911 SC brakes, 19mm MC combo question Options

[RoadGlue]

Hi everyone,

I've recently entered the world of bigger brakes, and hoped I could get some feedback here about proportioning valves. I have a set of 911 SC brakes ready to be mounted, and wondered if I'll get the correct brake bias by ditching the stock proportioning valve. I have the "T" fitting, but wonder if I shouldn't put an adjustable proportioning valve inline. Seems like it would always be better to have an adjustable proportioning valve, but is it absolutely necessary with that brake configuration?

Mmmm, brakes.

Thanks!

Randy

[jgiroux67]

I'm doing the same thing, sc front calipers and stock rear calipers with a 19mm MC. I have a feeling that the front is gonna do almost all the work. Whats the best way to set it up so its somewhat smooth and even?

[mrihop]

If you're going to do a 6 or 8 cyl conversion, go to the T fitting. Otherwise keep the proportioning valve in it.

[RoadGlue]

QUOTE (mrihop @ Apr 4 2005, 11:09 PM)

If you're going to do a 6 or 8 cyl conversion, go to the T fitting. Otherwise keep the proportioning valve in it.

What consequences would there be by taking the stock valve off when running a hopped up 2.0 4-banger? How does the extra weight of the 6-cyl or V8 factor into the equation? I'm running stiff suspension (180# rear springs, 21mm torsion bars, 22mm adjustable sway bar, poly bushings) if that matters at all.

Thanks for the answer, I just like to know "why."

Best,

Randy

[J P Stein]

I run S fronts which are the same size as the As. The stock rear cakipers were just along for the ride.....with an adjustable P valve wide open. M rears restored F to R balance.

[lapuwali]

QUOTE (RoadGlue @ Apr 4 2005, 11:18 PM)

QUOTE (mrihop @ Apr 4 2005, 11:09 PM) If you're going to do a 6 or 8 cyl conversion, go to the T fitting.  Otherwise keep the proportioning valve in it. What consequences would there be by taking the stock valve off when running a hopped up 2.0 4-banger? How does the extra weight of the 6-cyl or V8 factor into the equation? I'm running stiff suspension (180# rear springs, 21mm torsion bars, 22mm adjustable sway bar, poly bushings) if that matters at all. Thanks for the answer, I just like to know "why." Best, Randy

This subject has been discussed in detail, multiple times.

In short, a prop valve will only help you if your rear brakes are more powerful than available traction in back. There is some weight transfer forward under braking, though the effect is smaller on a 914 than on many other street cars because the CG is so low already, and the car starts out with a slight rear bias. With the SC brakes up front, you will have a great deal more braking power up front, so you'll have (on balance) less in the rear. This means any prop valve will be less useful, and probably not at all useful. This is why fitting a T is usually done when big brakes are added to the front, to try and add back some of the rear brake bias that's lost by fitting big front brakes.

It's pure speculation on my part, but I feel that fitting big brakes only to the front of a 914, esp. if you're doing anything to add weight to the back (like swapping in a V8) is not going to give you the best possible brakes. Adding big brakes also to the back and fitting an adjustable prop valve to tune them will very likely result in significantly better brakes that will also be more stable. I have yet to test this theory myself, though SirAndy has fit large rear brakes and found a substantial improvement.

[Joe Bob]

An adjustable valve wouldn't hurt....every car set up is different. Having the ability to dial in bias can be a good thing as long as it is tested....nothing worse than having the rears lock up and spin yer ass when in a turn. (IMG:http://www.914world.com/bbs2/html/emoticons/cool_shades.gif)

Except maybe the sudden impact at the end.... (IMG:http://www.914world.com/bbs2/html/emoticons/headbang.gif)

[Joe Ricard]

Just my .914 cent worth If you are screwing around with brakes on your car, you are also screwing around with the safety of others near you on the road.
Meaning BE CAREFULL. If you spun out infront of me causing an accident and I found out you did a change to your brake bias We would be working on Monitary settlements.

Again be absolutley sure you know what you are doing. Some of us here me included are full of bright ideas (just read my posts). Then test your set up in a parking lot or track to be damn certain you know what the car is going to do in wet, dry and loose sandy surfaced roads.

Lots of variables in brakes. M/C size caliper piston size, caliper pad size, pad compounds, Rotor size, The difference between front and back, Tire compounds, corner balance, weight transfer front to back while under hard braking, (if you can stand the car on it's nose with big front brakes the rears will have little fiction with the road. rears locking up will certainly send you in a spin.

[SirAndy]

QUOTE (RoadGlue @ Apr 4 2005, 08:46 PM)

Seems like it would always be better to have an adjustable proportioning valve, but is it absolutely necessary with that brake configuration?

nope. you'll end up running the adjustable valve full open anyways ...
with your setup, you want as much brake to the rear as possible, so you'll run the valve full open, which is exactly what the "Tee" does ...

i'm running 911 Carrera Brakes on all 4 corners with a 19mm MC and the T works great, perfect bias front/rear ...

(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Andy

[RoadGlue]

OK then, I seem to be on the right track and understand that I'm not to test my brake configuration out in the rain, on 101 (even though it really is a parking lot most of the time).

