john gill

Hello All
I am about to remove my race car gearbox g28/55 for inspection and or rebuild , this is because under race conditions it will lock out 3rd gear when coming down from 5 th 4th etc , there are no other problems other than this , it shifts fine up and only locks out 3rd upon downshifts and only 3 rd gear . So I am posting here for any hints from this vast pool at what to look for and any idea on any measurement for wear specifications .I am aware that it has given good service and a rebuild is due .

This sits behind my stroker engine , 5.35 litre that has 400 kw at the wheels , itbs etc , and it has done some work , (track) this issue above makes me completely non competitive , would love to convert to another gearbox (6speed) to go down the sequential shift route .

However the repair of the above is first , I do have spare unit to fit but am scratching my head as to why the gate locks me out of 3rd , this only occurs after a few laps , the gearbox temp does get over 100 degrees c .
I should be able to post any log pictures as the nexus r5 that controlls it make this fairly easy .appreciate any thoughts and contributions .

Thanks
John Gill

SwayBar

Does it only start misbehaving at 100c/212f?

Are you running a transmission cooler and pump?

What trans oil are you using?

400kw at the wheels = 526 rwhp - that is incredible, especially for 5.35L!

The totally destroys any expensive stock-manifold stroker engine by a huge margine - not even close. Too bad there are no ITB kits available.

john gill

Does it only start misbehaving at 100c/212f?
No becomes evident after about 3 laps temps are below 95 degrees
Are you running a transmission cooler and pump? yes activates at 85 degrees , but am thinking of adding another unit hence more oil

What trans oil are you using? 80 - 140 mobil trans oil

400kw at the wheels = 526 rwhp - that is incredible, especially for 5.35L!

yes it goes hard (scarily so ) much beyond my talents (or lack of ) as a driver

The totally destroys any expensive stock-manifold stroker engine by a huge margine - not even close. Too bad there are no ITB kits available.
I machines all this up myself , the tricl is with the Ap pwer itbs and the straight passages into the cylinder along with the many hours getting all the intact tracks to match .
The stock manifold loss was working a measured 100 wkw

Thanks for the responce
JG

GregBBRD

John:

Our Carbon Fiber synchros would be a fantastic upgrade.

More information would help, in attempting a "blind" diagnosis .(The "devil" is usually in the details.)
What RPM range dies this occur? (Bouncing off of redline or mid rpm shifting?)
What do you feel, when this occurs?
(Does the gearshift lever stop in the nuetral position or is there movement towards 3rd gear?)
No grinding, no noise?
No kickback on the gearshift lever? (Is the lever "rejected" or just hit a wall?)
Does it come "cleanly" out of 4th gear, when this occurs? (No change in feeling from either 4th or 5th?
Any gears "crunch"?
(Common for these style synchro to "crunch" when the transmissions get hot and shifted quickly, but have no crunch after they cool down.)
And what do you do when this occurs? (Back to 4th or go to 2nd gear?)
If you go to 2nd, (even if you don't let the clutch oit) can you immediately upshift into 3rd?

john gill

Hello Greg

Our Carbon Fiber synchros would be a fantastic upgrade.

Wow yes dont get out much am very intersted in this didnt know they wer in existance , will check your website for detial

