Clutch master cylinder matchup

[Bill Musarra]

Given a size of master cylinder and slave, my pedal needs to go to the firewall to get the clutch to release completly. Very slight grind on down shift. The slave is a non exchange item as the way it is mounted. To get it to release sooner which way should I go for trying to get the action I want. Should I go to a larger master? Will this give me a softer pedal as well? Thanks in advance.

Bill

 

[Ian Anderson]

Hi Bill

Larger master moves more fluid and will mean the slave moves further for the same pedal travel.

But it will also mean a pedal that is harder to press

Ian

 

[Phil]

Bill:
Since the fluid movement is directly related to the piston travel see if it is possible to move the pivot on your clutch pedal a bit towards the pedal, this might give enough travel to do the trick.
Also, if there is any free play in the pedal before you feel resistance of the clutch you may gain there by removing any excess, you should have approx 1/2 to 3/4" before starting to feel resistance.
If those simple steps don't work, you will need to upsize your master, but not by much, probably 1/8" larger will do the trick.
Hope some of this helps
Cheers
Phil

 

[Terry Oxandale]

Bill,

To help me learn off your experiences, could you tell me what size cylinders (master and slave) you are currentlly using. I'm going to try a short push type slave instead of the pull that I have, due to space issues. But I've no idea what the 930 likes in regards to this ratio. I've got a 3/4 inch master right now, but feel I'd be better off finding the slave first, and then move to an appropriate master later.

 

[Bill Musarra]

Thanks for replying guys. Your info is very helpful. I went back over my build log(what there is of it) and I was discussing the various problems I was having with the master. In that thread I noticed I had left off the balance spring for the pivot arm. This will save the throw out bearing. So if I can't find it I will have to get another. I also figured out that the stop I have on my Tilton pedal, is set to just give me the 13-15mm of travel that I needed on the external lever. So I will have to give a little lenghtening to the stop to allow another mm or two. That should fix the disengagement problem. The pedal is still a little heavy to me but not having a clutch for a good number of years, it may just be me. Don't have a feel for how 54 lbs should feel.
Terry, if you go to my build log,
http://www.gt40s.com/forum/gt40-build-logs/13742-drb-5-a-7.html
and go to page 7, post 123 and look at the spread sheet of Tom's, you will find a lot of helpful information. My setup is Tom's normal configuration. He has or had an MLP setup which I think is an internal slave, not sure, but it will give you a softer pedal, just a little further travel. I may go to it later on. I have too many other things to worry about right now. My master is 3/4(for now). Could go to a 7/8 if I don't mind a little further push on the pedal. Will have to see. Take a look at some of the post after that as there may be more info that will help you out. There is a pic of the balance spring in post 140

Bill

 

[Terry Oxandale]

Thanks for the info. Have you considered a smaller increment of increase, such as a 13/16 inch master? Past experience in even an 1/8 inch increase at this size can result in a significant increase in pedal pressure required to release the clutch.

 

[Bill Musarra]

Terry,
I guess I could, but just an increase in the fulcrom adjustment should do me OK. My pedal is hard enough as it is. Might be tempted to go lower and put up with the extra travel. Might have to move my seat a little forward for that though!!

Bill

 

[Dave P]

Bill:

Not sure but you may check your travel to cut off point. Which is the point the seal covers the intake hole in the master cylinder. you may be able to squeak a couple turns inward on the threaded rod and thus increase stroke a little after the cylinder starts making line pressure.

 

[Bill Musarra]

I want to raise an interesting question. The amount of force required to get release is in the 42-54LBS. range. Tom in another thread installed an MLP setup on his 40 and got the effort down to 42LBS. I asked where he found the setup but never got an answer. Is it an internal slave setup? I don't know about you guys but that is a lot of force compared to a lot of other car makes. With road courses(fast shifting) or for long drives(city,stop and go) there is a lot of effort in shifting the gears. Has anyone looked into the idea of using a power assist mechanism(hydraulic or electric) or is this an idea that can't work?? What are the differences in a swing pedal setup compared to the Tiltons as far as effort and pedal travel(never having seen one).
Most american cars, I will assume from the old days, use the mechanical linkages to multiply force and the pedal pressures are way down compare to the 930. I do remember the HD clutch setups were a little heavier than the stock units. Are there ways to make the force required less, or will I have to live with a smaller MC and longer travel?? How much will the effort go down(on a 930) if I went down from a 3/4 to say a 5/8 MC?

