Don't skip shift. Official from Acura.

[Olyar15]

Very interesting discussion. I've always wondered how transmissions work, and how synchros do their thing (or occasionally not to their thing). This thread has inspired me to do some reading and try to get my head around how all these gears in a transmission actually works. And how it factors into this TSB.

The basic components of a transmission is an input shaft and an output shaft. A number of gears are "attached" to the input shaft, so that when the input shaft turns, it turns all the gears at once, but the gears rotate at different speeds. These gears freewheel around the output shaft. The input shaft is connected to the engine via the clutch. The output shaft is connected to the road via the driveshaft, differential, and wheels.

The gear lever allows one particular gear to be locked onto the output shaft (or none of the gears, if the gear lever is in neutral). The other gears are still turning because they are attached to the input shaft, but simply freewheeling around the output shaft. It is during the act of selecting a gear that the synchros come into play. They help synchronize the rotational speed of a particular gear on the input shaft to the rotational speed of the output shaft. How they work is basically by use of friction. Hence, the greater the difference in rotational speed, the greater the amount of friction on the synchros and therefore the greater the wear.

When the clutch is engaged (clutch pedal not pushed in) and the gear lever "in gear", there is a direct mechanical connection between the engine and the road. Every part of the drivetrain is synchronized to turn at a particular speed, depending on the gear and road speed. There is no way for any one component to be out of synch unless something either breaks (ouch!) or something slips (eg. clutch or tires).

When the clutch is disengaged (clutch pedal pushed in) but the transmission still in gear, then the input shaft is disconnected from the engine but still connected to the output shaft. Therefore the input shaft will still turn at the same proportional speed as the output shaft, taking into account the particular ratio of the selected gear.

When the clutch is engaged and the transmission is in neutral, the input shaft will rotate at the same speed as the engine. The output shaft will turn at whatever speed the tires are turning.

When the clutch is disengaged and the transmission is in neutral, there is nothing that is turning the input shaft and it will eventually stop rotating because of the internal friction within the transmission.

So what does all this have to do with the TSB, double-clutching, synchro wear, etc.? Let's take a look at what happens during a typical shift, say from 1st to 2nd. You are accelerating in 1st gear. The engine speed, input shaft speed, and output shaft speed are all synchronized. So you push in the clutch and let off the accelerator. Engine speed drops, but input shaft speed is still the same as output shaft because the transmission is still in gear. You shift out of 1st, pass into neutral, and into 2nd. It is in that brief moment that the input shaft is decoupled with the output shaft, allowing it to start slowing down. Of course you won't know how much it has slowed down because the clutch is disengaged and therefore the input shaft is not controlled by the engine speed. Depending on how quickly you shift, the input shaft may have slowed enough so that by the time you select 2nd gear, the input and output speeds are pretty similar. Otherwise it is the job of the synchro to make sure they match. Again, the greater the speed difference, the greater the wear and tear on the synchro. However, since the difference in gear ratios between 1st and 2nd is pretty small, even if you shift as fast as you can, the wear on the synchros isn't too bad. Once you re-engage the clutch, the engine will be re-connected to the input shaft and the speeds of all the components synchronized.

Now, when you shift from, say 1st to 4th, the difference in rotational speeds is greater, and therefore the syncros face a greater load and more wear when it slows down the input shaft in order to engage 4th gear. The load on the synchros can be lessened by giving the input shaft more time to slow down. But the problem is, how much time is enough? Remember, if you have the clutch disengaged, the ENGINE SPEED IS IRRELEVANT. The obvious solution, then, is to re-engage the clutch when the gear lever is in neutral. Now you can control the input shaft speed.

The conclusions one can draw from this:

If you do a "normal" shift (push in clutch pedal, select new gear, release clutch pedal), then the synchros will experience more wear when skip-shifting (eg. from 1st to 3rd or 4th) because the input shaft has to alter its speed more than if you go through the gears sequentially.

Rev-matching (ie. matching engine speed to gear speed) will not help the synchros if you do so with the clutch disengaged. It will only help the smoothness of clutch re-engagement.

Rev-matching will help if you do it with the clutch engaged, while transmission is in neutral. Thus, double-clutching will help reduce wear on synchros.

