Are you still running the clutch foot rest and making a point of people using it...perhaps a go pro down there for analyses?

A go pro down there is a great idea, I will probably make this happen!

Just ordered ACT 4 puck sprung kit, will run OEM 1.8 flywheel for now.

I'll throw this out there as a remote possibility - Ran across a post recently where a guy doing a 1.8 swap couldn't get his clutch to engage. Turns out the ball that acts as the fulcrum point for the clutch lever on the 1.8 transmission is taller than the 1.6. He was able to shim it and get the clutch to start working - and someone else chimed in that swapping the slave cylinder rod across would have done the same thing.

Just wondering if that combination of parts is doing the opposite in yours - preloading the throwout.

We have a few 5 speeds but only one 6 speed. I doubt we would be able to measure anything around the fork precise enough but we can simply run a 5 speed for one race to see if things would get better.

I'm not sure if the 1.6 and 1.8 finger heights are the same - sorry, all this third hand information - but I'd be curious to grab a na6 and na8 slave and check the rod distance.

New information:

So we've switched to 1.8 flywheel (machined by NAPA) with new ACT 4 puck spring clutch, new slave cylinder, SS clutch line, swapped from 6sp to 5sp

The internals of the 5sp went kaput after 3 hours of test day and 3 hours of racing on Saturday.

We are really proud that it took us only 2 hours exactly to swap that 6sp back. We got back on the track for another 2 hours on Saturday and 7 hours on Sunday. The 6sp has survived but it's runs REALLY hot. Even on the same day the 6sp is MUCH hotter than the 5sp. I wonder if any 6sp is much hotter or if that's this specific one which generates too much heat or if we are making this up?

Fluid level in the 6 speed correct?

clutch disk rubbing on the boss of the input shaft?

clutch disk not sliding on the input shaft?

what exactly do you mean by "collapsing"?

6sp takes 2QT and that's what we put into it.

"collapsing" I mean we are loosing clutch grip without destroying clutch surfaces, we assume we are loosing clutch plate pressure

OOOOOH. Proper explanation would be helpful next time.

One or more of your drivers is riding the clutch or shifting incorrectly. Or your clutch isn't adjust properly. You're GLAZING the clutch. Much different than collapsing PP.

Also how'd you nuke a 5-speed in 6 hours. Ours lasted close to 100 hours of racing before 3rd got a little crunchy.

Please show pictures of the heat shielding where you protect the transmission from the exhaust running right next to it.

Well, I assume riding the clutch generates more heat than a solidly-clamped clutch. Maybe the extra heat is killing the trans?

I'd re-do your pedal push rod adjustment to make sure there's absolutely, positively, unequivically no pressure applied to the clutch when your foot is (hopefully) not on the pedal. When done properly, that rod should be ever so slightly loose and rattly against the interior of the clutch master.

I would second that (readjusting your master cylinder pushrod) . If its too long it covers the master cylinder fill passage so fluid cant easily flow in from the reservoir or out to return the slave to atmospheric pressure, With the addition of heat this gives you something that acts like a spring that is always pushing on the fork. That creates all sorts of crazy symptoms including lack of clutch disengagement (stiff shifts) and lack of engagement (slippage). The result is you are hard on the synchros shifting and the clutch starts slipping particularly once things get up to temp. You will think everything is broken until you get the rod adjusted correctly. Its a bit difficult to judge slack in that rod so keep loosening until it is distinctly loose, don't be OCD about getting it just right err towards loose.
We have about 20 race weekends on our junk car clutch and flywheel. We did swap to other disks but never because the lining was worn. Replaced it once because we didnt like the lining material and again because the torsional springs were getting loose . We run 1.6 clutch because its lighter and with a stiff plate has plenty of extra clamp force for unboosted motors. We do run a ACT stiff pressure plate not because we need it but because it was handy. I like the stiffer plate because you definitely know if your foots pushing the pedal. I also bend the clutch and brake pedals laterally towards the throttle to create a space to plant your left foot and to help heel/toe. Seems with the average cage position and stock pedal placement it was hard to get your foot between cage and pedal and onto the dead pedal especially while sliding into a turn. I found I would often dangle my foot until turn exit and some of those times it ended up on the pedal depressing part way because the the stock spring is so soft..

As far as "high performance" disks we avoid them because some (metallic in particular) are really high friction that may go up with temp/slippage. The high friction causes clutch drag that is really hard on the synchros and flywheel surface. Also the HP discs often have really crappy engagement characteristics that make pit and trailer approaches a pain. If you think about the internal clearances a clutch seldom completely disengages it just slips a lot and bounces back and forth off the flywheel/PP if you increase the plate friction the synchros become the alternate clutch. In endurance racing this little bit of added synchro work seems to show up quickly as really shortened trans life.

As far as trans heat we found that you see true fluid temps and levels after a lot (hour or more) of track time, this may also be a frothing time when the fluid reaches a volume that includes air entrapment? Its worth checking your fluid levels hot after a couple hrs. or at the end of the event. While its level on stands pull the fill plug and see if anything comes out. Catch what does and subtract that from your cold fill volume (maybe after it cools and degases). Oil getting too far up into the moving parts can cause a lot of heat generation and come out the seals. Also if you have clutch trouble, open the slave bleed in the same warmed up car and If fluid squirts out you have the master rod adjusted too long. That ejected volume was the "spring" which of course doesnt really compress and is a trapped volume causing static clutch release. A stock clutch will slip from this and it doesnt generally show up until the cars been lapping a long time sometimes an hour or two. As well you'll come in, cool the car, do something, and then associate the non slippage with the change or driver switch rather than the cooling. It come back but a bit sooner the next time. If the clutch is getting marginal you should be able to exaggerate it by shock loading at turn exit and or as you approach the torque peak. It can easily be confused with tire slip as its first developing especially if you don't have a torsen.

Infrared thermometers are so cheap now and you can shoot the whole underside and have a heat map to refer to when issues come up. Its really doubtful you are getting heat transfer through air or the trans that is severely overheating other parts beyond your foot.

Also more often than youd expect places will machine the disc surface without taking the same cut on the shoulder that mounts the PP. This will show up just like a fairly worn clutch and give away a lot of clamp force. Its a good idea to check this dimension,

If the PP springs really "collapsed" ie has lost its tension you'd see this as less tension when tightening the pp bolts. Usually a sign of bad flywheel machining, worn plate or rarely failed (collapsed) PP springs. Overextension could do that but heat transfer doesnt seem feasible to me. Overextension might be a bigger risk in the trans swap if slave volumes or pivot geometry is altered a lot? I seem to recall a clutch pedal stop either adjustable or fixed because I had to ensure limited travel when I installed one of 949 clutches to another car.

Id put money on the master rod. Life is so much better when that's adjusted right.