1989 240SX Hatch, early redtop SR20DET swap. new SPEC clutch, new problem(s)

[95lstegman]

for starters, i've searched like a banshee and found very little, although what i found i'll share shortly. this is very lengthy, and i'm sorry for that, but it's a very peculiar problem, and i want to give you as much info as i can because this has stumped every 240 guru i've asked, including Casey and Martin @ XAT, both very experienced guys with a lot of savvy in problem solving on these cars.

background:i've had this swap for a little over a year, and at first it was a headache getting the car and drivetrain up to date on maintenance, but since i finished that stuff off, it's been very reliable and a joy to own and drive. about a month ago, after upping the boost to 12psi on the stock turbo (with supporting mods), my clutch gave out, so i replaced it with a Spec Stage II clutch set. FYI, this is a windowed kevlar but otherwise full-faced disc with a sprung hub. no marcel spring that i remember seeing. i replaced the clutch disc and pressure plate, as well as the release bearing and pilot bushing, and resurfaced the flywheel. i did unbolt the engine mounts from the front subframe to get the trans back in. i lifted it up high enough that the mount studs came out of the frame all the way. after installing it, i disabled my wastegate actuator for 750 miles, and never once revved it over 3500rpm and i always rev-matched every up- and down-shift. it has been about 1,200 miles since i finished the break-in, and it holds all my 215whp/214wtq (needs leaning out past 4500rpm) with plenty of grab to spare. it slips easily and there is just a little chatter when slipping at very low engine speeds. it's rated for 380ft-lbs. also i've had Nismo motor and transmission mounts for a long time and never had any issues. also the whole car (including rear subframe) has ES poly bushings.

the crux:since i installed this clutch, there is a serious vibration that makes me afraid for my life almost in third and fourth gears. it occurs when decelerating or accelerating around 2500rpm, and, to a lesser extent, 2000rpm, although at 2200rpm, there is nothing, and by 2700rpm, it's gone. at 2000rpm, fifth gear also vibrates like the others. when cruising even at 2500rpm, there is still nothing. finally, between just over 55mph and just under 70mph, and only in fifth gear, almost any load condition will set off this vibration, except for modest deceleration. under acceleration, the vibrations at 2500rpm and the one in fifth at high speeds is not as bad as under deceleration, and under deceleration, the vibration at 2000rpm is not as bad as under acceleration. the 2000rpm vibration is worst in fifth, lesser in lower gears and i can't even feel it in second. the 2500rpm vibration is maybe a hair worse in fourth than third, but not present at all in any other gear. the high vehicle speed vibration only occurs in fifth gear. all of these vibrations only occur while driving in gear. pressing the clutch and/or taking the transmission out of gear have no effect. i have also tried to repeat these symptoms while sitting still, both with the clutch in and out, and with the clutch pedal down and with the transmission in and out of every gear, and in reverse. none of these makes the same problem. it does feel like something that is strongly bolted to the car, such as the rear end, driveshaft, or engine/transmission.

research:i heard from a seasoned veteran drivetrain tech that sometimes with a puck style clutch, the springs are not engineered to damp the clutch disc properly, and that in this vehicle that always presents as a harsh vibration in fourth gear only, and at 2500rpm. he said it was the rear end's pinion bearing having a harmonic vibration transmitted to it from the clutch, through the transmission and driveshaft.

my actions and opinions:first off, i've tried everything, from braking alone, braking while holding speed constant with gas, to shifting gears or changing speeds and coming back. these symptoms happen ALL the time, not just when it's wet out or when it's hot or after pumping the clutch or what-have-you. also, the vibration is significantly worse up a hill, and less bad down a hill, which i find interesting, and which corroborates the driveline tech's diagnosis (i think; summary of his input is below).i've since replaced the driveshaft bearing, which did nothing. since it presents differently with different gears, i presumed it was in the transmission. the driveline tech said only in fourth gear, which makes me wonder if maybe mine is just a little different and the other, similar symptoms are from the disc. i'm willing to live with this vibration, but i'd like to know it isn't detrimental to the car. judging by severity, i would think it would be. in any case, my hypothesis was that since the mainshaft's rear-most bearing is between fourth and fifth gears, perhaps that bearing is bad after having a rough time reinstalling the gearbox, which might lead to the gears at the rear of the mainshaft having vibration problems. i've also rotated the driveshaft on the pinion shaft flange each way, but to no avail.interestingly, i changed my rear end fluid for the first time ever (at 118,000mi) last week with Mobil1 full synthetic 75W90, and it seems as if the problem has actually gotten worse. i've been thinking of changing to something super thick back there, like 50wt motor oil or cheap 75wt gear oil or RedLine shockproof, to see if maybe the pinion bearing doesn't vibrate so much with thicker goo in there.the transmission oil is a mix of RedLine MT-90 and Mobil1 synthetic 75w90. it's been changed several times over the past year. i topped it off after changing the clutch.i also "checked" the u-joints by grabbing the driveshaft and violently shacking back and forth. no movement. also i sprayed them with WD-40 and immediately drove. no effect.i'm very comfortable with internal transmission work, but i've been racking my brains trying to crack this one without tearing up my car replacing everything. i'm back in college this semester and money is tighter.is there something i can do in the event that it is the rear end, like replace the pinion bearing and bring the gear teeth back into proper adjustment? i was actually thinking that maybe adding a small, weighted disc on the front of the diff's input flange might change the frequency at which the bearing and/or pinion shaft vibrate, but i haven't yet got a good idea for implementing this.also, i want to get the SPL rear subframe aluminum mounts. i would think this would make the rear end's movements really hurt me, but perhaps it could actually make it better? just wishful thinking maybe.
Modified by 95lstegman at 10:59 PM 9/9/2006

[180fan]

The SPL rear subframe collar/bushings will make the noise/shock more prevalent.

Is there any vibration below 2k or at idle?

[95lstegman]

not really. i mean it's not JUST 2000rpm, it's like 1800ish to 2100ish, but nothing below that. there was always a little vibration there, but nothing like this. i'm fairly certain now that it is in fact the failure of either the engineering or quality control at SPEC for the design of their sprung hub, and that the pinion bearing in the rear end is resonating. i wonder if a thicker fluid would dampen this motion, or if i could change the mass of the rear pinion shaft by adding a small metal disc to throw this off. or perhaps a thick piece of rubber wrapped around the pinion shaft? i don't know. i like the clutch for doing what it's supposed to do, but this is really stupid, especially for a sprung hub, non-puck style disc. the whole point of this disc was to avoid this kind of crap. if i'd known it was going to do something so stupid i would have bought the Stage 3.