My question is about the diaphragm style pressure plate.

Unlike many on this forum who have 383, 406 and even much, much larger cubic inch engines, I have a high revving 3" stroke small block. My hydraulic flat tappet cam pulls to 7 grand. Maybe even more but I don't want to break my mechanical tach.

Some of you might remember my post of last year asking if anyone had knowledge of the Schiefer Revloc clutch made many decades ago that I still have in my '69. Since no one knows about the Revloc, and I don't know how much life its disc still has left, I've decided to replace it with something more modern. And to get a lighter pedal as I'm not getting any younger and this Borg & Beck 11" has 'heavy pedal' in spades.

My only experience with a diaphragm pressure plate is with the OEM one behind a solid lifter rat motor I had in my '57 Nomad wagon. The pedal would routinely stick on the floor when shifted at 7200 rpms. I don't want that grief ever again.

So my 2 questions are:

1. Have the newest diaphragm pressure plates corrected that problem? If yes, what specific brand would you recommend.

2. With less than 400 hp at the crank, do I need an 11" clutch or would a 10.4" suffice?

 

I can't answer your question about the diameter, but Centerforce clutches have counterweights attached to the diaphram to prevent the pedal sticking to the floor stuff. I have a Centerforce II unit which is a direct replacement for OEM as far as dimensions are concerned. They also offer a Dual Friction clutch for more aggressive applications. I've seen several posts about having to change the ball pivot with these because of dimensional differences. Not a big deal if you know about it first hand, I'm sure others may chime in with comments on it.
I think it's an excellent clutch, no complaints from me, others have complained about the counterweights rattling a little when the car idles. Mine is pretty quiet. My hp is near yours although I don't pull the rev's you do.

 

I have run a Centerforce dual frction now for a few years. Mine is shifted at 7000 rpm quite often. I took the weights off the fingers, more advertising hype than anything. I found no difference in holding power or shifting. I run 500+ HP and it holds everything I can throw at it and I abuse it often. I know run a hydraulic TO bearing but did not for years and it never stuck on me.

 

Unless you are going with a 153 tooth flywheel and 10.5 clutch very little is gained by sticking a 10.5 on a larger diameter clutch faced 168 tooth.

Lot's of modern C&P's have very light pedals. But once you start getting over 6500 rpm I would be thinking all SFI rated parts and blow proof bell housings.

My Vette has shifted for years using 7500 as my red line and that is with both a 3.750 and 4 inch stroke

 

Thanks for the replies so far.

Yes, I certainly was intending to change flywheels to the 153 tooth size if the forum consensus was that 10.4" was adequate for my hp level.

I was thinking of a lighter weight 17# steel flywheel to replace the OEM iron one from the '64 pickup truck it came in. But am uncertain if doing so would be a benefit since I have the roadracing 2.20:1 first gear ratio to contend with. 3.70 rearend ratio.

And adding a blowproof bell gives peace of mind.

 

I run a 15 pound aluminum flywheel with an 11 inch clutch.

 

I've had issues with my lighter thinner flywheel and the stock linkage pedal setup. For some reason there is not alot of room for deviation from stock flywheel thickness without running into problems with pedal travel. my .02

 

There is if you go to the stock GM longer pivot ball.

O have the block saver plate which moves the bell housing back even father. My flywhell started out as a billet steel 30 pound and got machined thinner by 8 pounds down to 22# and still gets the SFI rating. 168 tooth 11 inch.

When you decrease the diameter to 153 the weight would need to be somewhat more to store as much energy. If I was you I would try around 30#

 

That's probably what my OEM weighs, no?

 

You would have to ask an expert. I've heard 40# and I've heard 30 pounds. The problem is TQ is a factor of cubic inches and you really don't have any.

 

I thought I was!

But apparently those who populate this particular forum do not have high-revving, small cube engine, diaphragm pressure plate experience. Of course, those who have graciously responded, excepted.

Could you point me to a forum more suited to answering my question?

 

I don't know that much about stock setups. I'm not a flat rate mechanic. I only work on higher end race cars. I don't know very much corvette trivia.

