EOC bump starting -> synchro wear? - Fuelly Forums

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Old 06-19-2008, 08:26 AM   #1
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Question EOC bump starting -> synchro wear?

When I neutral coast with the engine on, I can rev-match before stepping on the clutch, so when I shift it goes right into gear.

When I bump start during EOC, not only can't I rev-match, but it's got a lot more RPM for the synchro to make up, and I can feel it in the shifter. The synchro has to accelerate the clutch from a dead stop to 1500 or 2000 rpm. Is this going to wear my 5th gear synchro really terribly?
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Old 06-19-2008, 08:56 AM   #2
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The synchro is only used for smoothly engaging the actual gear. Once you're in gear, the synchro really isn't used until you go to change gears.

If it's not shifting smoothly, you are risking damage to the synchros. They're made of soft metals and can't stand up to shock nearly as well as the gears. I usually gauge which gear I need to be in by watching the speedometer which has output when the key is switched to accessory (at least it is in Saturns).
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Old 06-19-2008, 09:14 AM   #3
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Not an option on all vehicles. On all of my vehicles if the transmission is not in park the key will not turn to accessory. Also, in Accessory on all of my vehicles the steering wheel is locked.

-Jay
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Old 06-19-2008, 10:07 AM   #4
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Try this:
Clutch in, move shifter to highest gear
Quickly pulse the clutch up-down. Just high and long enough to turn the engine over, but not enough to jolt anything.
Then, choose your gear, rev-match, and go.
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Old 06-19-2008, 10:50 AM   #5
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It seems that I've failed to adequately explain my concern.

At the end of an EOC, I put it in gear. The act of putting it in gear so I can bump start requires the synchro to do a lot of work, because the clutch plate is not spinning at all. Is that asking too much from the synchro? Will it wear too prematurely?
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Old 06-19-2008, 10:54 AM   #6
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I've thought of a clearer way to explain it and point out my concern.

Bump start procedure:
1. Coasting along, neutral, engine off. Decide it's time to restart.
2. Clutch in
3. Move shifter to highest gear -- THIS is when the synchro has to do a lot more work than it normally would do.
4. Quickly pulse the clutch up-down.
5. Then, choose your gear, rev-match, and go.
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Old 06-19-2008, 10:57 AM   #7
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Ah, I see. Hmmm... No idea.
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Old 06-19-2008, 11:26 AM   #8
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Quote:
Originally Posted by theholycow View Post
I've thought of a clearer way to explain it and point out my concern.

Bump start procedure:
1. Coasting along, neutral, engine off. Decide it's time to restart.
2. Clutch in
3. Move shifter to highest gear -- THIS is when the synchro has to do a lot more work than it normally would do.
4. Quickly pulse the clutch up-down.
5. Then, choose your gear, rev-match, and go.
Thinking about it Cow, I can't see how the wear from bump starting would be unusual or any worse than normal. The synchro is there to assist in matching revs between the driveshaft and the transmission. During EOC with the transmission disengaged, the input shaft will not be rotating whilst the gearshaft/output shaft will be. The synchro will simply rev match when you use your gear selector to pick the gear you will use to bumpstart the engine.

Seems pretty innocuous to me, FWIW.
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Old 06-19-2008, 11:42 AM   #9
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The synchro matches the gear to the input shaft, not the driveshaft. Normally it would have to rev match a small difference between the gear you want to enter and a spinning input shaft.

As designed, they expect the input shaft to be pretty near the right speed already; it's not often that normal drivers need to go into gear with the input shaft at idle speed and the driveshaft at driving speed, as we do during a P&G with the engine on. As such, I've mostly been trying to rev-match before putting the clutch in.

With EOC and bump-starting, now we're talking about a completely dead input shaft.

However, since nobody else is worried about it, I guess I don't have to either. I stand to lose nothing, my only risk is being an incosiderate jerk; my lease ends before my warranty ends, and I doubt it will wear out before I return the car even if I am wearing it abnormally fast. The only real risk is that the next owner will have a worn-out 5th gear snychro a lot sooner than normal.
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Old 06-19-2008, 01:13 PM   #10
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Quote:
Originally Posted by theholycow View Post
The synchro matches the gear to the input shaft, not the driveshaft.
Well, by "driveshaft" I didn't mean that thing that runs under the car from front to back, but the shaft that is carrying power into the transmission from the engine. Your terminology is probably correct/better.


Quote:
As designed, they expect the input shaft to be pretty near the right speed already; it's not often that normal drivers need to go into gear with the input shaft at idle speed and the driveshaft at driving speed, as we do during a P&G with the engine on. As such, I've mostly been trying to rev-match before putting the clutch in.

With EOC and bump-starting, now we're talking about a completely dead input shaft.
We're talking about a few hundred RPMs of difference between "normal" and EOC. Doesn't seem worrisome to me.

I used to shift without the clutch. I always thought that would accelerate the wear and tear on the transmission...but in 8 years of operation (before I got rid of it) I didn't have any issues. I think you'd really have to work hard to damage the transmission.
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