is looking for. Others have sourced a manual trans ABS unit. The guy who posted that had his own thread tat may also prove helpful. Hope that leads somewhere. :)

26729

http://forums.nasioc.com/forums/showthread.php?t=1863125

[Goldwing]

Perhaps testing that wire, off a working car while in gear will yield the proper signal?

[Bob_n_Cincy]

Bob, I noticed a possible solution in the thread below, though it means carrying around a little extra or doing some testing. Look for the post like the picture, and read on from there. The long and short of it is to keep the auto tcu and run the gear selector switch off of one of the clutch switches. Once the clutch is out, the tcu reads in gear and sends whatever signal the ABS unit is looking for. Others have sourced a manual trans ABS unit. The guy who posted that had his own thread tat may also prove helpful. Hope that leads somewhere. :)

Thanks Goldwing.

The TCM has 40-50 wires going to it. I want to delete it.
All the guys that have done this swap just keep the TCM.

If I can figure out this on wire, I can ditch 20-30 wires.
It might be something as simple as a brake signal.
Thanks
Bob.

[Jaime]

Shouldn't grounding the "Manual Transmission" pin on the ECU eliminate the need for the TCU? There are a few sensor wires that route to the TCU instead of the sensors, but that's really easy to fix. If the ECUs are the same for A/T and M/T, I can't see why it wouldn't be easy to eliminate the TCU and still have ABS.

[Goldwing]

It's probably as simple as grounding the pin, or applying 5 or 12 volts.

[Bob_n_Cincy]

Shouldn't grounding the "Manual Transmission" pin on the ECU eliminate the need for the TCU? There are a few sensor wires that route to the TCU instead of the sensors, but that's really easy to fix. If the ECUs are the same for A/T and M/T, I can't see why it wouldn't be easy to eliminate the TCU and still have ABS.

As understand it. Grounding the "Manual Transmission" pin on the ECU does eliminate the TCM fault.
The issue has nothing to do with the ECU.
There is a signal from the TCM to ABS that must be there for the ABS to work.
If I can't figure this out I will get an ABS out of a Manual Trans car.
Thanks
Bob

[Jaime]

Interesting... Pre-2005 the TCU does connect to the ABS module, but 2005 and later has no connection. I lucked out, because I'm doing the same thing you are, but with a 2005.

[Bob_n_Cincy]

Interesting... Pre-2005 the TCU does connect to the ABS module, but 2005 and later has no connection. I lucked out, because I'm doing the same thing you are, but with a 2005.
In 2006 there was a canbus connection to the ABS from either the DCCD or TCM.
I don't want to open that can of worms.
26733

[Quiny]

I was planning on using the ABS from my 2002 donor. I have made up the bracket and dieted the harness being careful to include everything I will need. I am now second guessing myself, has anyone successfully tested a working system yet? and would you do it again?

[Goldwing]

To me it seemed like the path of least resistance was to keep it. Just recreate what was already there in the first place. It might not be the leanest setup, but worth it for me, probably having something to do with being a new parent. My brake lines are installed, I've started plugging stuff in, but I haven't tested anything yet. Hopefully, I'll power it up soon, but it'll be a while before any real testing for me.

[Junty]

To all the smarter than me people who have posted many interesting discussion relating to 818 and ABS. My chassis (131 RHD) is currently onboard an ocean liner expected to arrive 37 degrees below (New Zealand) soon. I'm definitely not really smart enough to post here - however I am definitely going to include ABS in my 818S, and I have been studying everything anyone posts about ABS and researching elsewhere for what I trust is hopefully a full proof plan for success.
My limited experience with Mid/rear engine vehicles was some time ago with both Fiat X19 and Carrera 3.6. Both these vehicle's I liken to the layout of an 818 with an engine adding significant weight/traction over the rear axle when compared to any standard front engine rear drive car. Both these cars have the same size callipers and disks on the front as the rear - so with the foot hard on the pedal the result is very rapid deceleration without either front or rear hinting at locking up - the Carrera certainly squats in the back and the rear wheels contribute significantly to the braking. If I compare to 90% of street/production cars including the Subaru range - they all have significant more brake force/size in the front compared to the rear.
So my 818 plan is to have stock WRX front callipers and disks, and upgrade the rear to the most significant STI callipers and disks I can (without stretching to Brembo's or WW). I would ideally like to adapt the rear to accommodate the same 4 pot front callipers - but I suspect this will be a little mores difficult to do although still an option worth looking into more.
After this logic of upping the rear - the balance should be closer with the added weight over the rear from standard. I would love it if I could get the rear to hint at locking up under very heavy braking before installing the ABS...
Part 2 is the ABS. I intend upon swapping the front LH and rear LH sensors and position from the ABS controller. Within the Subaru the diagonal distribution must have been developed to provide some brake to front and rear and RH and LH sides in the most incredibly unlikely event of 1 out of the 2 circuits within the Master Cylinder to fail. This is most likely well planned for an original WRX slowing from 40mp/h on a street with its 4wd helping to offer a little balance also. However in an 818 likely to be travelling significantly faster on the track or autocross course, in the same incredibly unlikely event 1/2 the master cylinder was to fail - I would never wish for brake force on the Front LH and Right Rear or vis versa as my only braking, as this would result in an uncontrollable 360 degree spin for sure?
So my logic/plan would suggest that in this same unlikely and unfortunate event, by swapping the circuits on one side we should theoretically retain either the front or rear as a more even stopping force? I do not intend upon needing or running any bias - increasing the rear system should suffice. I look forward to reading about someone successfully plumbing the Subaru ABS and posting their results soon.