I was going by what you said: I have a high revving 3" stroke small block. My hydraulic flat tappet cam pulls to 7 grand

Your motor is not high revving because 7 grand is nothing especially with only a H-flat cam.

My honest oppinion. I would stay with a 22 - 30 ish pound because of your smaller cu. 10.4 inch 153 tooth flywheel and buy Something like a RAm clutch. Summit racing has them good for up to 500 or so hp for under $400 or the HP model for under $600. But I would call Ram, Hayes, Zoom, Centerforce and tell them that you want light clutch pedal and not having to go hydro clutch.

In all my years and in all the cars I have been in at tracks and on the street I have never experienced a pedal on the floor

My present clutch is a diaphagm. I would not recommend this DFX to you.

The DFX® line is the competition-proven clutch package for select import and domestic applications. This DFX series is engineered for competition standards of quality and craftsmanship. The DFX series is aimed at high-end applications that continually demand increased integrity and holding-capacity. DFX scores big, not only with holding-capacity and strength, but also provides some of the best drivability from a race-inspired clutch. DFX retains an easy pedal effort, thanks to Centerforce’s patented ball bearing actuated diaphragm, for smooth engagements without detriment to the vehicle’s release components. The Centerforce DFX performance clutch system is SFI approved for competition with Nodular Iron pressure plate rings and reinforcements designed to help prevent drive strap breakage and safety.

 

Thank you for your insight, gkull. I do appreciate your taking the time with my question. But I was not directing it only toward you. I thought more forum members would respond that, perhaps, once had a similar diaphragm PP problem. I swear I was not imagining the clutch pedal sticking on the floor, it actually happened. Yet, it appears, I'm the only one to experience such a thing.

I did find a reference in my dog-eared book, Basic Clutches & Transmissions by Hot Rod technical library, circa 1968.
"This type of pressure plate (Rockford diaphragm) is not recommended for use in cars where the engine will be turned over 7000 rpm because centrifugal force tends to hold the levers 'in' after a 'power on' shift."

The reason I posed my question was to find out if any progress had been made in the 40+ years since that tome was published.

I have not talked directly with PP manufacturers yet. But I did send in my question to Summit's Professor Overdrive. This was his response: "That problem was more likely caused by either A) wrong length throwout bearing, or B) Misalignment of bellhousing to crankshaft."

When I asked him why those 2 suspected problems did not manifest themselves at 6500 rpms, his reply was: "I am saying that those are the two most likely causes. The wrong TO length can over center the pressure plate, causing it to stick. We both know that is pushing a diaphragm clutch beyond it's intended use. My suggestion...stay with a B&B and use a hydraulic TO bearing. Diaphragm clutches are intended for street use, which I would limit to around 5000 rpms, realistically. If it's going to be run harder than that, the Borg & Beck is the way to go.

 

I'll go along with Gkull on this. I have driven many a mile with a diaphragm clutch and shifted amny times well above 6500 rpm and never had one stick to the floor. Now these were not stock GM clutches and yes there has been better clutches made in the last 40 years for sure. I don't agree with them on only shifting them at 5000 rpm or lower. There are plenty of diaphragm clutches shifted every day above 6500 and never a probelm. i had a B&B clutch once. It was the stiffest clutch I had ever driven and eventually cracked the fork. I tossed it in favor of a diaphragm and never looked back.

My brother in law had a built up 302 Z28 that shifted many times at 8500 rpm with a diaphragm clutch and never had it stick to the floor. I will agree with the tech guy that something was not adjusted correctly or the PP was over center with to much travel.

 

I've never had my pedal stick with this Centerforce but I have experienced it in exactly the same fashion as you describe many years ago with the three finger clutch design in a 57 Chevy. I used to have them specially made with heavy springs, you almost needed two feet to depress the pedal! At over 6k it would almost guarantee to stick between shifts. Maybe the technology has advanced and they've figured out how to make these diaphram clutches better so they don't stick; they were noted for that years ago even more so than the 3 finger type. I'm pretty sure you won't have any issues as others have also commented.

