jwillman

I have owned my 79 for 5 years and been driving it daily for the last several months.

It occurred to me that not ever having driven a new 928 I have no idea what the shifting felt like in a new car/ fresh tranny versus my car.

I don't race or speed shift and I don't have grinding but as I drive it more I notice it is not sliding seamlessly into gear like my 2003 Hyundai 5 spd.

Its a bit firm into 2nd and 3rd and less so into 4th and 5th. I don't have to force it but it does seem to need a patient touch. Maybe I baby it because I know its a 36 year old car and components.

Car has 170K miles on it and assume its the original tranny. I am using Mobile 1 75-90 gear oil and changed it last fall.

Is this just a sign of the syncros getting worn? Is there something I should be checking / adding to the tranny to assist?

hacker-pschorr

When I'm driving an early car I randomly grab reverse when at stop lights to see if there is any grinding.
A transmission with good syncrhos will "hide" issues until the syncros are worn and they start to grind.
Reverse has no syncrho, so I've always felt this is an easy way to test and make sure the clutch isn't dragging.

If that transmisison has never been rebuilt at 170k, you are doing something right!!!

granprixweiss928

In my 81, My first to second shift is "harder" to do than those of 3rd-4th-5th as well. If I do it too fast it will grind a little. Kind of have to baby these early trans. especially when cold.

Same goes when at a light. I never sit there in gear with clutch depressed at a light, I always put in neutral and let clutch out. When the light goes green, I depress clutch, then have to wait a sec for the drive shaft to stop spinning, before grabbing 1st, or it can grind a little then also.

mark kibort

thats wise advice... plus blips on downshifts were the destructive forces can be much greater.
i always would use 2nd gear to stop the driveline before putting it into reverse or 1st. the higher gear synchros have less load on them to stop the driveline with the synchro itself.

the flyin' scotsman

I didnt know either until I drove another older 5 speed than my GTS.

From that driving event I knew my GTS could shift better so when I did the TT rebuild I also included the front and rear gear shift bushings.

The new bushings plus fresh gbox oil made all shifts very smooth............the rear bushings can be changed without dropping the TT.

jwillman

Thanks for the input. Front and back bushings and coupler have been done.

I will try some of the other techniques as well.

GregBBRD

??

Jetdriver69

After the 82 gearbox was rebuilt by GB, the difference was night and day.

Before I had clashing in 2nd and 3rd almost constantly, obviously the synchros were gone. Going into reverse was also problematic.

Now the shifting is very smooth with absolutely no clashing in any gear.

I modified the new CMC by shortening the piston rod 4.5mm, cut a slot in the rod end and cut 1 winding from the spring.

Now even going into reverse immediately from a cold start, no problems at all.

You can pull the CMC piston out through the drivers side firewall to do the mod, no need to completely pull the cylinder. Just drain the fluid or you will have a mess inside the car. About a 2 hour job total.

jwillman

Can you explain clashing? I know how tough tis is trying to explain.

I have no grinding into any gear.

What I feel is a hesitation like meeting some resistance and then it slides into the gear.

I can down shift with no issue.

First time I changed the gear lube I had a good inch long collection of metal powder on the magnetic drain plug.

No idea how long that gearoil had been in there.

At this point I am not dropping the tranny out of the car. Just trying to figure out where my car is against new/ GB rebuilds and if there is anything I can do to prolong the live of the tranny as is

GregBBRD

These transmissions have synchros and sliders that rely on friction to "match" gear speed to shaft speed.

The longer you spend going over the synchro with the slider, the more wear.

When I raced 911s with this style synchro, the synchros lasted virtually forever...because the shifts were very quick.

The downside to fast shifting in a 928 is the shear mass of what needs to be matched (in speed). The internal shifting components (brake bands, stops, energizers, and engaging teeth on the gears) can get (and frequently are) damaged in a 928.

Damaged internal pieces will generally be obvious. The same gear may upshift fine, but not downshift well....or downshift well, but not upshift as well.

In an early 928 transmission, the trick is to shift in one continuous motion, quickly (but not fast). If there is resistance, crunching, or grinding, there is an issue.

And as mentioned above....check the clutch operation frequently to make sure it completely releases.

mark kibort

Yep. and the upshifts are easier because you are bringing the rotating mass down in speed, that already has forces bringing it down.
the downshifts are harder, if you dont match gears with pre-rev and partial clutch release, because you are bringing the mass of the drive-line components up in speed and that takes a lot more force.

Originally Posted by mark kibort View Post
the higher gear synchros have less load on them to stop the driveline with the synchro itself.

the reflected inertia in the taller gears is much less than it is in the short gears. This is why the short synchros wear first, not only because they are used more.

the foundation for this is similar to the point i made last year about the forces to the engine and drive line, being greater miss matched rev downshifts in the taller gears, than the engine can produce at WOT.

GregBBRD

Yeah....

Actually, when the vehicle is stopped, the engine is at idle, and the clutch is depressed, the energy required to stop the rotating mass is exactly the same, regardless of which synchro you choose to do that job. Remember, the clutch disc, the torque tube shaft, and all of the gears (on both shafts) are rotating until the clutch is depressed. The only thing not rotating is the pinion shaft, reverse gear, and the hubs with the sliders attached.

In reality, because the lower gears are spinning significantly slower than the higher gears, there would be less wear on the synchro and the slider if the lowest gear possible (1st gear) was selected to stop the transmission, before putting the transmission into reverse. Stopping the transmission with a higher gear and then putting the vehicle into 1st gear is actually wearing two different synchros and sliders to do the same job. This, obviously, would wear both of the synchros and sliders used...a very poor idea.

I'm not saying that it's not a good idea to use one of the higher gear's synchros, to put the transmission into reverse, on a transmission that has significant synchro wear (less used, therefore they have less wear), it's just that your logic to do so is very flawed.

James Bailey

Yea I rarely down shift into fifth ......

WallyP

"Reverse has no syncrho,"

Don't know about the early boxes, but the later transmissions have syncromesh on reverse.

It can be hard to separate the dual-disc clutch adjustment problems from the syncro problems. Best to get both right.

GregBBRD

All transmissions, pre 1985, have no reverse syncro.

Makes it pretty easy, on these cars, to verify if the clutch is releasing properly.

Simply depress the clutch, count to 1002 and see if reverse can be directly engaged (without stopping mass by engaging another gear first) without grinding.

Try this both hot and cold. (When cold, the gear oil is thicker and will help reduce the energy from the spinning mass.)

If it can be engaged, clutch definitely releases.

It's a very good, quick test, to verify clutch operation.