More information would help, in attempting a "blind" diagnosis .(The "devil" is usually in the details.)
What RPM range dies this occur?
Only occurs coming back down the gears form 4th , doesn't happen coming up from 2nd ,e.g. turn 3 at qld raceway ,which is hairpin after a long straight , coming down from 5th 4th and then wont find 3rd it goes into neutral but wont find the gate to 3rd can find 5th or 1sr from this position if not careful .
(Bouncing off of redline or mid rpm shifting?) this where I have run up the gears to redline 7000 rpm and then back down due to encroaching corner
What do you feel, when this occurs?
It fells as though 3rd has disappeared , note not noises form gearbox just cannot locate the gate to 3rd
(Does the gearshift lever stop in the neutral position or is there movement towards 3rd gear?) no movement towards 3rd at all , as above can go to 1 st or 5gth if not careful , found 1st at one and had a lockup spin .
No grinding, no noise?
none
No kickback on the gearshift lever? (Is the lever "rejected" or just hit a wall?) just hits the wall it is as if the gate has to 3rd has gone
Does it come "cleanly" out of 4th gear, when this occurs? (No change in feeling from either 4th or 5th?
Any gears "crunch"? none at all
(Common for these style synchro to "crunch" when the transmissions get hot and shifted quickly, but have no crunch after they cool down.)
And what do you do when this occurs? (Back to 4th or go to 2nd gear?)
If you go to 2nd, (even if you don't let the clutch oit) can you immediately upshift into 3rd? yes coming from 2nd is not a problem

Thanks
JG

GregBBRD

So, you are gear braking to help slow the car, into this particular corner?
Are you able to heel and toe?
Are you using ABS?
Are you trail braking into the actual corner or braking and downshifting in a straight line, before corner entrance?

What kind of shift mechanism are you using?
What kind of engine/transmission mounts are in the car?

Is the large internal spring for side to side shifting preloading original ?
Has the external adjusting eccentric ever been moved? (Under the large nut on the right side of the transmission.)

john gill

THanks GB your questions answered below

So, you are gear braking to help slow the car, into this particular corner?
Yes after the first miss it is then continuous
Are you able to heel and toe?
Yes up until the point of the lockout
Are you using ABS?
No all wheel speed sensors are there feeding into the nexus r5 for data, pedal box (tilton ) install with bias adjustment
Are you trail braking into the actual corner or braking and downshifting in a straight line, before corner entrance?
Yes this is the only way that I know

What kind of shift mechanism are you using?
Hans unit bolted to the torque tube
What kind of engine/transmission mounts are in the car?
Stock rear , and the a solid rubber units not the hydraulic originals

Is the large internal spring for side to side shifting preloading original ?
As far I know , this unit has been assembled and in the car for over 10 years now
Has the external adjusting eccentric ever been moved? (Under the large nut on the right side of the transmission.)
Not by me , but if it was do not know what is was

GregBBRD

Have you checked the rear shift coupler for bushing deterioration and the rear set screw to make sure it has not come loose?

john gill

Hello Greg

Yes it is good , I have made an inspection plate over the area to service inspect regularly and make it easy to work on , as I have had this issue in the past .
Although this time it is secure .

GregBBRD

The complete Hans adjustable shifter is fairly complex.
Have you checked to see if anything has some loose?

john gill

Yes it is all tight and secure up to the connection on the gearbox .

GregBBRD

This sounds more and more like an internal issue, which was your initial thought.
The 993 and 964 transmissions are extremely similar in their synchro/engaging teeth/slider/hub design.
The track cars would beat up the engaging teeth on the slider and this would become very difficult to move over the hub it rides on. This could certainly be an issue in a 928 transmission, also.
3rd gear synchro could certainly have galled on the engaging teeth hub....this is somewhat common, with this synchro design. (Internal moly coating on a steel or bronze synchro, depending upon what gear they are on....1st to 3rd are steel. 4th, 5th, and reverse are bronze.)
This synchro design has a much tougher time getting cool lubricant into the synchro/hub interface....causing this galling. Carbon fiber synchros virtually eliminate this issue, as the carbon fiber material requires almost no lubrication (I wish we had these, when racing 993 and 964 RSR's...we threw away many gears and synchros after every 45 minute sprint race. For the 12 hour and 24 hour events, it was a virtual given that all the gears and synchros would be junk.

Darklands

Greg,
are you pleased with the quality of the new production batch of carbon syncros?
You told last week you wait for the delivery.

GregBBRD

So far, extremely pleased.
They look fantastic, just studying them, out of their boxes.
I will check fitment, tomorrow.