Downshifting 915 question

[Charles Freeborn]

Here's one for you transmission experts.
Is it likely to damage a 915 box downshifting from 5 th to 3rd (skipping 4th)?
At my home track at the end of the front straight turn 1 is a hard right in 3rd, and depending on tracffic, sometimes 2nd gear. Will I cause damage by not running through the downshifts in order?
Thanks.

[Jon B]

Charles,

There's no harm in downshifting a 915 out of sequence, as long as the car is traveling at an appropriate speed for the gear you select.

[heimtun]

While I'm no 915 expert.
As with any trans, I would also suggest double clutching and blipping the gas to match engine speed. UMMV

[NYNick]

I find it difficult believing you're even asking this question.

[DRACO A5OG]

Get a Gate Shift Kit to help prevent this from happening again by accident.

[CCM911]

The only rule with downshifting is to be sure to match wheel speed to engine speed.

[Charles Freeborn]

Perhaps I should clarify.
In my car I'm topping out at about 115 mph on the front straight. Approaching the chicane, which is a tight right-left combination (as most chicanes are) I lift at about 700' brake hard beginning at 500', heel - toe the downshift to prepare for the first turn, which is a 45'ish mph turn in my car. So I'm dropping about 60+mph in the space of 500', with hard braking and shifting in the middle. A lot is going on.
My 911 is relatively low power comparatively, so I try to carry as much speed into the first turn as I can. By skipping the downshift to 4th I get a few milliseconds more time to maintain momentum. Drivers of higher power cars slow even more and take the turn in 2nd, because they've got the power to rotate their cars around turn 2 with throttle. I have to do it with momentum, and by braking late I make up distance here. Track direction is right to left on this diagram:


Not being a transmission expert, I'm only asking if there are gears and pulleys in there that don't like to be handled like that. It's not a matter of missing shifts (though I would like a gate shift, my car shifts very well with new bushings and couplings), or not matching speeds (I do all that). Transmission is currently in good condition with fairly recent synchros and sliders replaced, and I'd like to keep it that way. I change the gear oil every year with Swepco and overall maintain the car very well, but it is over 40 years old...I was hoping for a reply from Matt or another transmission expert. Perhaps I should simply contact him directly to avoid the sarcasm.
-C

[Matt Monson]

There's nothing wrong with that shift. Just watch for nicking reverse.

Ps. I would make the argument that Jon B knows more about gearboxes than I do.

[Charles Freeborn]

Many thanks Matt. Yes, grazing reverse can be an issue with these old girls. After a few tries I got the coupler adjusted correctly and haven't had that problem.
-C

[NYNick]

There's no sarcasm here. I still can't believe you're asking the question.
Other than the obvious, which you covered in your clarification, and the reverse issue, and the fact that you're an experienced tracker, I, uh, oh never mind.

[911pcars]

With scheduled gearbox overhauls, you can do anything you want (within reason). When downshifting from any gear, the synchromesh is speed matching the different rotating speeds of the input and output shafts. Downshifting 2 gears means the synchros have that much more rotational speed differential to speed-match. Thus, more synchro wear. Rev-matching is after-the-fact.

heimtun's suggestion is a good one. Double-clutch when downshifting. This reduces stress on gearbox parts and when performed correctly, will minimize chassis unbalance during the transition from decel to accel. Rev-matching is after-the-fact.

MHO,
Sherwood

[Charles Freeborn]

Originally Posted by 911pcars

With scheduled gearbox overhauls, you can do anything you want (within reason). When downshifting from any gear, the synchromesh is speed matching the different rotating speeds of the input and output shafts. Downshifting 2 gears means the synchros have that much more rotational speed differential to speed-match. Thus, more synchro wear. Rev-matching is after-the-fact.

heimtun's suggestion is a good one. Double-clutch when downshifting. This reduces stress on gearbox parts and when performed correctly, will minimize chassis unbalance during the transition from decel to accel. Rev-matching is after-the-fact.

MHO,
Sherwood

Many thanks, that's what my instincts were telling me. As said, not having first hand knowledge of construction of a 915 I was not sure how gear stacks, synchros, etc are oriented and what velocities there are going on in there. Having learned to drive in the early '70s with cars that had either worn or no synchros (1st gear especially), double clutching is second nature to me.
-C

[Jon B]

Originally Posted by 911pcars

When downshifting from any gear, the synchromesh is speed matching the different rotating speeds of the input and output shafts. Downshifting 2 gears means the synchros have that much more rotational speed differential to speed-match. Thus, more synchro wear.

The output shaft is governed by road speed. If the car is slowed to an appropriate road speed for the selected gear, there is no greater difference for gears to synchronize by skipping some, and no increase in synchro wear.

[911pcars]

Originally Posted by Jon B

The output shaft is governed by road speed. If the car is slowed to an appropriate road speed for the selected gear, there is no greater difference for gears to synchronize by skipping some, and no increase in synchro wear.

Probably better explained in this YouTube video which I've embedded in other discussions on "why double-clutch"?


If interested, the videos that follow help illustrate what's happening inside a typical gearbox.

Sherwood