[Bob_n_Cincy]

To all the smarter than me people who have posted many interesting discussion relating to 818 and ABS. My chassis (131 RHD) is currently onboard an ocean liner expected to arrive 37 degrees below (New Zealand) soon. I'm definitely not really smart enough to post here - however I am definitely going to include ABS in my 818S, and I have been studying everything anyone posts about ABS and researching elsewhere for what I trust is hopefully a full proof plan for success.
My limited experience with Mid/rear engine vehicles was some time ago with both Fiat X19 and Carrera 3.6. Both these vehicle's I liken to the layout of an 818 with an engine adding significant weight/traction over the rear axle when compared to any standard front engine rear drive car. Both these cars have the same size callipers and disks on the front as the rear - so with the foot hard on the pedal the result is very rapid deceleration without either front or rear hinting at locking up - the Carrera certainly squats in the back and the rear wheels contribute significantly to the braking. If I compare to 90% of street/production cars including the Subaru range - they all have significant more brake force/size in the front compared to the rear.
So my 818 plan is to have stock WRX front callipers and disks, and upgrade the rear to the most significant STI callipers and disks I can (without stretching to Brembo's or WW). I would ideally like to adapt the rear to accommodate the same 4 pot front callipers - but I suspect this will be a little mores difficult to do although still an option worth looking into more.
After this logic of upping the rear - the balance should be closer with the added weight over the rear from standard. I would love it if I could get the rear to hint at locking up under very heavy braking before installing the ABS...
Part 2 is the ABS. I intend upon swapping the front LH and rear LH sensors and position from the ABS controller. Within the Subaru the diagonal distribution must have been developed to provide some brake to front and rear and RH and LH sides in the most incredibly unlikely event of 1 out of the 2 circuits within the Master Cylinder to fail. This is most likely well planned for an original WRX slowing from 40mp/h on a street with its 4wd helping to offer a little balance also. However in an 818 likely to be travelling significantly faster on the track or autocross course, in the same incredibly unlikely event 1/2 the master cylinder was to fail - I would never wish for brake force on the Front LH and Right Rear or vis versa as my only braking, as this would result in an uncontrollable 360 degree spin for sure?
So my logic/plan would suggest that in this same unlikely and unfortunate event, by swapping the circuits on one side we should theoretically retain either the front or rear as a more even stopping force? I do not intend upon needing or running any bias - increasing the rear system should suffice. I look forward to reading about someone successfully plumbing the Subaru ABS and posting their results soon.

Junty,
We are thinking very much alike.
I also want the ABS for safety reasons. Plan on having key switch to turn it off if desired.

Because of the rear weight bias and low CG, I am putting the same size calipers all around.
Here is a picture of my rears. I need to do some machining on the bracket to make it fit.
I'm using a 11.4" vented rotor and dual piston calipers in back
http://thefactoryfiveforum.com/attachment.php?attachmentid=30722&d=1404090563

I haven't thought about your ABS mod yet.

[Junty]

Thanks Bob,
Although much consideration has gone into my plan - I still have my rear hubs completely disassembled awaiting powder coating and bearings etc. However after seeing your rear, I pulled a front caliper and test fitted to the rear assembly. It appears to be much closer than I anticipated, so I'm now following suit and modifying to install front calipers on rear (and front). My WRX calipers however are the bigger JDM 4 pots - but they still appear to almost bolt straight onto the rear assembly. Might require minor movement of back plate holes - but only +/- 5mm. I'm very excited about this revelation - as it will follow similar concept to other well respected mid/rear engine vehicle's. I doubt any bias is going to be needed at all. Cheers Junty...

[Bob_n_Cincy]

I hope to have this done in the next couple of weeks. I let you know how it goes.

