Transmission Issues?
 

I have seen a solid mount offered for a 3rd gen and am considering making something like that for a 2nd gen. the trans in my ITS car currently seems to occasionally have difficulty going into 3rd downshift. I have tried double clutching and all that, but nothing seems to work unless I hesitate in the neutral gate before I put it in third. I have had the trans apart numerous times and the brasses and shift forks are fine, actually very fine. I am thinking that maybe the engine is deflecting the transmission enough to make this shift a bit troublesome, it seems to shift fine at low rpm's as where there would not be a lot of torque load on the drivetrain, or when the engine is somewhat stabilized and not torqueing over. I have Mazda’s comp mounts on the engine and the stock rubber bushing in the rear.

So my question, would a solid mount be too hard on the trans and have any tendencies to crack the tailshaft ect... Or is there a different problem all together.

Thanks fellas!!!

The transmission mount is not the problem. Since the shifter is mounted in the tailshaft housing it could jump around and never change the length of the shiftrods in the tranny. Sounds like a worn out 3rd gear synchro. I have rebuilt alot of these trannys and the majority of problems stem from trying to speed shift the upshift and using the trans to stop the car on decel (forcing tranny into a lower gear). In my opinion these trannys do not do well when try to force them into doing things they aren't designed to do. I rebuilt the tranny in my Pro7 car when it was first built. I ran it for 4 seasons and 2- 12 hour enduros without any shifting problems. Slow down the upshifts and properly double clutch the downshifts and you shouldn't have any problems after a rebuild.

Good luck,
E

A solid mount on the tranny without solid mounts on the engine will cause problems. Besides, none of them are allowed in IT.

If the syncro is OK I would change the type of oil used in the tranny. Some synthetics actually work too good and don't let the syncros spin up fast enough.

I've always used stock mounts. The only trannys I built that had issues were my father-in-law doing the 3/reverse upshift (twice) and a guy who rented his car out and the renter would do a 4-2 shift to stop the car instead of using the brakes going into turn 11 at Sears Point. Man that guy could screw up a tranny and driveline. I still don't think the mounts will affect the shifting.

I used Redline 75/90 NS, and that did make a difference in a couple cars that had rough synhros

hummm, ok, the brass rings are all in good shape. The parts that are somewhat worn are the synchro ring that is part of the gear, or I think that is what it is referred too. When you guys rebuild these do you replace any or the gears themselves usually? Thanks for the help so far, much appreciated.

for the most part the trannys I have pulled apart were good. They just needed brass rings and bearings. My father-in-law destroyed the 5/R slider and hub both times. The renter guy hammered the 1-2 slider and hub to the point where you could see heat discolorization. If the bearings are good I would do all the brass synchro rings and go from there. I also like to use valve lapping compound on the ring and work the slider to smooth out deformities. Then clean really good before assembly. They usually shift like butter fresh. Also I chek the tolerence of the shift rod (long rod with the foot) to the shift shaft ends ( U shaped forks on rods to shift forks). That will take some play out of the shifter. I have seen a few Miatas get locked in 2 gears because of this. I still haven't figured out how to get 1 and 5 engaged at the same time. I've seen that on 3 daily driven customer cars, not even auto-x or track day cars.


Edit- I have been thinking about the mounts. If the mount is bad enough that the transmission is bouncing around, you will have trouble getting a gear. I don't think solid engine mounts and stock tranny mount will create an issue

You're talking about the dog teeth. These teeth can get rounded and not grab properly. Only answer is to replace the gear and probably also the clutch sleeve too.

I would also suggest using a less slick oil. Synchros can't synchronize if they can't grip the gear, and that's what can happen with the GL-5 oils with a lot of additives.

Dave

ahh yes, dog teeth, that is what I was refering to, thanks! If the gear needs replacing, and I bet it does, do you or anyone else know where to get stock gear parts for reasonable cost? Is this a part that mazda would have? Thanks!!!!!

Mazda has them but I wouldn't consider them cheap. Dunno if any aftermarket mfrs make stock gears, I doubt it.

You might call an RX-7 shop that does tranny rebuilds and ask if they'll part with a good used gear or two.

Dave

Here's a trick from the old time Datsun racing guys. Use a 50/50 mix of gear lube and automatic transmission fluid in the manual tranny. The ATF provides plenty of lubrication, but has lots of detergents that get the crud out of the synchros.

I have done this in six cars over the last twenty years, and always see an improvement in shifting performance. My 7 has well over 200,000 miles on it and shifts like a dream.

If your synchros are the cause, and they aren't completely worn out, then this might be an option for you. Good luck.

By the way, the above info comes from "How to hotrod and race your Datsun" which is a really nice book to have if you own any type of import racer. :)

Sometimes on the track I would get a little too excited and try to downshift into third while I was still going faster than 90 MPH. You can try to rev match and double clutch all you want at 95 MPH but you will have a hard time getting it into gear. Chances are good the syncro will be toast after one race driving like that. (it was for me)

Now I take a breath and wait for the speed to come down then I blip the throttle and the gear slides right in with fingertip force.

It really doesn't pay to try to shift faster than the tranny wants to.
Sometimes the problem is the nut behind the wheel.

hummm...good advice. I remember that if I would hesitate in the nuteral gate between 4th and 3rd, on the downshift, it would cooperate a bit better. I have had it all season, and some pretty shift intensive tracks and the synchros are still in very good shape, its the ole gears, the dog rings that are round. edmcguirk i think you're on to something. Thanks!!

And thanks to everyone else, all great advice....

There's no need to downshift very fast! You're under braking and merely have to be in the correct gear when you're ready to get on the power again. No sense in jamming the tranny down through the gears and then having to wait anyhow for the car to finish slowing before getting back on the power.

^ Exactly...

^not to mention its the best way to pop a motor....at least on a piston it is, not sure on the rotary.

i heat ya, the only thing that makes it difficult is that at the end of a straight, some of them, if I wait to do the downshift, an allow enough time for the trans to cooperate, then I am blipping the throttle on the downshift just before my initial turn in, makes it a little more complex. I thought maybe there was something with the trans I could do, but guess I will just have to deal with it. oh well.......

My current downshift habit is to pop the tranny into neutral when I first move from gas to brake.
I do not engine brake at all.
I wait until I am just about to get off the brake and then I blip and hold the throttle, push in the clutch for the first time, put it in the lower gear, let out the clutch, remove my foot from the brake, and start to get on the gas.

Once the tranny is in gear, the engine and transmission is totally under control of the throttle position and I can make a smooth transition to power by right foot control alone.

Even a tricky trail brake could be handled that way but I usually try to finish up the shifting while I am still in a straight line. Then I can enter the trail braking portion of my turn in without thought of gear shifting.

(although to be brutally honest, my car has not been quite predictable enough to fully commit to a strong trail brake)

RX7+mad shiftin skilz=$$$$