Clutch master to clutch slave sizing

[mayhamfx]

Once again I am attempting to make an external slave work with my car. The last try ended in a horrible pedal feel and this try is heading the same way. I think I know whats wrong. I have a 7/8 master and a 7/8 slave so master cylinder travel equals slave travel which makes the pedal like rock and a super short stroke. I guess my only choice is to go to a 3/4" master.

Can anyone tell me what travel a stock LS1 clutch needs to disengage?

[texfc]

Mayhamfx, here is how mine is going, if it?s any help to you.  The slave is attached to the fork 5.75 inches from the pivot ball.  I measured the travel at this point from where the TO bearing first touches the fingers to the point where the clutch is free.  It was .68 inches.  On the master side, the piston travel was 1.00 inch from the pedal full up to the lower stop.  With both the master and slave both being 7/8 inch bore, the ratio will be 1:1, so giving some free play at the top, and some over travel at the bottom, I hope it will turn out the way I like the clutch to feel.  The big question in my mind is if it will have clearance in the tunnel.  The Granny?s guide says cut 1.25 inches off the fork, I think I took about 1.5 inches off to be sure.  I had hoped to put the engine in this weekend, but I found out Friday night that I have the wrong engine mount, it?s the manual steering mount and I need the power steering one. 

[carlb]

Mayhamfx, here is how mine is going, if it?s any help to you.  The slave is attached to the fork 5.75 inches from the pivot ball.  I measured the travel at this point from where the TO bearing first touches the fingers to the point where the clutch is free.  It was .68 inches.  On the master side, the piston travel was 1.00 inch from the pedal full up to the lower stop.  With both the master and slave both being 7/8 inch bore, the ratio will be 1:1, so giving some free play at the top, and some over travel at the bottom, I hope it will turn out the way I like the clutch to feel.  The big question in my mind is if it will have clearance in the tunnel.  The Granny?s guide says cut 1.25 inches off the fork, I think I took about 1.5 inches off to be sure.  I had hoped to put the engine in this weekend, but I found out Friday night that I have the wrong engine mount, it?s the manual steering mount and I need the power steering one. 

What's the advantage of an external slave cylindar?

[mayhamfx]

What's the advantage of an external slave cylindar?

When the POS starts to leak you don't have to pull the drivetrain out to fix it. (he says doing it for the second time this year)

[texfc]

+1 to mayham's answer.........also if it leaks, it's not all over your clutch and pressure plate, and it's away from all the clutch dust, which IMO is why the internal ones tend to fail early.

[mayhamfx]

Which fork are you using?

[mayhamfx]

These are the facts I have been able to determine today. Acorrding to the Howe website using a 7/8 master with the bearing I had will give .580 of travel per 1 inch of stroke. I measured how much stoke I had with the current setup and got 7/8". So .580 X .875 = .507 of travel with the Howe bearing. I had left about .100 clearance in front of the bearing so that mean theoretically I need about .400 of travel. I searched on LS1tech for the stock f-body slave stroke and only found one reference by someone who said they had about 7/16" (.4375) travel and was that correct to which they had no answers. So I can only assume that some thing in the .400 will work.

From what I can find there should be between .062 and .125 clearance from the clutch face to the throwout bearing. So assuming agian with this info approximately .500 of travel should surfice.

With the clutch fork I tried on the last external setup I had 3"  between the bearing end of the fork and the pivot ball and 4.5" between  the pivot ball and where the slave mounted. giving me a 1.5 to 1  mechanical advantage. Meaning to get the .500 of travel I need I should have .750 (.500 X 1.5 = .750) of stroke in my slave. To move a 7/8" slave .750 requires .451 cubic inches of fluid (.4375 X .4375 X 3.1416 X .750) (radius squared times Pi  times stroke)

Next I adjusted my pedal stop and got the maximum pedal travel I could and came up with 1.25

The smaller the master cylinder the easier the pedal will be to push. Basically the only choice of master I have is 7/8" or 3/4", I have a 7/8" and it feels like crap so I must try a 3/4". At 1.25 of master stroke it will give me .552 cubic inches of fluid, more than I need. I could move my mounting point on the fork out further but the tunnel gets in the way. 11/16" master would be ideal but it's not made.

[texfc]

The fork I'm using is AC Delco # 387001 used in a lot of applications, but as a referance it's from a 1973 307 Chevelle.  One option you might consider is going back to the Mazda master.  It is a 5/8 inch bore and assuming your 1.25 inch master stroke would deliver .383 cu/in to the slave.  That would move the 7/8 slave .637 inches.  If you need the TO bearing to move .500 and the fork is 3" from the piviot to the bearing centerline (same as mine) then it would need to attach 3.82 inches from the piviot. (1.274 ratio) Hope my math is right.

[mayhamfx]

One option you might consider is going back to the Mazda master.  It is a 5/8 inch bore and assuming your 1.25 inch master stroke would deliver .383 cu/in to the slave.

Already grabbed my old Mazda master off the shelf and measured it, the maximum stroke is only 1" so it wont work.

Yes your math is correct.

[mayhamfx]

OK on to pedal effort. Going by the info I posted earlier, with the Howe bearing I would have had a "hydraulic" advantage of 1.724:1 and no mechanical advantage. From what I can see the 2nd gen has a pedal ratio of about 5:1 so multiplied together I get a total ratio of 8.62:1. With my first attempt with the Wilwood slave I had a mechanical advantage of 1.5:1 and no hydraulic advantage, plus the pedal for a total of 7.5:1. This time I cut the fork down to 1:1 so my total ratio dropped to 5:1 (just the pedal). Going to a 3/4" master I get a 1.359:1 hydraulic advantage and if I go back to the 1.5:1 fork ratio I get a total of 10.192:1 ratio. (5 X 1.359 X 1.5)

Is any one agreeing with this logic.

[texfc]

Sounds good to me.  I found in the FSM that Mazda says the pedal ratio in the FC is 6.35 to 1.  I know your running an LS engine so it's about and inch futher back than my SBC.  I assume the tunnel width is the same on the vert and the coupe?  You stated that a clutch fork of 1.5 ratio was the longest you could use before hitting the wall (4.5 inches from the pivot)  Are you using the same fork, that has a bend forward about halfway down the arm?  Just trying to tell if my setup will fit inside the tunnel.  Do you happen to have a photo showing the clearance?  I got my hydraulic line in today, if I get some time tonight I'll try hooking eveything up and see how the pedal feels.  My setup is 6.35 pedal x 1 master/slave x 1.92 fork = 12.19 total

[mayhamfx]

I saw that the FD has a 6.35:1 ratio but I just measured my FC and got 5:1 ratio. I believe I'm using the same fork.



I've moved my rear mount out an inch from where it show in these pictures to straighten it out some.

[texfc]

Thanks a bunch for the photos.   Yes, looks like the same fork.  Did you swap the bleeder and inlet ports on the slave cylinder?  On mine, to put the bleeder on top puts the inlet to the outside.  I didn't know they were interchangeable.

[mayhamfx]

Did you swap the bleeder and inlet ports on the slave cylinder?  On mine, to put the bleeder on top puts the inlet to the outside.  I didn't know they were interchangeable.

Yes I swapped them it's just two holes in a cylinder.

[Yarp]

I love reading Grahams posts. Its like being back in shop class all over again.

I was planning on using the slanted Fbody bellhousing in hopes of not chopping on the clutch or or slave at all.