duragg Posted January 11, 2013 Share Posted January 11, 2013 Trying to understand the dynamics of high RPM shifting. Specifically why does my 1983 FS5W71B tend to grind slightly when shifting from 1st to 2nd at 7300rpm. When I rebuilt this tranny, i carefully lapped in the new brass synchros. My theory is that at very high RPM the tranny fluid builds up and is acting like a bearing surface, such that the syncro ring is "hydroplaning" for lack of a better term and can't do its job right away. Tried regular tranny juice and then Redline MT90 if I recall (no change). Might try something thinner next or the GM Synchromesh synthetic stuff. Seen a lot of posts about different juices, but wanted to understand the mechanics of the delayed syncro function which could shed light on a partial cure. Maybe they just all do that. With just 200whp and a 4.38 rear end that 1-2 shift is pretty important for me... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2013 Share Posted January 11, 2013 Do you have a light flywheel? It will help quite a bit. Less inertia means it slows the motor down a lot quicker. For oils I would go Synchromesh, SWEPCO 201, or Ford synthetic MT fluid. All are expensive, and I've tried 201 and Ford and they were about the same for me, I've heard others say that Synchromesh and Ford are about the same. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 17lb shaved but its a 240mm. Next round I would go much lighter. I've read dozens of threads on trans juice and others clearly indicate it makes a difference. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted January 11, 2013 Share Posted January 11, 2013 Try adjusting your pedal or clutch fork for a little more disengagement. Quote Link to comment Share on other sites More sharing options...
RebekahsZ Posted January 11, 2013 Share Posted January 11, 2013 Try adjusting your pedal or clutch fork for a little more disengagement. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 GM part number #12377916 is the Gm Synchromesh with Friction Modifier that nearly all the other forums rave about. I will change to that today and report. I have the throw about maxed out but will double check. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2013 Share Posted January 11, 2013 (edited) I've got an adjustable slave, spring and fork off of a 70/71 for sale if you want more adjustability. In the parts for sale area. Edited January 11, 2013 by JMortensen Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 I seem to have plenty of adjustability and certainly no problems in other gears. Just when trying to SLAM 2nd from a 7000++ RPM 1st gear sprint, my only hope in life is to shift quick and keep it spooled up. When I do that 2nd is notchy and sometimes grinds. Tried regular gear oil, then MT90. Just bought the GM SyncroMesh Friction Modified (AKA: GMSMFM) stuff. Spent a few hours researching last night and the other forums all swear by it (as much as any concensus could exist). This includes the NSX, Honda, Mazda and many Mustang forums. One member contated Penzoil about their SM oil but he displayed an email from Penzoil Tech who said the FM product was a GM exclusive. So, at $14 / quart I just swung into the dealer, dodging the Zombies and got the true GMSMFM. Always wondered if FM is more friction or less friction... Quote Link to comment Share on other sites More sharing options...
johnc Posted January 11, 2013 Share Posted January 11, 2013 Do you have any pressure plate or clutch made by Cetnerforce? Their pressure plates/clutches do not release quick enough at high rpm and cause a lot of problems with Nissan transmission synchros. Its inherent in their design, especially the ones with the little weights on the diaphram spring. Quote Link to comment Share on other sites More sharing options...
johnc Posted January 11, 2013 Share Posted January 11, 2013 BTW... the trans fluid most likely is not the issue, unless the fluid you are using has a lot of sulpher (GL5). Redline MT90 is the gear oil of choice for Nissan transmisisons with Swepco 201 a strong second. Both can be lighted with 50% Delo transmisison fluild if you want. Don't pay attention to other forums where the transmisisons are a 20 year newer design. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2013 Share Posted January 11, 2013 I'll disagree with John on the fluid. I've had several people who were running MT-90 say they really felt a significant improvement switching to 201. And yeah, thinning it down isn't a bad idea. That's how I used to run my 280zx 5 speed. I think synchromesh is already thinner. Friction modifier in LSD terms gets rid of the stiction that causes clutch chatter. Don't know what it would do in a manual trans. You've got it though, so give it a shot. You might see an improvement. Quote Link to comment Share on other sites More sharing options...