I like your Idea on the ABS. Make sure you switch the wires and the brake lnes together.
Bob

[Junty]

Hey all. My above post about other mid or rear engined sports cars that have the same front and rear callipers and disks without any proportioning valves definitely holds true. I'm nearly finished my build, car has spent best part of a week on Dyno and prior to becoming legal with compliance and getting registered I've merely completed 3 1/2 miles of road test laps around the block. We need to be very careful here in New Zealand as they are likely to impound the car if stopped by the officers in blue - and depending upon circumstances if classified as a 'boy racer' they are likely to crush the car instead of fines!
So we really cannot afford to be caught at all here.
Anyways each time around the block, Im gaining a little more speed and confidence. The last is/was the most telling - under hard/heavy braking the system certainly slows very quickly, and I have managed to get the front to start locking up - long before the rear. Im running 2008 WRX 4 pot callipers up front and the subaru matching 2 pot WRX 2008 rears - full ABS system, but no fuse in system until compliance completed. And no proportioning valve anywhere.
This simple test allows me to absolutely concur with Bob - and I will most certainly be swapping the rear 2 pot callipers and rotors for the 4 pots - same as fronts. I would dearly love to get the bigger rears starting to lockup long before initialising power to the ABS system. I doubt anyone has a car that will start to lock the rear wheels, and we certainly need a lot more braking with the weight over the rear axle!

[Quiny]

I did not use the ABS from my 2002 donor but I did use the stock calipers, if you check my build thread you will see in one of the go-kart videos that the fronts do lock first with the proportioning valve wide open. I have since started to adjust it to even things out. Still fine tuning on every test. Keep in mind as well the adjustment is counter-intuitive (screw in vs. screw out).

[Oppenheimer]

It certainly seems like a better idea to even out the balance naturally, than to resort to reducing the front bias with proportioning valve(s). If its as simple as using another set of front calipers on the rear, so they are all 4 the same, that sounds like a pretty sweet solution (even if it involves some effort to make those calipers fit on the rear). I can understand FFR wanting to go with the prop valve though, to keep it single donor.

[Bob_n_Cincy]

It certainly seems like a better idea to even out the balance naturally, than to resort to reducing the front bias with proportioning valve(s). If its as simple as using another set of front calipers on the rear, so they are all 4 the same, that sounds like a pretty sweet solution (even if it involves some effort to make those calipers fit on the rear). I can understand FFR wanting to go with the prop valve though, to keep it single donor.

Oppenheimer
That is exactly what I did.
http://thefactoryfiveforum.com/showthread.php?12534-MRG-MotorSports-818S-Build&p=173658&viewfull=1#post173658
Bob

[C.Plavan]

I'm in the camp of do not change anything until you try the first solution. There are tons of 818S's running the stock proportion valve without going to bigger rear brakes. You want the fronts to lock up before the rears.

I could not get my Wilwoods to lock up with the pressure full front. I did not like the feeling at all at the track either. I just changed to a more aggressive pad, and now pedal pressure is perfect, and I can lock up the fronts out on my test loop easily with little pressure at the same valve setting. I will probably have to back out some front pressure next time at the track.


Just as a FYI-
Screw out (Counter Clockwise) all the way for maximum 57% reduction in stock pressure.
Screw in (Clockwise) to add more pressure to front brakes.

http://www.wilwood.com/PDF/DataSheets/ds488.pdf

[Scargo]

I got the same answer as what is being said and discussed here, from two brake experts. First, an engineer that used to work for AP and then Stoptech. I also was told that a proportioning valve will slow braking response and that a balanced system and adjustable bias on the master cylinders, using a racing pedal box, like the Tilton 600 and 900, is the best way to address braking pressure bias. I submitted all the pertinent data about my R to Stoptech and they said the weight distribution under braking is very close to 50/50, so I could use the same caliper sizing front and rear.
I went with, Front & Rear: STR40/28-32 (28 &32 are the piston sizes), 328x28mm rotor
Recommended Master Cylinder Size: Front: ¾”, Rear: ¾”
Recommended Pedal Ratio: 5.61
You can also juggle brake pads to get the best results.

[TouchStone]

Here is what I'm doing based on a suggestion from Wayne. I haven't tested it but it makes sense to me.

Rather than matching the original Subaru design of having the brakes paired (front left / rear right) and (front right / rear left), I will be pairing them (front left / front right) and (rear left / rear right). This allows a single proportioning valve on the input to the ABS module for the front pair, rather than two prop. values after the ABS module. Changing the wheel outputs like this means that the ABS sensors will need to be rewired to match this new configuration (Don't want to activate ABS on a wheel that isn't slipping).
37927

[AZPete]

When you can road test your set-up please post the results. I've got my ABS plumbed differently but not yet tested. Lots of guys are waiting for results.

[TouchStone]

I'd love to and I will, but I'm many months out from a drive.