Porsche 915 Transmission Video - Case off Demonstration

[Gordo2]

Made a video showing what's happening inside the 915 transmission as you shift between gears.

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Enjoy.

Gordo

[MARISOL78sc]

Great Video! Very informative.

Paul

[timchar]

Nice work Gordo. What's the phrase, "picture is worth a 1000 words". Really helps give perspective to guys like me. Followed your transmission rebuild, great work. Tim

[proporsche]

Gordo2..nicely done..good as a learning tool ....

Ivan

[tdw28210]

Super cool. Thanks Gordo.

[356911930]

Nicely done!

Rahl

[gazzerr]

Awesome! Thanks Gordo!

[0396]

Thanks, first time I've ever seen anyone do a video showing the gears running.

[Knockdown]

Cool Video, Thanks!

[Bill Douglas]

Thanks.

[Gordo2]

Thanks for the kind comments folks.

I actually made another video attempting to explain what happens when you double clutch, and why it helps reduce synchronizer component wear.

I have a firmer understanding how it works & helps now that I've torn the trans down and could observe the components moving together - but it was still difficult to try to concisely explain while reducing my tendency to "ahhh... uhhhh... crap" my way through the voice over.

For the curious minds:





Gordo

[Bill Douglas]

Thanks for the educational Gordo. I really didn't have a clue what went on in there.

[DRACO A5OG]

Good Job Gordo!

Hey I like your CMS bracket, welcome to the club :-)

[Duc Hunter]

I am working my way through Gordo's videos. Very informative. My 915 is supposedly recently rebuilt. It is very stiff shifting. Some times no matter how hard you push it wont go into gear (could be 1st, 2nd, 3rd, 4th). You can pull the clutch out, put it back in, and it goes right in like butter. Other times it just drops in gear like its a G50. I bought the car with this tranny (car was not running). My local mechanic says they get stiff like this when people rebuild them with new parts, and spring etc. I think he said the lock rings etc get really stiff. When he use to build 915's for his race cars he used matched parts that were good and used as they worked better. He says a properly rebuilt 915 can shift almost as good as a G50, not like mine.

It doesn't crunch going into gears. The synchro's feel fine as they engage. Any thoughts on this behavior?

[tobluforu]

Wow, I never really knew how that all worked until now.

[Sunroof]

curious to know if the entire gear area is fully submerged in transmission oil or the gears drive the fluid up for lubrication. Could not see the tranny fill port.

Great and informative video.

Thanks
Bob
1973.5T/915

[Gordo2]

Originally Posted by Duc Hunter

... My 915 is supposedly recently rebuilt. It is very stiff shifting. Some times no matter how hard you push it wont go into gear (could be 1st, 2nd, 3rd, 4th). You can pull the clutch out, put it back in, and it goes right in like butter. ... Any thoughts on this behavior?

Duc,

I observed what may be the same condition you are describing as I was playing around with my trans with the case off.

I think I understand what's occurring, but I'm not sure why or what can be done to prevent it.

If you want to understand what's happening, I think you first need to understand how the synchro's work. The following info is from my trans rebuild thread:

Gordo's 915 Transmission Rebuild (video explains how the synchro assembly works)

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How Does the Synchro Assembly Work?

(1) As the shift sleeve ("slider") moves to engage a gear, it begins to slide over the outer edge of the synchro ring.

(2) The synchro ring rotates and compresses a small amount as the slider begins to move over it. The synchro ring's initial rotation is independent of the rest of the 1st gear synchro assembly.

(3) The initial rotational movement of the synchro ring is limited by the energizer block's raised nub. As the synchro ring rotates and comes in contact with the energizer block, the energizer block begins to push against the brake band. The brake band can't rotate since it is semi-fixed to the synchro hub with the anchor block. The force of the energizer block pushing against the brake band and presses / levers the band against the inside of the synchro ring.

(4) At this point, 1st gear is fixed to slider (due to the friction between the slider and synchro ring and the locking of the brake band) and the rotational speeds of the slider and synchro ring becomes equalized / synchronized.

(5) Once the rotational speed of 1st gear matches the speed of the slider (synchronized), the tension between the synchro ring and energizer block is reduced, and the brake band is unlocked. This allows the slider to move further over the synchro ring, and the slider teeth to begin to engage with the teeth on the synchro hub.

(6) As the gears are mating, the 1st gear assembly can rotate freely (slightly) to allow the slider and synchro hub teeth to fully align / mesh and engage.

(7) At this point, the synchro ring uncompresses and expands into a small recess formed at the center of the slider's teeth. This locks the slider and synchro hub together; keeping the gear from popping out.


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When the Shifter / Gear Feels Locked Out / Blocked and Won't Engage

The problem seems to be associated with step (5) in the above process.

Keep in mind, the brake band intentionally (by design) blocks the shift sleeve from engaging the synchro hub's dog teeth prematurely (to avoid gnashing), but should release once the two components have equal rotational speeds / are synchronized.

Meanwhile, I found that sometimes the brake band would not release / unlock to allow the shift sleeve to move the rest of the way over the syncro ring and engage the synchro hub's dog teeth.

When this happened I found that I needed to manually rotate the synchro ring (spin it in the opposite direction) to unlock the brake band, and then re-engage the gear.

I assume something similar occurs when the engine is providing input to trans:

- You attempt to shift into a gear, but it feels locked out (because it is...)
- As this occurs, you release tension on the gear shifter (which backs the shift sleeve off of the synchro band - allowing it to expand), and release and depress the clutch again
- As you release the clutch, the rotational (centrifugal) force of the input shaft unlocks the brake band (in essence)
- You then push the clutch in again, re-engage the gear and it shifts without issue.

Again, I observed this in my transmission as I was playing around with it, but have no idea why it occurs or what can be done to preclude it. I was hoping that the issue was associated with my low speed / simulated demo shifting, and that I wouldn't experience the problem in normal driving / shifting - but wasn't too confident that would be the case...

I did notice that it only seemed to happen with 1st & 2nd gears and not 3rd or 4th. I replaced the 1st / 2nd brake bands, but re-used them on 3 & 4 - so there may be something to that.

Sorry, no recommendations toward anything that can be done to address it, but will think about it more (especially if mine shifts that way).


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Originally Posted by Sunroof

curious to know if the entire gear area is fully submerged in transmission oil or the gears drive the fluid up for lubrication. Could not see the tranny fill port.
Bob
1973.5T/915

Bob,

The gears on the output / pinion shaft are roughly half way submerged in the trans oil, as is the differential. They splash lubricate the rest of the trans components as they spin.

Gordo

[Bill Douglas]

Another one for you Gordo... My 915 had a rebuild sometime ago, but ever since then it will only go into 5th if I do a really fast shift from 4th. If I'm slow it won't go in and I have to try again but fast. Weird I know. Any ideas?

[JackMan]

subscribed, ty Gordo, looking forward to more goodies from you

[cabmandone]

Great video brother! Now just remember all of this so at some point in the future we can redo mine.