Master cylinder sizes and piping

I have the Wilwood 6 pots on the front and 4 pots on the rear with 15" tires and a Porsche 996 slave cylinder. I am in a quandry for the proper size masters and piping size. I have done a search but can't find difinitive responses. Would like some direction. I have several masters that were purchased off Ebay and would need rebuilding. I am leaning toward giving up on these as some are Wilwood and aome are Tilton, some are 7/8" and some are 1". They can be resold. I got some "standard" piping from the local parts shop, but alas they come in preformed lenghts, and they aren't exactly the right lenghts. That means to cut and flare, which is not what I want either. Most flare kits will not do the job correctly(found out by trying my hand at it). If the flares aren't correct, you have to cut again and flare again. Will let the shop guys do it as they are racers and do it a lot. My problem is they are drag guys and their requirements are different from mine. So would like some info from the "experts" to pass along to my guys. What do yo think?

Bill
 
Bill, for master cylinder sizes to use go to the Tilton Engineering web site and fill out the request form with all of your car and brake info and they will get back to you in a few days to advise on what sizes to use. The lines are a pain I know as I tried my hand at using stainless lines a while ago, kept breaking the flaring buttons. If I were to do more I'd get my hands on one of those hydraulic flaring kits.

I THINK the lines themselves are 3/16" but it's been a while, clutch is 1/4" as it is more concerned with volume than pressure. I'm sure others will chime in and correct me if my memory has failed.

Brian
 
Bill
If you are concerned about lengths of available lines you can always buy a coil, I believe it comes in 25' length from NAPA. I think McMaster also sells it.
The only problem is you need to buy a bunch of inverted flare nuts for your connections, and the line is tough to get really straightened.
I would run 1/4" lines for the clutch and rear brakes, and it is generally ok to run 3/16 in the front.
The problem with running a small diameter line for the clutch is sometimes the fluid takes its time returning, and the clutch will engage slowly after you have released the pedal. This happens more in winter weather, but running a 1/4 line will insure it is ok all the time.
If you really don't want to use double flared lines you can also opt for stainless lines with a single flare. The fittings are a little pricey, and they use a little backup spacer to hold the flared line in conjunction with the nut, but they last forever, rarely leak, and are good for about 5000 psi which your system should never see anyway. I believe they are called JIC fittings and McMaster also sells these. The lines are a bit tougher to bend, but look great, and there are various fittings available such as line to pipe thread, tees, elbows, mostly everything you might need.
I would take Brian's advice on the master cyl. sizes, as every system is different, and matching the cyl. size to your calipers piston size and number of pistons is important. Also a larger rotor diameter, size of the pucks etc. affect the overall performance.
A smaller master cyl. may give you too much pedal travel that you may not be comfortable with, a larger cyl. will give you a harder pedal, but may not stop the car well.
Last but not least do the math on your pedal ratio, most run 6 to 1 or thereabouts.
Where the pedal is 12" long from pad to mounting pivot, and the master cyl. connection is 2" up from the pivot is a 6 to 1 ratio. An input of say 100 lbs of foot pressure will give 600 lbs. of force to the master cyl., Take the radius of your master..lets say a 3/4" master cyl. is being used so the formula would be .375 X .375 X 3.14 = surface area ( .4415 sq. in.). Divide that into the 600 and you get your line pressure of 1359 psi. A bit high but maybe necessary with small pistons in the calipers. Going to a 7/8 master will drop your pressure to under 1000 psi so small changes can make a big difference here.
Hope some of this helps
Cheers
Phil
 
Bill, all good advice here regarding MC and pipe sizing for brakes. Regarding your clutch MC and pipe sizing, you want to make sure you have an appropriate matching of MC size v. slave cylinder. In other words, a fairly linear relationship. If your MC is sized too big (large bore) for your slave cylinder then you end up with a really stiff clutch pedal and too much movement on the clutch arm - this can damage, and even break off, the pivot bearing/post. Not a good thing for a freshly installed engine and gearbox. While it's true that you can generally adjust the throw on the MC (and thereby, the throw of the slave) by adjusting the pick up point on the clutch pedal, it's desirable to keep the clutch MC piston plunger properly positioned relative to the piston bore (straight on, basically).

Phil's advice on utilizing a 1/4 line is definitely spot on.
 
I have the Wilwood 6 pot front,4 pot rear calipers on my Cobra, and use Wilwood master cylinders at 3/4" front and 5/8" rear. It's very easy to modulate, and will stand the car almost on end, if you get on them. I'm also using a Wilwood .78 slave for the clutch, and a 1" master. The 3/4" master that Wilwood recommended didn't have enough throw, and made the clutch feel too soft. I used 3/16" stainless tubing throughout the system. I sourced all the tubing and fittings from Inline Tube - Preformed Stainless & OEM Brake Line Sets and used AN type fittings throughout the car. AN flares are much more forgiving than SAE type flares, and can be disassembled numerous times without causing any leaks or other trauma.
 
We here in Nascar land use .750 for the clutch,1.00 for thr front and .937(15/16) rear.Utilizing 3/16 line, remember this is for a 3400lb car w/ 800hp running (short track and road course) (non superspeedway) 6 puck Brembo's front and 4 puck in rear.Also,i haven't heard anyone mention pedal ratio?That needs to play into the equation.That,most of the time is driver preference but is sometimes vastly different from driver to driver.So,i don't think there is a simple answer to the op's question on what sizes to use only a baseline.Unfortunately he may need to do a little trial and error to find the optimum set up,and don't forget the brake bias cable/knob assy.My .02

Good Luck,
Bill
 

Ross Nicol

GT40s Supporter
Bill
I agree with the 1/4" clutch lines and 3/16" brake lines.Don't worry about larger diameter for the rear lines I don't reckon it's necessary.The pedal ratio and resulting pressure in the lines was explained well, but not what happens at the wheel cylinders.The line pressure acts on the area of the wheel cylinder pistons so it is important to enter this in the equation when your working out a starting point for MC sizing.You didn't say you have a balance bar on the brakes but I think it has been assumed.If you are running a single MC on the brakes you will need a pressure reducing valve for the rears.I used the Tilton email help and gave them all my cars details, but found I still had to trim sizes for my own preference.If you intend driving the car on the track and (you will no doubt be standing on the pedal coming into corners) front to rear bias is critical as rears locking before the fronts will spin you off, probably backwards.A road car is much less critical but it is nice when you get the required pedal pressure to match a modern car with booster. I made all my own lines using a high quality USA made double flaring tool ABW Part # 70092. Takes a while to perfect the flares but it can be done.As I have found on the track the most effective brakes are to be had when you get the fronts to lock just before the rears.Unfortunately this is not easy to set as all these things affect the adjustment, road/track condition, tyres, tyre temps, speed/weight transfer etc. If it sounds like a minefield well yes that's not a bad description.
Hope this is helpful
Ross
 
Top