bigbore468 Posted January 11, 2013 Share Posted January 11, 2013 BTW... the trans fluid most likely is not the issue, unless the fluid you are using has a lot of sulpher (GL5). Redline MT90 is the gear oil of choice for Nissan transmisisons with Swepco 201 a strong second. Both can be lighted with 50% Delo transmisison fluild if you want. Don't pay attention to other forums where the transmisisons are a 20 year newer design. Completely agree with John that your 1-2 shift issue is probably not the trans fluid... You mention everything is in spec regarding your transmission, so other things to check out would be the clutch and release system as others suggested. Something you did not mention is confirming the bellhousing is properly aligned to the block; which will allow the trans input shaft to spin freely and not bind up against the pilot bushing/bearing. Check to make sure you don't have too much airgap (overcentering the fingers) between the clutch, discs and flywheel in the release position. Warped clutch disc(s) can also cause the low gears shift issue you have. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 Well then... this will be fun, won't it. Quote Link to comment Share on other sites More sharing options...
NewZed Posted January 11, 2013 Share Posted January 11, 2013 Did you rebuild with Nissan baulk/synchro rings or aftermarket? I've seen internet reports about "problems" with the less expensive rings although details were sparse. And did you record the ring/gear clearances, just for comparison? Was the 2nd gear clearance similar tothe others, or on the high or low side? I had a similar high RPM shifting problems with an old junk yard transmission and found that the Swepco 201/ATF blend was significantly better than both MT90 and Valvoline 75/90. I ended up at 75/25 blend. Just one more anecdote. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 Back for a moment to my original question about why does the syncro match the rpm at low speed, but not at high speed. I was pulling on the stick... so the 2 items were certainly being presented to each other. At that high RPM as the juice get slung out and makes a film that the synchro floats on momentarily, before breaking through and stopping the relative motion... is my neat little theory. Does a thinner sauce allow that to happen just a bit faster? This has been reported on Hybridz and other forums. Not a Centerforce, its is the Arizona Zcar clutch. Bearings and brass rings from Drivetrain.com. Lovingly lapped and carefully assembled. Tolerences were all measure to that manual spec. All spins nice and easy. This car being a big experimental test-bed, I hope this little test will yield some result (nothing or something..) Quote Link to comment Share on other sites More sharing options...
Leon Posted January 11, 2013 Share Posted January 11, 2013 Do you have a light flywheel? It will help quite a bit. Less inertia means it slows the motor down a lot quicker. For oils I would go Synchromesh, SWEPCO 201, or Ford synthetic MT fluid. All are expensive, and I've tried 201 and Ford and they were about the same for me, I've heard others say that Synchromesh and Ford are about the same. I'm not sure how a lightweight flywheel would help, unless he has a lot of clutch drag with the clutch pedal to the floor. In that case, the clutch hydraulics need to be adjusted. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 11, 2013 Share Posted January 11, 2013 I see where you're going. If the input shaft were truly freed from the crank, the flywheel wouldn't have any effect at all. IME it does make a big difference, so I can only surmise that the input shaft speed is always influenced by the flywheel even if you have a fresh bearing or bushing in there. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 Trying to remember my anatomy,.. the tranny input shaft has a bearing surface that rides on that brass bushing in the crank? I suppose it mostly just floats there, but in reality there is probably some transfer or energy from the 7000rpm motor into the decelerating transmission. So at the least, my clutch should engage as far "Pedal Out" as I can stand it to maximize release. Quote Link to comment Share on other sites More sharing options...
johnc Posted January 11, 2013 Share Posted January 11, 2013 If the pressure plate drags in release, the synchros have to do more work getting the clutch slowed down. There is always some drag on the clutch during shifts becase it still slightly contacts the flywheel and pressure plate. Minimizing that as much as possible will extend synchro life and give more positive shifts. Quote Link to comment Share on other sites More sharing options...
duragg Posted January 11, 2013 Author Share Posted January 11, 2013 (edited) And the Throwout bearing would be transmitting some rotational energy as well, not much but some. No, wait, that is wrong. T/O connected to cage side spinning on tranny input shaft and linked to fork. I can't visualize that part of it. T/O pushing on clutch housing, with its little bearings riding on tranny input shaft? Add it all up and it is something. The higher the RPM the more the chance of outrunning the Syncro. Making sense to me. Chasing that deamon just a little higher in the range should do the trick. Edited January 11, 2013 by duragg Quote Link to comment Share on other sites More sharing options...
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