Bill

 

[Terry Oxandale]

I figure an effort (rounded to 2 decimal points) that is 70% of what you'd have with the 3/4 MC. Expect a comparable increase in pedal movement though.

 

[Bill Musarra]

I have been doing a lot of work on the car and now am ready to redo the clutch problem. I tried to adjust the clutch with a 5/8" MC, and found that no adjustment would keep the gears from grinding. I figured it had to be the extra distance required that I needed for it to work. Swaped out the 3/4" MC and got the same result. I then looked in the view hole of the clutch and found the "ears" of the throwout bearing were not engaged. It is a close fit and looks can be decieving. This resulted in me having to halfway pull the trans and readjust the fork around the ears. With this settled I decided to stick with the 3/4" unit. I found out that with the "balance spring" in place the lever at the slave end stuck out about 4mm. I addid a small throtle spring and it still didn't quite close the gap. This may seem like only a little bit of difference but the 3/4 MC only travels 24mm. So wasting a few mm can make a difference. In my case the clutch would not reslease with 16 mm of travel at its max(needs minimum of 12). So I took the balance spring off and set the slave(with the help of a medium size C clamp) to 0 mm. This allowed the adjustment screw to be utilized to close the gap between the working side of the lever and actual rod that controls the fork. I then put a big C clamp on the working side of the lever and got the backside of the lever to travel 17mm and got clutch release.
My question of you 930 guys is this. Suposedly the balance spring is to keep a preload off the TOB. Constant pressure will cause it to wear out acording to the Porsche guys. On the other side you have the spring pressure of the slave pushing the fork against the TOB to maintain contact and limit the travel required for release. If you look back at Tom's spreadsheet the numbers tell me that I need all the tightness I can get. Read another way I can't aford any slack in the system. The TOB rides normally on the pressure plate fingers and seeems to be turning just from the friction of the system. Now there is no "balance spring" on the G50 boxes(that I am aware of). And the setup is strickingly similar in setup. So I am asking whether I should go without the spring or just add it to this 0 clearance setup to remove "most" of the pressutre on the TOB. When you add the spring, it will pull the lever back but not all the way back on its own. That is why I added the small throttle spring. but adding it also didn't pull it back to a zero setting on its own. These springs add to the amount of pressure required to mash the clutch pedal. Probably more than the 54# for the G50.
Opinions??

Bill

 

[Bill Musarra]

Here is the visual

 

[Jim Cowden]

Bill

They havnt run springs on clutch forks since the mid 70s.
Piss it off, it only makes light contact it wont rear it out.
All modern cars are in constant contact.

Your pedal effort may come from your p/plate design.
Newer designs have the pivot ring further to the outside of the plate and gives a lighter pedal.

The lever in the picture looks fabricated.
Is the ratio the same length as the std porsche lever or have you changed it.
I am assuming you are using long bore tilton master on the clutch and not their short bore version.

Jim

 

[Bill Musarra]

Jim,
To answer your questions.

