How bad are automatic transmissions? - Fuelly Forums

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Old 12-21-2006, 07:10 PM   #1
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How bad are automatic transmissions?

I was pokin around the TN forums and came across a couple dyno runs with the same car before and after a manual transmission swap. What's remarkable is the increase in low end torque at the wheels, at 2k rpm the m/t results in 100 more ft/lbs at the wheels compared to the automatic, and peak torque is about 30ft/lbs greater. I've heard automatic transmissions are bad, but a 25% difference in peak torque along with nearly twice as much low end torque seems nuts! I've heard of a 15-20% loss in economy, but it seems like a manual transmission vehicle may have an even larger advantage, maybe 20-40% depending on how bad the auto is and good the manual is. SVOboy's gaslogs seem to show an ~30% increase fuel efficiency, before/after the swap.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 12-21-2006, 08:22 PM   #2
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It depends on the type of automatic, but normaly I figure you loose 20% with an automatic, so what I want to know is how newer automatics are getting better mileage, like the Yaris, and Fit are pretty close when you compare auto to manual as far as mpg, but I wonder about the dino.
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Old 12-21-2006, 11:10 PM   #3
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I know it's said in the honda world that a b16 auto would be smoked by a d16z6 any day. Now, there's a huge difference in those engines, such as DOHC vs SOHC, 170 HP vs 128 HP (or something such for the z6), so yeah, I know in that regard they suck as well, but I'm not sure by how much. And yes, my mileage has be decent lately. I think more amazing is that I'm still getting ~49 mpg this winter when last winter I got low 30s, :-)
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Old 12-22-2006, 03:36 AM   #4
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The Autos used in the Nissan Bluebird (like my car) were pretty good. There was almost on difference between Auto + Manual in the MPG figures. This was because, I suppose, they had overdrive and 'lock-up' when above a certain speed, preventing slip at the torque convertor.
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Old 12-22-2006, 05:10 AM   #5
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As I understand it, an automatic transmission tries to keep the engine making peak power and bleeds off the unnecessary power through slip (friction & heat). With a manual transmission, the driver determines where, when, and how the power gets used.
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Old 12-22-2006, 06:44 AM   #6
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Here's an anomaly:

GM's MT full size trucks are often rated worse EPA MPG than AT trucks. I've often wondered whether GM rigs its tests somehow (for example by using engine braking instead of pushing in the clutch and braking normally), in order to sell more ATs. Given this curiosity, it ought to be easy to hypermile one of their MT trucks.
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Old 12-22-2006, 06:48 AM   #7
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It kind of shows that a well trained human brain can do better than a transmission ECU any day. But at least the programming and number of gears available in the autos is getting better.
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Old 12-22-2006, 07:30 AM   #8
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Originally Posted by TomO
It kind of shows that a well trained human brain can do better than a transmission ECU any day. But at least the programming and number of gears available in the autos is getting better.
This is true because not only is the slushbox itself wasteful, the gear set points have to be completely turnkey.

Many of things we do in improving the FE of our driving technique with a manual are very hard to automate. Automatics confound our ability to use many of those same techniques.
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Old 12-22-2006, 10:13 AM   #9
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I think that newer auto transmissions with overdrive are just as efficient at highway speeds once the torque converter locks up (if it stays locked up). As long as the converter is locked up, its getting no worse mileage than a manual trans. I have also noticed that many have gearing higher than a comparable 5 speed trans (less rpms at high speeds). Maybe because they can get by with simply unlocking the TC if the driver wants to pass another car/go up a hill while someone with a 5 speed trans may have to downshift to 4th to accelerate as fast.

I put about 200 miles on a 2006 Nissan Sentra rental car (AT 1.8 liter) back in September. It had an onboard MPG display and it was reading 42-43 mpg at 60 mph (and only about 1800 rpms). When I disabled the 4th gear (pushed the OD button) it went down to 37 MPG (at 60 MPH). And when I selected 2nd gear, the mileage went down to 25 MPG (at 50 mph- the engine was turning pretty fast and I didn't want to stess it too much by doing 60).

For city driving with stop and go traffic, a manual trans is much better.
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Old 12-22-2006, 11:55 AM   #10
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Originally Posted by Erik
For city driving with stop and go traffic, a manual trans is much better...
...for fuel economy...not your leg.
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