Thanks for all the great info,

Randy

[Steve]

You should be able to slam on your brakes when no one is around and see what happens.
Preferably on a long side street with no obstructions around.
I'm running the 19mm MC, 911S aluminum calipers up front and 914-6 rear brakes.
I'm also running the T instead of the proportioning valve.
I've slammed on the brakes at Autox's and on the street and I feel the car brakes perfectly.
If i slam the brakes hard enough I can almost get the front brakes to lock up.

I wouldn't spend the money and hassle on a proportioning valve unless you need it.

[andys]

If there is a tested and established combination of parts known to work, then I suppose that would be ok. However, I see no way of obtaining proper or optimum brake balance without some from of adjustability built into a retrofit system (or perhaps swapping $$ parts until you reach good balance). Anyone with roadracing experience knows the value of this. (And) Yes you can have too much front brake.....A symptom of this becomes evident when under hard braking, the car feels like it wants to swap ends or becomes directionally unstable. Crank in some more rear brake, the car becomes stable on entry and you'll shorten up your braking distance. Too much, and of course you'll start locking the rears. BTW, the rear rotors on my Carrera brakes are slightly larger in diameter than the fronts (282mm F vs 290mm R). FWIW.

Andy

[Eric_Shea]

Listen to JP and James...

Even a "T" won't do much for your rear brakes. A "T" and some late model rear M-Calipers (ironically at match for what your A-Calipers came with (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) ) would be best... Now you're without a parking brake. You'll also need to machine a 5mm spacer to sit under your new vented rotor or get 5mm washers for behind your caliper mounts.

Look around for 914-6 rear calipers. They would be the same (piston/pads) as the rear M-Calipers but you'd still have the intigrated hand brake. I know of one lucky teener that just got a pair for $400 bucks!! BASTARD!! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) (at least he's having me rebuild them...)

[lapuwali]

QUOTE (andys @ Apr 5 2005, 12:09 PM)

If there is a tested and established combination of parts known to work, then I suppose that would be ok. However, I see no way of obtaining proper or optimum brake balance without some from of adjustability built into a retrofit system (or perhaps swapping $$ parts until you reach good balance). Anyone with roadracing experience knows the value of this. (And) Yes you can have too much front brake.....A symptom of this becomes evident when under hard braking, the car feels like it wants to swap ends or becomes directionally unstable. Crank in some more rear brake, the car becomes stable on entry and you'll shorten up your braking distance. Too much, and of course you'll start locking the rears. BTW, the rear rotors on my Carrera brakes are slightly larger in diameter than the fronts (282mm F vs 290mm R). FWIW. Andy

Adjustability is indeed a good thing, but the point being made here is adding an adjustable prop valve will generally not give you any useful adjustability. With stock /4 rear calipers and SC front calipers, the valve would be wide open and still too front-biased. Thus, there's no way to get proper balance with a setup like that, no matter how much you spin the adjuster, and a T is just as good as an adjustable prop valve.

The only thing wrong with the stock 914/4 brakes is the calipers are heavy and the rotors are a bit undersized and not vented. Using most 911 calipers doesn't help the weight problem, and the non-vented rotors only seem to be a problem on the track. Using all stock (including the MC!) 914/4 parts with good pads, within everything in tip-top shape (rubber lines aren't swelled up, calipers aren't sticky, etc), I'd bet would be perfectly satisfactory for most people on the road. All too many people (including the DAPO with my car) have switched to 19mm MCs as an "upgrade", and now are hell-bent on adding bigger calipers, all to simply return the system back to the same hydraulic advantage it had with the stock MC.

[RoadGlue]

QUOTE (lapuwali @ Apr 5 2005, 01:53 PM)

All too many people (including the DAPO with my car) have switched to 19mm MCs as an "upgrade", and now are hell-bent on adding bigger calipers, all to simply return the system back to the same hydraulic advantage it had with the stock MC.

That's true! However when you change over to 911 suspension you have no choice but to upgrade to bigger brakes. Trust me, I tried to mount the stock 914 calipers, but they didn't seem fit! (IMG:http://www.914world.com/bbs2/html/emoticons/tongue.gif) j/k If you're still running 4-lug suspension, simply making sure your brakes are in good working order along with using a better pad compound will probably do the trick for most drivers.

[jgiroux67]

Ya, i went 5 lug so I had to go bigger brakes. So with the A calipers up front and stock in the rear will it realy be that noticable of a bias? Should I go with the T fitting? I would like to keep the hand brake function so I realy don't want to go the the m's.

[Dave_Darling]

When you test, remember that the "worst case" scenario is one where you have the most weight being transferred forward. That would be--going downhill, with a keg in the front trunk, in the dry and on good pavement, with good tires.

At the very least I would test going slightly downhill in the dry and on good pavement with whatever tires I was going to run. If the front brakes lock up first in those circumstances, you should be OK for anything but the "beer keg" scenario.

--DD

[Eric_Shea]

Josh... at the least go with the T fitting.

[Eric_Shea]

Who "ever" takes their keg out? That's what I wanna know... (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/beerchug.gif)

[jgiroux67]

How much do the T fittings run and where to get them?