 

Yet I received a reply via PM from a CF member who concluded it by saying, "By the way, 7 grand is screaming."

So it seems each has his own personal prospective what 7 grand shifting is.

I like to think of myself as an educated consumer. If I'm going to tear into my driveline to replace this Scheifer Revloc relic of a clutch setup before the upcoming driving season, I want to spend my hard-earned $$$ on new technology that will last me the rest of my driving days. So far Centerforce is the name that crops up the most. I suppose I'll query them first unless someone comes along with a quote from their own PP's literature saying they have conquered the pedal sticking problem with their fill-in-the-blank patented technology.

 

I am using 153 tooth GM # 14085720 - its a 15lb nodular iron flywheel that was used on the 302 Z-28 motors. Clutch is Hays # 85-103 10.4 in Diaphram on the 360ci in my '69. Its 400+ hp

I just got the car running after a 2 yr project and like the clutch setup very well. Its firm and crisp - I am running a TKO600 (2.87 1st) along with 3;73 ratio out back.

My car does not see 6000+ rpm as its a hyd roller but I don't see a problem spinning this clutch/flywheel past that - I'd use a scattershield though

Good luck, Ed

 

I ran every malfunction scenario i could think of through my head. The clutch is disengaged, motor at higher rpm, tranny input shaft spinning or not spinning.

My best bet would be: The bell housing is out of alignment. You know off center. that causes binding of the tranny input shaft to the crank end bushing or bearing. It could also cause binding of the throwout bearing holding the depressed pedal to the floor. Older weak pressure plate or clutch fork/ z-bar return springs could not return the fork and pedal to static position.


Block and bell housing shift or even mass produced motors are not perfectly aligned.

When you get the new C, PP, & flywheel. are you familiar with how to align the bell housing?

My blowproof bell on my 79 Vette. I bent the rod arm on my magnetic base for my dial indicator and place it on the flywheel. I then rotate the motor over and measure the bell housing opening. I have the adjustable allen locking offset guide pins to tweek the bell housing left - right up - down. You center the bell housing in relationship to the crank shaft center line.

 

ED,

Forgive me for dismissing out of hand as not particularly fit for a roadracing vette ANYTHING that says 'truck' in its descriptive info, or 'especially good for 4x4's.' I normally just keep on looking. But it seems to be working for you, so best of luck. Now that I know, I won't rule it out for my application. Thanks!

Comp DIA 10.5 x 1-1/8-10 Hays 85-103 Performance Clutch Kits

HAYS COMPETION TRUCK CLUTCH KIT Includes Pressure Plate, Disc, Throwout Bearing and Alignment Tool

Gkull,

That dial indicator procedure for Lakewood's blowproof bell has not changed one whit since I installed one in my '57 Nomad. At the time, 1969, my best choices were over the parts counter genuine Chevy performance parts. But they didn't offer a safety bellhousing so I made a long distance trip to Honest Charley's in Chattanooga to buy one. Being in the military at the time, I knew how to follow orders and I followed Lakewood's to a T.

I've been working in the major wreck department of a truck repair shop for ~20 years and regularly replace busted up big rig bell housings using the very same technique. In fact, it's also required on the flywheel. So, I'm not some wet-behind-the-ears nebish.

But that is neither here nor there; just a clarification of my experience with one OEM diaphragm PP, and I didn't want to repeat it by foolishly buying another that might do the same thing.


Not me but a machinist friend checking out my work of assembling my old school .090" over 283.

 

No harm intended. I've torn up some motors, clutches, and trannies.

You never know the experience level on the other end of cyber space.

personnally i have only used 168 tooth flys and flex plates since the 80's. Easier on the starters and when it comes to longevity more surface area is better.

I actually drilled a lower attachment point on z-bar to firewall clutch rod to make my mechanical movement shorter. My seat is custom mountaed all the way back to the batter box and flush to the floor so my feet are nearly straight. I only have to move the clutch pedal a couple of inches to disengage.

Why I would not recommend my clutch is not just the cost, but it is overkill and noisy to a small extent. On a quiet car it would be noticable.