Therefore, if you do want to skip-shift and want to minimize wear on the synchros, you can double clutch on the upshift. Push in the clutch pedal, shift to neutral then let out clutch pedal, let engine revs fall to an appropriate range, then push in clutch and select the new gear.

Blipping the throttle on downshifts will also not help the synchros unless you double-clutch, but it will help lessen any driveline shock when re-engaging the clutch.

Hope this helps. Now I just have to practice double-clutching.

 

[FXMDBilly]

Some measures like WPC Treating the synchros and sliders will greatly improve the strength and durability of the synchros.

www.wpctreatment.com



Billy

 

[Dan Ciezniewzki]

I always skip shift from 1-3 when the tranny is cold for the first couple of miles and have ALWAYS heard this is the way to go, especialyl with Italian cars as they are not known for reliability. 2nd gear takes the most beating of any gear and until temps are up enough I want as little wear on 2nd as possible.
I also do this at slower speeds and the revs match as not to bog the engine or tranny

 

[TURBO2GO]

I always skip shift from 1-3 when the tranny is cold for the first couple of miles and have ALWAYS heard this is the way to go, especialyl with Italian cars as they are not known for reliability. 2nd gear takes the most beating of any gear and until temps are up enough I want as little wear on 2nd as possible.
I also do this at slower speeds and the revs match as not to bog the engine or tranny

Dan, then you should absolutely be double clutching.

 

[Aiken Drum]

There's a lot of conflicting info on this thread. I definitely have a problem with one thing:

At one point, someone asks if rowing through the gears with the clutch depressed is okay, as opposed to directly skip shifting. Someone else says it's okay, and then someone else says the second person is wrong and it's not okay. I think the second person was right and the third should not have contradicted him.

Here's how I understand it, and I want someone to correct me if I'm wrong:

First, you disengage the clutch. Duh.

Now, while the clutch is disengaged, the one and only thing that can be active (or not) is the synchros. As you row from the most recent gear that actually drove the wheels, through the other gears, at each stop you're using the synchros to synchronize gear+shaft rotation and lock them, albeit briefly, with the collar that you're pushing into place with the gear shifter. Yes, you're producing a little synchro wear at each stop, but you're also getting things a step closer to what you'll need at the final desired gear. This seems to me like it should indeed be better than one harsh synch directly from a low to a high gear. If nothing else, it distributes the wear and tear across the whole range, instead of putting it all on one.

Then, with the shifter in final position, you rev match and engage the clutch, just once, one hopes with little or no clutch wear.

(Note: I'm not comparing disengaged rowing to double-clutching--I'm only comparing it to a skip shift and thinking it should indeed be at least somewhat better.)

 

[goldNSX]

First, you disengage the clutch. Duh.

Now, while the clutch is disengaged, the one and only thing that can be active (or not) is the synchros. As you row from the most recent gear that actually drove the wheels, through the other gears, at each stop you're using the synchros to synchronize gear+shaft rotation and lock them, albeit briefly, with the collar that you're pushing into place with the gear shifter. Yes, you're producing a little synchro wear at each stop, but you're also getting things a step closer to what you'll need at the final desired gear. This seems to me like it should indeed be better than one harsh synch directly from a low to a high gear. If nothing else, it distributes the wear and tear across the whole range, instead of putting it all on one.

Then, with the shifter in final position, you rev match and engage the clutch, just once, one hopes with little or no clutch wear.

Yes, that's my view of things. With this technic you're distributing the work on all participating synchros not just on the final one.

As for the synchro wear with a cold transmission it's like getting out of bed in the morning . I shift at low rpm's (<2500 rpm) and give it a little bit more time as the oil is stiff. If you're not in a traffic-jam the gearbox oil has reached his operation temps after 10 km's or minutes. I can feel it on my 5th gear getting smoother if the oil is warmed up.

Shifting from 1st to 3rd is not recommended.

 

[Kyras]

Bump. I think more NSX owners need to know this skip-shift issue.

Thank you for the bump. I bought my '05 beauty in March and had it shipped from Chicago to So Cal. This is the first time I've heard of this.

 

[TURBO2GO]

Bump again, all NSX owners should know this.