I am running the 74 series which are the longer units.
The pressure plate/clutch unit are from Kennedy and are the standard for the 930. As most of the 930s are from the late 70s that is what mine is. The lever and slave(996 I believe) are custom done by the manager of Jim Ellis Porsche who has built several mid engine setups. His help has been invaluable. the ratios are stock Porsche 2:1. The fork is untouched and thus stock. The trans itself comes from a major builder of Porsche trans for racing. For an interesting story of this trans see 930 saga.
http://www.gt40s.com/forum/gt40-tech-powertrain-transaxles/18166-930-saga.html
I have lost his email address somehow or I would contact him and get the skinny on the balance springs. I have seen the parts exploded drawings and they show the spring. It is not a weak unit and adds a lot of force to the pedal for disengagement. Normally the TOB rides on and surrounds the edges of the fingers of the pressureplate. It sits freely and is easily spun as it sits there, making the engagement of the ears to the fork an easy task as long as you watch and adjust their location as they come together. The TOB is a multi piece unit and is able to move with the pressureplate spinning(the ears remain stable with the fork). That means there is contact all the time(without a load), and I wonder how long the bearing would be able to take it. The pressure of the slave is equal to the balance spring and that comes out to be around 40#. When mashing the pedal that is added to the pressure of the pressureplate(total now about 80#). With the spring on the lever, the slave is balanced about 4-5 mm away from the zero point in the pic above. I have been told by Porsche mechanics that the adjustment pin should be off the short bar lever, by 1-2mm. That adds up to 5-7mm off zero. Then I have to get to 12 - 13mm for disengagement. That brings the total to about 20mm. The slave can go to around 28-29mm total. At the zero point the lever is at right angles to the slave piston. As it is pushed away the angle decreases and the amount of travel required to get to 12-13 increases.The MC remember can only go to 24. That is why I have trouble getting the clutch to disengage unless I can get the lever to the zero point. As I add springs, even light weight ones, the pressure required to push goes up. These are added to the amount of pressure of the pressure plate. What bothers me is that any pressure of the TOB on the pressure plate will cause it to be moving all the time(under load) and may well cause it to wear out prematurely. Plus my left leg may get hypertrophied from all that pushing!! Pulling the trans in my car is a bitch. So do you think the bearing is up to it?? I think I will give Kennedy a call tomorrow for their take on it.

Bill

 

[Jim Cowden]

Is your pedal ratio an issue.
It will give high pedal pressure and lack of disengagment if the ratio is not right.
Have you compared it to a porsche.
If everything else is porsche and your ratio is the same then it may come down to a clutch and not a hydrolic issue.

From memory you have a steel line from master to slave and just short flex line to cylinders.

I had similar issues on an M3 that I helpout on.it had a heavy clutch due to the 550rwhp output and from day one it was a pain to disengage.
It was just of the floor a couple of mill.
About a month ago it welded the ceramic plate to the flywheel when it had a little clutch slip.
We replaced it with a different setup and the clutch is picking up a few inches of the floor.
The issues related back to the pressure plate.

Heres an idea
Get a porsche and at a given point on the lever on the box measure the amount of movement it has compared to yours if you are on the money then it is likley the pressure plate or who knows maybe the clutch linings a .5 mm to thick, it dosnt take much to cause an issue.


Jim

 

[Bill Musarra]

Jim,
The clutch is a standard clutch setup from Kennedy, nothing special. The pedals are the old style aluminum units and from memory have 5.5:1 ratio. The new pedals from Tilton are "adjustable" from 4.5 to 6.0(the cheap ones). The more expensive ones are 5.29:1, 5.44:1, 5.61:1, and 5.75:1. In their tech drawings the pedal pad height is adjustable.
I happen to have the day off so I will take a trip out to the Porsche dealer and have a talk with some of the techs there as well as talk to Kennedy this afternoon.

Bill

 

[Bill Musarra]

After talking with the mechanics at the Porsche dealership about the balance spring they gave me the latest info. Seems that when they came out with the 933, the next generation of the 930, they decided to leave the balance spring off. The reasoning is that with the spring off, the setting for the clutch, once set is a set it and forget it. As the clutch wears, the mechanism will move in concert to keep it in conact with the TOB. So there will be no play in the clutch. I have just adjusted the lever to 1mm away from the slave face and the clutch releases nicely. I am able to put the clutch pedal where I want it(in front of the brake pedal, even with it, or in front of it) and it still releases easily. The pressure on the pedal is tolerable in that my leg shouln't get sore from a leisurely drive. Once I have a feel for all this it will go into the shop for the final suspension settings and hopefully a trip to Road Atlanta and other tracks in the area... I have set the stop mechanism so that there is a little extra movement in the clutch and should not experience any grinding issues.

Bill