How stiff should a diaphragm pressure plate clutch setup be?

[DRASTiK 22]

I'm currently assembling the car.  I haven't fired it up quite yet, but after installing the clutch setup I'm finding that the clutch requires an extremely high amount of force to push.....even for my 19 year old son, it's very tough.  With no seat in the car, I have to anchor myself to the steering column mounts just so push the pedal.  Otherwise it will just move me to the back of the car and laugh at me.

 

It is a smooth motion, and there doesn't seem to be any binding.  From rolling the car around the shop, it is definitely engaging and disengaging.  It just requires a very high amount of effort to move the pedal.  Is this normal?

 

The factory z-bar linkage is being used with all new linkage components (except the z bar itself), and roller bearings on the pedal assembly.

 

The clutch kit is http://www.specclutches.com/688207/

 

4 speed top loader, 302 late model block (347 stroker) with the aftermarket bracket to mount the z-bar to the block.  The transmission install went smoothly and no problems were apparent until I pushed the pedal.  I've adjusted the rod so that it won't fall out of the fork when at rest, but there is a minute amount of play in it.

 

I have removed the clutch pedal spring from under the dash also.

 

If I disconnect the adjustment rod from the fork, the pedal operation is smooth as silk and easy to move (no binding).

 

I'm not a big guy, but I'm not a weak one either.  I can squat 300lbs, so I don't think it's an issue of just having weak legs.

 

Have any of you experienced this? 

[barnett468 418]

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not all pressure plates are created equal . . they have different pressures and different leverage ratios . . if your engine is below maybe 350 hp, you can use a pressure plate that is just slightly heavier than stock which will still be very easy to depress.

 

i personally hate diaphragm pessure plates in cars thsat did not originally come with one . . i am a mcleod dealer and i helped them with their ford 3 finger pressure plates . . a 10.5 with 1850 lbs of pressure will be slightly firm but far from hard to depress and will handle 350 hp . . if you dump the clutch on it with 350 hp then i would use around a 2100 lb plate . . pressures over 2250 on a 3 finger ford plate start to be pretty firm but still reasonably doable . . 2400 lbs can require 2 feet from an average person.

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[Rsmach1 71]

The key to your issue is no seat, try it with the seat installed.

[barnett468 418]

The key to your issue is no seat, try it with the seat installed.

 

lol, yeah, a seat would be a good place to start

[DRASTiK 22]

Thanks.  I'll leave things as-is until I get a seat in there and then see what it feels like. 

[Rsmach1 71]

No problem, keep in mind although stage 1, it's still a performance unit so will be heavier than stock.

[Midlife 814]

The key to your issue is no seat, try it with the seat installed.

+2, and I would install the underdash spring.  It is called an over-the-center spring in that it assists (and reduces) pedal effort going down and coming back up. 

 

The lack of seat means you have no leverage which which to apply your leg against the clutch; your rump simply gets moved backwards in response.

[barnett468 418]

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actually, the clutch pedal can EASILY be fully depressed with just the over center spring connected, therefore it provides very little assistance in reducing pedal force but it can cause an odd non linear feel in diaphragm pressure plates, especially upn releasing the pedal.

 

it also actually INCREASES resistance until the pedal is depressed around 1/2 way which is why the pedal remains jammed against the stop if the linkage is not attacjed to it.

[DRASTiK 22]

I knew it would be stiff, but I've never dealt with one that was this stiff in any vehicle.  Having said that, I've also never dealt with one of these setups (mechanical linkage, top loader, etc.), so I'm not sure if I have a problem that needs to be addressed, or if the amount of force needed is just 'how it's going to be' in this scenario.   I didn't disassemble this car, the PO did.  Therefore I have no way to compare it to a stock setup because I've never driven one.

 

As for the pedal assembly spring, the research I've done suggests that the spring should be removed when using a diaphragm pressure plate.  The original spring that I have is broken anyway (missing part of the upper coil so it won't mount correctly), so I just left it out and didn't buy another one.

