Clutch pedal loss at high RPM

[BuddyP]

Have roughly 500hp motor mated to LS7 pressure plate and disc. Ran a couple track events without issue. Last year I had the tranny out for motor reasons. At next rack event I noticed clutch pedal loss on the track to the point I couldn't shift up or down after making a lap on the track running the car at high RPM. Once slowed down on track predal came back, same if I pumped the pedal. It didn't get spongy, it just went lower to the floor. I had since replaced the stock LS1 master for a Tilton. When getting on the car hard here locally I've noticed a slight loss of clutch pedal again. My fear is I'll have issues on the track again.

In doing some more research I ran across a post discussing this exact issue taking place with the LS1/LS6/LS7 clutch with "self adjusting pressure plate". Has anybody heard of this or had this issue? Not a big fan of pulling the tranny back out if I don't have to.

Here's the link I found.... see post #8
http://ls1tech.com/forums/manual-transmission/1623068-clutch-problem-wont-shift-after-high-rpm-hydraulic.html

Just want 2nd opinion before I spend more $$ I don't have.

[1BADBET]

You ran it at high rpm track days with no issue, then after the trans was out and then put back in the issue started correct? ....the problem might be something that changed in between the trans coming out and going in. That's the first place I look when I have a problem, if I didn't have this problem before what did I change that could cause this (its not always the problem but best start either way).... Only thing I could think of that you might have changed is the fluid, now I know that might sound dumb but hydraulics are more and more complex these days and systems are engineered to run with a very specific fluid. My Uncle has been doing transmissions for 40+ years, did a VW trans and the customer didn't want to pay the 20-25 bucks a quart for the trans fluid direct from VW so he put regular dexmerc in it and the thing went no where.... Didn't move. Drain fluid, spend almost $150 for 6 quarts of the VW stuff at a dealership and boom, problem solved, the thing runs like a top and shifts perfect.

I have no idea if GM did something like to make the pressure plate work properly but it doesn't sound like its out of the realm of possibility. I'd go see how much the GM fluid is and if it's cheap enough I'd try that. In all honesty if that's the case it might be what's causing all those guys to have problems too, and if they replaced the gm pressure plate yea that would solve it because the replacement wasn't designed for specific engineered fluid.

Just a thought... But I'm not a trans guy.

(Most guys who build transmissions aren't either! You wouldn't believe the insanity that has come through my uncles shop from the well known big name guys).

[BuddyP]

Yes that is correct...and why I noted it. But not sure what would've changed? This was my first thought. I didn't have anything apart on the trans. Only thing was pilot bearing replaced with bushing. But was suprised to find how much this happens when doing a search on the subject. None the less, good points, i'll start with fluid change.

[Centerforce]

The LS7 clutches are notorious for doing what you described, but it is odd that the issue just started after the trans was removed. Could be a coincidence, but then again it could be something else. Keep us posted with the results after the fluid change.

[1BADBET]

My uncle said that a few times they had to have a dealership reflash computers after taking the trans out to get the electronics working right. I don't know if the pressure plate is electronically controlled though. Might be something to look at too...

Make sure you bleed the system too.

[andrewb70]

My uncle said that a few times they had to have a dealership reflash computers after taking the trans out to get the electronics working right. I don't know if the pressure plate is electronically controlled though. Might be something to look at too...

Make sure you bleed the system too.

The computer has no input from the clutch. Transmission sends VSS, controls the reverse lock out solenoid, and also the skip shift, if plugged in.

Andrew

[AMC Racer]

Back in the day, diaphragm pressure plates were notorious for release issues at high rpm, so the Long-style was the go-to option for best high rpm performance. Modern diaphragm PPs seem to have mostly exorcised that demon ... but maybe some still have issues?

... but if it was fine before you changed from pilot bearing to pilot bushing, maybe should have stuck with a bearing and maybe have an alignment issue ... but doesn't explain the low pedal? Is the slave leaking, air in line, hose routed near the exhaust (boiling the fluid), restriction in line to the slave or fittings or something else change?

[BuddyP]

well for what it's worth, my "new" pilot bushing was wore completely out. It was press fit when I installed it, but fell right out of flywheel when I touched it. Also ID meaured .638 while input shaft measured .588. Not sure why if affected pedal effort but the whole reason I went with bushing was to not have failed pilot bearing issues. Try to do something right and this is what I get. Now input shaft on tranny needs to be looked at. Has a bit of side to side play, not sure how normal that is..... very aggravating.

[parsonsj]

Lingenfelter sells a kit for this problem:

http://www.lingenfelter.com/product/L360080197.html#.V08vtVeJVvU

I ran it on my Z06, and it solved the pedal sticking problem. If you're doing more than annual trips to the track, I'd look into this kit from Katech, which replaces the weak stock throwout bearing and master cylinder:

https://store.nexternal.com/katech/heavy-duty-masterslave-cylinder-kit-p331.aspx