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-   -   manual transmission damage with EOC? (http://www.fuelly.com/forums/f8/manual-transmission-damage-with-eoc-4933.html)

MetroMPG 06-11-2007 09:10 AM

manual transmission damage with EOC?
Received this comment through MetroMPG.com, and not being an expert in transmission design, I thought I'd bring it here for discussion, and then point the writer here.

My initial response is - if the description of lubrication is right, then yes, the potential exists to do damage to a manual tranny from very extended ICE-off coasting. However, in practice, the length of ICE-off glides is generally short enough that it's pretty unlikely that all lubricant will be flung from the output shaft causing it to effectively run "dry" and be subject to significant increased friction.

The comment seems to have been triggered by a statement I made in my ICE-on vs. ICE-off coasting experiment that 40% of my driving on that route was done ICE-off. This may have created the impression that I was coasting uninterrupted for long stretches, rather than several times per km (which "reloads" the lubrication each time).

Anyway, his message to me:


I have a comment about your "Coasting experiment: engine off VS. engine idling" post. Another thing to consider about that driving method is wear on the starter, clutch, and mostly the transmission.

The problem is that most if not all manual transmissions do not have a pump for the transmission fluid. The fluid is circulated throughout the transmission by the rotation of the gears, turned by the input shaft.

When driving along in gear the output shaft is turning at wheel speed and the input shaft is turning at engine speed. Whatever gear you are in is turning in the middle and flinging transmission oil onto the rotating parts.

When you coast along in neutral [with the engine running] the output shaft is rotating at wheel speed but the input shaft is only rotating at idle, which is presumably a lower speed than if you were in top gear. When in neutral all of the gears in the transmission rotate along w ith the input shaft, so even though you have less RPM there are a number of gears to move the oil so it's ok.

When you cost with the engine off [in neutral], the output shaft rotates at wheel speed but the input shaft does not rotate at all, so there are no gears turning. The problem is that the rotating output shaft is no longer getting any lubrication, so if you do this technique a lot (and you are stating 40% of driving on certain trips) you will most likely have premature wear in the transmission.

In something like a metro where getting a new transmission either from a junkyard or JDM or any number of other sources it's probably not a big deal, but for people who drive cars with more expensive parts but are also concerned with mileage this may no t be such a good idea.

- somebody with 4 Metros and a Honda Fit :)
BTW, I fully agree with his initial point that there's likely to be more clutch & starter wear. (Not to mention slightly more drivetrain & tire wear.) Whether it' enough to worry about is another matter.

Any comments on the lubrication issue from the assembled GS gearheads?

Silveredwings 06-11-2007 09:32 AM

Interesting points. I think they are valid, though I don't know at what point it would become a problem.

psyshack 06-11-2007 10:20 AM

More times than not the final drive shaft and gear are in the lowist spot in a MT FWD tranny. Its been my experiance that this kicks more than enough lube up on the other parts to keep them lubed up. Ive never had issue with coasting a MT or AT tranny. Have been doing it for years.

Almost all tranny distruction is cause by redline gear jammers. Clutchs also take a beating in there hands. And then theres the folks that hold a car on a hill with the clutch. That will smoke a clutch and throw out bearing in nothing flat.

I flat out dont worry about tranny damage.


MetroMPG 06-11-2007 10:25 AM

Thanks for the info, psy. It's worth noting that you haven't exactly been doing this in junkyard Metros either.

omgwtfbyobbq 06-11-2007 11:17 AM

Huh... That's odd. My Bentley shows that the output shaft and input shaft are always bathed in oil, so even if the input shaft isn't spinning it's always lubed, and the spinning output shaft should be enough to lube the ring and pinion above it. I suppose if the oil level doesn't reach the output shaft by design or lack of changes, it may be trouble, but I know the transmission in the truck and rabbit are always being lubed when the wheels are spinning.

CoyoteX 06-11-2007 12:13 PM

There are very few manual transmissions that will be hurt by engine off coasting. Just look at the cars being towed behind RVs they never have trouble out of the transmissions. The ring gear is big enough that it throws more than enough oil at anything important when the engine is off while the car is moving.

GasSavers_DaX 06-11-2007 01:17 PM

Agreed on starter and clutch wear. For Hondas anyhow, I disagree on his analysis.

He forgot to mention that regardless of during EOC when gears 1-5 are not turning, the countershaft and the differential are still turning. The majority of the lubrication in the transmission takes place due to the oil being slung around by the diff. This is for a few reasons - the ring gear (bolted to the diff) is the largest "gear" in the transmission. It's teeth get to the lowest point of any gear in the box, thus it is the one that slings the oil. Not only does the diff sling the oil, it is essentially the "pump" inside the box. Strategically placed narrowing channels are around the ring gear in the cast housing. While the transmission is turning, the ring gear pushes oil into these channels, which force oil to the support bearings on both shafts. It also forces oil into the hollow shafts through the oil guides and lubricates the needle bearings under the gears.

While it may not sling around as much oil while EOCing, it still lubricates the components enough for no worries (in my mind).

Hope that helps. :)

psyshack 06-11-2007 02:45 PM


Originally Posted by MetroMPG (Post 56672)
Thanks for the info, psy. It's worth noting that you haven't exactly been doing this in junkyard Metros either.

Sorry,,, i didnt know metro's where so diff. than 98% of the other MT FWD trannys ever built.

MetroMPG 06-11-2007 02:48 PM

No, I wasn't saying the Metros are different - I was saying that since you're not driving a cheap car (Metro), you're obviously comfortable in your assessment of the risk of (not) damaging your vehicles.

psyshack 06-11-2007 03:05 PM


Originally Posted by MetroMPG (Post 56732)
No, I wasn't saying the Metros are different - I was saying that since you're not driving a cheap car (Metro), you're obviously comfortable in your assessment of the risk of (not) damaging your vehicles.

The risk assessment is from years of owning and driving cars. New and used. Ive only had three bad transmissions in my life. And two cars where bought knowing the trannys where goners. And both where GM. The third was a GM law suit car. 1977 Buick. With a Pontiac T-100 tranny in it. With a Olds 350 under the hood. The Buick only had 60k miles on it when it went south. After seeing the tranny they put in the full size Buick that should have been in the Chevette or T-100 I really have to question GM's ablity to do much right. So maybe the is a reason to be worried about a Metro tranny. Gm also has went out of there way to cheapin up the Allison in the HD trucks.

Hope you find the answers....


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