A very theoretical question. on final drive | FerrariChat
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A very theoretical question. on final drive

Discussion in 'Technical Q&A' started by 24000rpm, Nov 4, 2022.

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  1. 24000rpm

    24000rpm F1 Rookie

    Aug 31, 2012
    4,653
    China and California
    #1 24000rpm, Nov 4, 2022
    imagine a rear wheel drive car, say, 360 modena.
    suppose you have only 1 gear in the car.
    suppose your RPM-limited top speed is 80mph
    suppose you are not willing to exceed 80mph, overwhich you are not comfortable.
    supppose the friction between tire and tarmarc is unlimited ( i.e. there will be no wheelspin in whatever situation)
    suppose there is no wind resistance.

    Now you changed your differential, and the final drive changed from 4:1 to 4.5:1.

    the question: does this change of differential gear ratio reduce your 0-60mph time by 0.5/4 = 12%?
     
  2. daytonaman

    daytonaman Formula Junior

    May 1, 2007
    986
    Australia
    Full Name:
    howard pigdon
    #2 daytonaman, Nov 4, 2022
    other way around
    if you went from 4:1 to 1:1 then you would be doing, well, very fast
     
  3. daytonaman

    daytonaman Formula Junior

    May 1, 2007
    986
    Australia
    Full Name:
    howard pigdon
    #3 daytonaman, Nov 4, 2022
    but your 0-60 would be very slow
     
  4. wmuno

    wmuno Formula Junior
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    Dec 24, 2007
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    Wilmette, Illinois
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    Bill Muno
    #4 wmuno, Nov 4, 2022
    Drag race cars typically use numerial higher rear axle ratios to get the car moving as quickly as possible from rest in first gear. However, the fastest acceleration occurs when the engine is operating at the RPM that produces the maximum torque. In your example, if the higher rear axle ratio prevents the engine from reaching the RPM for maximum torque, then the acceleration time would be reduced. In normal circumstances, the fastest acceleration occurs when one shifts to a higher gear at the RPM for maximum torque. The torque curve drops off as the engine reaches its red line.
     
    24000rpm likes this.
  5. daytonaman

    daytonaman Formula Junior

    May 1, 2007
    986
    Australia
    Full Name:
    howard pigdon
    #5 daytonaman, Nov 5, 2022
    yes totally misread the question
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,932
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #6 Steve Magnusson, Nov 5, 2022
    With your simplified assumptions, I think the equation would be:

    Time for 0-60 with 4.5:1 gears = (4.0/4.5) * Time for 0-60 with 4.0:1 gears = 0.889 * Time for 0-60 with 4.0:1 gears = 11.1% faster

    Are you trying to change a 360 into a dragster? ;) These modern 6~8 speed transmissions with one, or two, very large overdrive top gears are great at giving both awesome low-end acceleration and good freeway driving behavior, but just changing the rear diff ratio on an existing car to improve low end acceleration can make freeway driving very annoying/almost unacceptable.
     
  7. 24000rpm

    24000rpm F1 Rookie

    Aug 31, 2012
    4,653
    China and California
    #7 24000rpm, Nov 5, 2022
    I have a custom differential laying around that will put the top speed of the 360 to only 255km/hour (as tested on track on another 360)
    So I was thinking that, maybe I can improve acceleration by installing this.
    I rarely go to freeway. its pretty much 100% city drive. Also, the track I usually go to , the long straight is only about 600 meters, so I can't reach 300km/h anyways.
    So ............... I think the only thing concerns me now is that I have to be careful when downshifting because the TCU will over-rev the car, or won't it?


     
  8. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,932
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #8 Steve Magnusson, Nov 6, 2022
    Sure, if your applications are limited as you say, why not. Here in TX, we have a fair number of 75 mph and 80 mph toll roads that come into everyday use so could be a little "buzzy" (and fuel thirsty) for that use. In the 360 OM that I checked, it shows the stock diff ratio as already being 4.444:1 -- is your custom differential near a 5:1 ratio?
     
  9. INTMD8

    INTMD8 F1 Veteran
    Owner Silver Subscribed

    Jun 10, 2007
    6,815
    Lake Villa IL
    #9 INTMD8, Nov 6, 2022
    Percentage in gearing reduction wouldn't be an equal percentage gain in acceleration.

    Some losses will be had in accelerating the mass of the engine/driveline. (shorter gearing will be more RPM/sec so more inertial loss unless you reduce reciprocating mass).

    And of course this is also assuming this doesn't result in traction issues, which it may, especially from a dead stop.
     

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