 

After I finish installing the wiring in the dash area, I'll get a seat bolted in and go from there.  I appreciate the replies.

[1969_Mach1 333]

I originally wanted to use a diaphragm type clutch in my 69 Mach1.  I didn't do it because I was warned they don't work very well with the stock clutch linkage.  I don't know about using the clutch assist spring that is under the dash with a diaphragm clutch.  By nature of the design, diaphragm clutches tend to stay inverted (in the disengaged position) at high RPM when you press down on the clutch pedal..  That plus the assist spring might cause a problem.

[barnett468 418]

I have no way to compare it to a stock setup because I've never driven one.

 

You can literally depress the pedal to the floor with your hand with a stock pressure plate if you are at least moderately strong.

[stangs-R-me 120]

I have all stock 351W linkage and Mustang Steve Ball Bearing conversion kit on the pedal shaft.

 

Went from a stock Ford 3-Finger Clutch & PP to a Centerforce Dual Friction and removed the assist spring.

 

Clutch before was always pretty heavy, clutch now is almost as light as a typical late model Mustang ... absolutely no complaints ... LOVE IT !!

 

Doug

[1969_Mach1 333]

I use stock clutch linkage for a 351W 4sp setup and a Borg Warner stock 3 finger clutch and pressure plate for a 390.  The pedal effort isn't bad at all.  With just moderate effort I can push the pedal down with my hand.

[DRASTiK 22]

Just to wrap this thread up, the previous posts were very helpful.  After putting the driver's seat in, the pedal effort was nothing to be worried about.  It's a bit stiff, but not nearly as bad as I thought it was.  The car runs and moves under its own power now and the clutch operation is smooth and functional.

[DRASTiK 22]

Well, maybe I spoke too soon.  I went to move the car completely out of the garage under it's own power for the first time, and heard/felt a pop while holding the clutch pedal down.  The pedal went very soft.  Here is a pic of what I found.  The rod went completely through the release lever.

[stangs-R-me 120]

Been there & done that too.   It was many years ago, so I don't remember if it due to fork being worn out or what.   Replaced it and the pusher rod with a new parts and have been good since.

 

Forgot all about it until I saw your pic !!

 

Doug

[DRASTiK 22]

I hope that's all I need to do.  I have ordered a new fork, and the rod is brand new already.  When I pull the trans out, I'll inspect the clutch, plate, and retainer thoroughly.  I've already spoken to SPEC.  They seem to be willing to RMA the clutch if necessary.  They insist that it should not be much more firm than an OEM clutch, so I'm not quite sure what the deal is. Obviously, a good bit of pressure is being applied to the fork.  The t/o bearing has free movement up to the pressure plate, and all looks normal from outside the fork hole.  I'll have the trans out in the next few days and see what I can find out.  

[1969_Mach1 333]

I'm sure you'll get it sorted out.  My 2 cents, I'm not a fan of diaphragm clutches with the stock linkage simply because the geometry of the stock linkage was designed to work with a 3 finger clutch and not a diaphragm clutch.  Then for those that really push their motors to high RPM's, by design of the Bellville spring in a diaphragm clutch tends to stay inverted at high RPM due to centrifugal forces.  Back when I was in high school is wasn't uncommon for a clutch pedal to stay on the floor when shifting at high RPM with diaphragm clutches.  When the engine RPM came down the clutch pedal would come back up. 

[barnett468 418]

+ 10

[DRASTiK 22]

Thanks for the replies. 

 

What 3 finger 'long style' clutch would you folks recommend for a powertrain setup like this? 

 

It's more powerful than stock, but it's not an outrageous build either.  The engine builder estimates ~450hp. 

 

This will be a street driven car with a trip to the drag strip on occasion.  Not for competition, but just for kicks.

 

Edit:  attachment below is not from me.   

[DRASTiK 22]

Well, I removed the clutch from the car this past weekend.  Didn't find any signs of problems anywhere.  I replaced the release lever, and have shipped the pressure plate, clutch disc, and throw out bearing back to SPEC for them to check it out.  We shall see.