So what's the guilty party? the planetary transmission, or the torque converter?

Fortuna Wolf

Veteran Member
Joined
Apr 28, 2006
Location
Wilmington, NC
TDI
2002 Jetta TDI Auto Sedan
We all know that slushboxen are inefficient compared to a manual. Even at highway speeds when the TC should be locked and the 01m has a taller gear ratio the auto transmission incurs a significant efficiency hit. Is this because of the inherent nature of the planetary gearbox?
Or is it because of the torque converter still being notoriously inefficient while locked?
 

El Dobro

Top Post Dawg
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Feb 21, 2006
Location
NJ
TDI
2017 Bolt EV Premier, 2023 Bolt EUV Premier
I would say the torque converter, because of the fluid coupling. Once the converter locks up though, there is no slip.
 

El Dobro

Top Post Dawg
Joined
Feb 21, 2006
Location
NJ
TDI
2017 Bolt EV Premier, 2023 Bolt EUV Premier
You may have to use a scanner to make sure the converter is locking up and that the transmission worked itself all the way up to top gear.
 

GoFaster

Moderator at Large
Joined
Jun 16, 1999
Location
Brampton, Ontario, Canada
TDI
2006 Jetta TDI
For lower efficiency when rolling down the road in top gear with the converter locked ... The fault is neither the planetary gears nor the torque converter. It's the way the transmission holds itself in each gear.

Part of the efficiency loss is that automagic transmissions rely on hydraulic pressure to hold the clutches engaged (every gear has a hydraulically-operated multi-plate clutch associated with it - that's how it selects between the various forward and reverse combinations). To produce that hydraulic pressure, there is a pump. That pump requires power to operate it. This loss is not found in manual gearboxes, because once a synchro is engaged then the teeth transmit the power, and there is no hydraulic pressure required to hold it in gear. Manual gearboxes have no oil pump, no circulating oil system at all.

Another part of the loss is that among all the multiplate clutches which are NOT engaged, the plates are still there spinning next to each other, and there is some minor frictional loss associated with this. This loss is not found in manual gearboxes. If a synchro is out of engagement, the teeth don't touch each other and there's no friction between them ... period.
 

Fortuna Wolf

Veteran Member
Joined
Apr 28, 2006
Location
Wilmington, NC
TDI
2002 Jetta TDI Auto Sedan
Go Faster, thanks.
Does increasing the line pressure then decrease the fuel efficiency?

El Dobro, I'm very in tune with what my transmission is doing. I know what gear its in and whether its locked up by feel.
 

GoFaster

Moderator at Large
Joined
Jun 16, 1999
Location
Brampton, Ontario, Canada
TDI
2006 Jetta TDI
Increasing the line pressure beyond what is necessary for the clutches to not slip will increase power demand on that hydraulic pump ...

Too low line pressure will cause slippage of the multiplate clutches, and although this slippage will also waste fuel, that will be the least of your worries ...
 

overbite

Veteran Member
Joined
Mar 10, 2004
Location
Eagle Lake, MN
TDI
1996 passat
so DSG - no loss from fluid pump because of the clutches or is there since there is some oil down there in the wet clutch system? May be were some of the efficeny is lost to manual even though the 6 speed has a lower final. That is what really got me there, and now I am lose !?!
 
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