hanseng1

I'm struggling to resolve some shifting issues and was hoping folks here may have some guidance.

Car is a C5Z with stock drivetrain, except for a Quartermaster 8.5" clutch and Tick MC. Clutch has been in for 18 months, has roughly 50% wear and everything worked fine until last December when the "grinding" started.

Beginning last December at Road Atlanta I started to get grinding on downshifts, particularly into 3rd, especially in hard braking zones like 10a. I assumed the synchros were going (transmission had 60k miles, and had seen 3 years of regular track duty). I shipped it off to Tick Performance for a rebuild in March, and now I still have the grinding on downshifts. Up-shifting is fine, and shifting into 2nd doesn't seem to grind; seems to be mostly into 3rd and 4th gear.

At this point I have to assume it's not the transmission, since it was grinding before and after the rebuild. What else could cause this issue specific on downshifts? I have the clutch set such that there is perhaps .25" shaft travel past where the rear wheel are able to free-spin. Giving it revvs to 4k or so with the clutch disengaged doesn't result in any movement, so I assume the clutch is fully disengaged.

Any ideas where to start with the troubleshooting at this point? How far can I go on clutch adjustment before I damage and pressure plate? If I recall correctly, slave piston clearance was around .19" when reassembled in May, and the clutch discs measured at roughly 50% wear.

Thanks in advance.

PRE-Z06

When you say 50% worn are you saying half way between the min and max spec of disc thickness?

hanseng1

Yes, each disc is half way to minimum thickness per the Quarter Master specs.

This clutch was installed February last year and I ran probably 9 weekends before the grinding started in December. If the transmission has been refreshed and it’s still grinding (actually worse now....) my next guess goes to clutch. But confused why the clutch would cause grinding only on downshifts. And I get a bit nervous adjusting the pedal too far for fear of damaging the pressure plate “fingers”.

PRE-Z06

I assume you’re rev matching on the downshifts? Sounds like it’s fully disengaging as you said and it’s not grinding the upshifts is why I ask.

rbl

If it is transmission you should feel it in the shift ****. Let someone else drive it and see if they get the same result.

Perhaps you are just trying to fast and even though the clutch has disengaged the trans has had a few seconds to settle. Slow the process a second or so and see if the result is the same. If yes, my guess is you have a bad transmission - rebuild or no rebuild.

hanseng1

Following up on this, last weekend I was at Road Atlanta, hoping to at least figure out what's causing some of my issues. I think I have a better understanding of the transmission grind now.

To start the weekend, I did a fresh bleed of the clutch system. Then I raised the pedal (Tick adjustable MC) about .25". Saturday morning the grind was gone. I ran two sessions with no issues, though those sessions were both very short (power steering temp issues limit me to 5-6 laps per session). However, things got progressively worse in the afternoon. Sunday morning the grind was back, though not as bas as originally. Did a new bleed at lunch with success. Also worth noting the clutch pedal feel was getting progressively worse.

So, confident it's the clutch and not the transmission.

Now with the car home and in the air, I noticed the remote bleeder line is fairly close to the header, and the rubber coating on the braided line was brittle. At this point I'm guessing the headers are cooking the clutch fluid. So next step will be to relocate and wrap the lines as best as I can, then take another stab at adjusting the pedal. Hopefully this cures the issue once and for all.

Thank you all who chimed in to help.

PRE-Z06

Ah, you never said it was heat related though yeah that makes sense now as it wasn’t dragging when cold. Definitely keeping it away from exhaust will help, but also running high temp fluid just like with the brakes will too. I experienced issues banging gears when launching at 5k+ rpm at dragstrip because of all the heat induced from slippage boiled it, what clutch fluid do you run?

hanseng1

The heat issue wasn’t apparent until last weekend as a consequence of paying closer attention to trans/clutch behavior throughout the course of the day.

i use rbf600 for fluid. Figured that should be more than adequate, but obviously not if the header is cooking the lines.