posted by
someone claiming to be Pia
on
Mon Mar 17 17:26 CST 2003 [ RELATED]
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Sorry if this may be a basic question, but why do manual transmissions get better mileage, all things being equal, as compared to automatic transmissions? Thanks in advance.
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Here's another one. My wife and daughter do not like to drive a manual. So my bucket seat fits nobody's bucket but mine.
Good post!
Rick
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An automatic transmission sends power to the rear wheels by internally pumping oil. This pumping heats the oil, which is why an automatic transmission needs an oil cooler. The heat is lost energy. Thus, a small amount of the power produced by your engine and sent to the automatic transmission is converted to heat and lost forever.
That same power, if sent to a manual transmission, makes it to your rear wheels and pushes the car slightly farther.
--
Don Foster (near Cape Cod, MA)
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As an extreme example, the Abrams M1A1 main battle tank has a 1500 shaft horsepower turbine engine hooked up to an Allison 6-speed al=utomatic transmission. Only 1000 hp is delivered to the drive sprockets! (Your mileage may vary)
(And to impress you shade tree mechanics - the entire engine and transmission can be pulled out in under an hour. Just need simple hand tools - and a 3+ ton lift!)
(More fun Abrams stuff: before they plug in a replacement engine, the engine is "ground hopped". Hooked up to fuel and DC power, and fired up sitting on the ground. Just wanna make sure she lights up before you plug 'er in.)
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Does the wasted heat contribute to global warming? Seems that if not the heat itself then at least the wastes from the extra fuel Would have a negative effect on the environment. Automatic transmission users are anti-environment!! I knew there was some reason not to be shiftless.
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01 V70 2.4 M5
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"Does the wasted heat contribute to global warming?"
It certainly does not contribute to global cooling.
However, you must view this in context. ALL the energy that makes it to the rear wheels to drive the car eventually becomes heat from frictional losses -- noise, air friction, mechanical friction, and more importantly, friction at the brakes. ALL energy in the fuel burned in the engine -- every femtoWatt -- eventually becomes heat radiated into space to increase the temperature of the universe.
Entropy rules!!!
"Automatic transmission users are anti-environment..."
Not necessarily. They tend to spin their tires less.
A car with a standard tranny may be capable of better fuel efficiency, but that does not mean that the driver is capable of better fuel efficiency. (Hell, I remember being young and stupid once. I'm no longer young.)
"I knew there was some reason not to be shiftless."
That requires a shift in thinking, though it may have no bearing on how we clutch opportunities for improvement.
--
Don Foster (near Cape Cod, MA)
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Another thing is that since the automatic transmission loses energy due to slippage, it results in slower acceleration. So the driver may be pressing harder on the accelerator (using even more fuel) to get the desired level of acceleration. Also, when waiting at traffic lights, the engine has to push against the torque converter (assuming that the driver of the automatic transmission car leaves it in D), which takes more energy than freewheeling (either declutched or in neutral) that it would be doing with a manual transmission.
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Pia -
You got a lot of good answers. Your question is not a bad one at all. To sum it up: The automatic system has some slippage between engine and drive shaft. Then there's a slight power loss because the trans itself uses some. Also, the ratio in the rear axle is different. 3.73:1 on the autobox cars, 3.31:1 on the sticks, in general. A few models are different.
Does your Volvo have a tachometer? If not, it's a nice upgrade, and easy, too.
Ask some more stuff.
Regards,
Bob
:>)
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Since nobody else said it, I will: manual transmissions are much more fun!
Better mileage is just icing on the cake.
--
-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854
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posted by
someone claiming to be Scott
on
Mon Mar 17 17:45 CST 2003 [ RELATED]
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Generally speaking, less power is lost through the drivetrain in a manual transmission car. With an automatic transmission, the engine has to drive an oil pump (torque convertor) which in turn drives the transmission (it's a hydraulic system). The torque convertor must reach a certain stall speed (in engine RPM's) before the transmission input shaft begins to turn. With a manual transmission, the engine is essentially coupled directly to drive wheels when the transmission is in gear and the clutch is engaged. Far more efficient power transfer, and thus better fuel economy and performance.
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There is not as much power loss in a manual transmission. Also, the clutch makes a "hard" connection between the engine and rear drive opposed to a fluid coupling in the automatic's torque convertor which has a lot of slip. Notice this on your tach, cruise at a speed and watch the tach as you press on the gas and you'll see the RPMs increase a couple hundred while the speed stays the same.
This is kinda oversimplifying it but you should get the idea.
Justin B.
83 Turbo
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All the thinigs Justin said are true, and the only bit I want to add to that is that you can drive a manual and force an upshift and thus lower engine revs, but with an automatic, you can't do that. You have no idea how many times in my automatic with overdrive I've been at around 30-33MPH which is revving around 1500 RPM and I'm not really accelerating and I just want to coast, but it refuses to shift into 4th gear (od) which would effectively lower the RPMs even more and save gas.
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'
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My OD kicks in about 40mph on level road. It drops out when the car slows down below 35mph +/-.
The horsepower curve is pretty steep at the lower rpms, so I think that the horsepower available at the 1500rpm minus 31%, even with a 3.73 rear axle ratio, is low enough to seriously lug the engine.
The old original overdrive units, my family had one on our 1950 Mercury, was a manually engaged (T-handle under dash) and had free-wheeling. When you backed off on the gas, you were in neutral, coasting along. Aah, technology!
Regards,
Bob
:>)
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posted by
someone claiming to be Scott
on
Mon Mar 17 19:27 CST 2003 [ RELATED]
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"The horsepower curve is pretty steep at the lower rpms, so I think that the
horsepower available at the 1500rpm minus 31%, even with a 3.73 rear axle ratio, is low enough to seriously lug the engine."
My '85, with a manual transmission, will protest if I try to engage the overdrive at less than 2000rpm. The automatic transmission has a taller overdrive gear (0.69:1), but because of the shorter rear axle the end result is that the reduction ratios are about the same. I think. :-)
"The old original overdrive units, my family had one on our 1950 Mercury, was
a manually engaged (T-handle under dash) and had free-wheeling. When you
backed off on the gas, you were in neutral, coasting along. Aah, technology!"
Never driven an automatic Volvo, but have noticed in other automatic cars that the tach drops to around 1000rpm if one backs off the gas at speed. I guess that must be the stall speed of the convertor?
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They usually shift into the highest gear when you let off the gas. If you select 3 on the shifter you can feel it shift down and brake the car.
--
'90 744 tic
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posted by
someone claiming to be MittenHed
on
Mon Mar 17 19:43 CST 2003 [ RELATED]
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Actually, as my tortured memory sweats and groans, that isn't stall speed. Stall speed is the speed that the engine will max out at if you hold it flat to the floor in gear with the brakes on. The engine will rev just so high and no higher. Stall speed. Heats things up right quickly, by the way.
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i've checked the stall before on my AW7x with lockup and it's around 1800-2000, somewhere around there. I have no idea how accurate the tach is, but the tach reads about 1900, so I figured +/- 100 sounds good. ;)
and yep, good way to heat things up. that's why as soon as i hit the stall speed i backed off the gas. :)
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'
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