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Why do manual trans have more gears than auto?

Discussion in 'Chit-Chat' started by 60wag, Aug 15, 2006.

  1. 60wag

    60wag SILVER Star

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    Why to manual transmissions tend to have one more gear than the auto version in similar applications? Why wasn't a 5 speed manual good enough for the FJC? The auto is 5, right? So the manual has 6. For years the automotive standard was a four speed auto or a 5 speed manual. Why not a four speed manual with 4th gear being an overdrive?
     
  2. rusty_tlc

    rusty_tlc Dain Bramaged Member

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    Which years?
    Three speeds column shifts, both auto and manual, were common when I was growing up.
     
  3. fj40charles

    fj40charles

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    probably for gearing... An auto will have a torque converter mulitplication of 2-3X. So you auto will have an effective first gear of something like 5:1.

    I don't know about the fj cruiser, but on my nv5600 behind my Cummins, the first gear is a 5.6:1. It is a 6 speed, but for street use, it is a 5 speed with a low granny gear.
     
  4. Gumby

    Gumby Supamod Staff Member s-Moderator

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    Benz now has an 8 speed auto
     
  5. rusty_tlc

    rusty_tlc Dain Bramaged Member

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    Are autos more complicated to build?
    I've never seen inside an auto so I have no idea how they work.

    But that would explain a lot.
     
  6. Tigerstripe40

    Tigerstripe40

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    MUCH more complicated.

    goto howstuffworks.com and search for automatic transmission.
     
  7. brian

    brian SILVER Star

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    well, an auto has to do all the work that a manual does, plus two jobs that the driver handles on a manual, IE the clutch and shifting work. all the while fitting in pretty much the same size and shape as the manuals.
     
  8. Brian in Oregon

    Brian in Oregon

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    A clutch is either in or out, with a 1:1 engine/transmission input speed.

    A torque converter is a fluid coupling, and allows the engine and transmission input shaft to vary their speeds.

    The automatic allows the engine to get into a power band, and "slip" via the torque convert (fluid coupling) and not stall, unlike a clutch.

    This does come at a price. Which is that the torque converter never quite achieves a full 1:1 lockup. It always slips, and thus is always less efficient than a clutch.

    Which is why in the late 1970's, lockup converters started coming out. These torque converters would lock up when the engine and transmission input were close to unity.

    Recently, some automatics have been getting as good of mileage as sticks, and in some cases, better. One example is my wife's 2004 Toyota RAV4. The stick model gets 27 mpg highway, while the auto gets 28 mpg highway. The auto is programmed to keep the engine in a more efficient power band.

    More and more automatics are being made with more gears. This is in response to some of the newer technologies that are trying to get rid of conventional transmissions. Like variable belts, pulleys and even variable cone type drives. All of which have problems with vehicles over certain weights or HP, so a multiple gear auto makes sense, with the goal of keeping the engine in a narrow power band.

    Multiple gear automatics aren't new. The old post-WWII GMC 6x6 2-1/2 ton M135 and M211 army trucks had a 7-speed automatic in them. It just took a while for this to come around in cars.
     
  9. NMuzj100

    NMuzj100

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    The simpler, smaller and lighter design of a manual allows for more gears for the same size,weight and cost considerations of an automatic in the same model. Basically to keep the cost difference at a reasonable premium they have to shave a gear. The torque converter allows you to get by with a higher first gear so the compromise isn't too bad.
     
  10. cary

    cary

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    For years the automotive standard was also points & condensor, carburators, cast iron blocks, no seatbelts, drum brakes and AM radios only.

    It is called progress. With better quality control and alloys, we can make smaller gears that handle more power and leave more room in the tranny to have 5, 6 & 7 speed trannies. Having more gears allows engines to stay within their most efficient range more of the time for better acceleration and gas mileage.

    Remember, when we had 3 speeds (heck we had two speed power glides), there were not enough gears, 4 speeds always required a compromise, even in a muscle car. You had to choose, do you want a car that accelerates (4.11 or 4.56 diff), a compromise (3.90, 3.73 or 3.55 diff), or a freeway diff (3.25, 2.92, 2.79). 5 speeds helped a lot with this, but six speeds really hit the cake. You get to have a transmission that offers you close ratios for the acceleration of a speed tranny with a short rear end, and still have a great freeway gear without having a huge gap.

    In my experience even with an automatic, there is still a big benifit from going from a 4 speed to a 5 speed. Test drive a 2002 100 series, then drive a 2003+ with a five speed. The 2003 accelerates much better, and shifts much smoother when passing as it doesn't have huge jumps from gear to gear. In comparing the two BMW's we drive in a drag race (1994 525i with a 4 speed auto, and a 2001 X5 3.0 with a 5 speed (close to identical power to weight ratio)), the X5 takes a 1.5 car lead out of the box, while the 1994 has to get spooled up with a higher first gear, then they are dead even to about 60mph. Yet the X5 turns over lower RPM's on the freeway.

    According to most articles I have read, 6 speeds is where you really start to fall off in terms of gains in more gears. I believe a 7 speed automatic was worth something like a 1-1.5% improvement in gas mileage, the point where the increase in cost and complexity is quickly outweighing the increase in efficiency. 8 speeds are getting into the "I have more than you do" camp.
     
  11. Gumby

    Gumby Supamod Staff Member s-Moderator

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    CVTs are doing a pretty good job handling the torque and hp these days. Especially in economy cars, but even in big sedans like the Ford 500.

    They are expensive, very difficult to repair, large in size and heavy as all get out, athough the last two have been getting a lot better.

    The fluid in the Ford's CVT is crazy expensive and a capful of regular ATV down the dipstick hole will ruin the CVT.
     
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