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Effect of changing differential

  1. Nov 24, 2015 #1
    If I am going to change my truck differential and find some suitable one for it, What could be the possible effects on vehicle performance of that differential..??

    For now I haven't plan for it. Just want to know the basic Idea of what could be affected.
     
  2. jcsd
  3. Nov 24, 2015 #2

    Mech_Engineer

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    Do you mean you're going to change the differential's gear ratio?
     
  4. Nov 24, 2015 #3

    rcgldr

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    A higher gear ratio increases acceleration (if not traction limited), increases rpm for a given speed, and lowers the speed at which upshifts need to occur.

    A higher gear ratio may require a larger differntial, since the driving gear will generally have fewer teeth engaging the driven gear if the gear ratio is increased and the differential size remains the same. This is why car guys may switch from an 8 inch differential to a 9 inch differential if going to a 4.10 gear ratio differential.
     
  5. Nov 24, 2015 #4

    cjl

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    It only increases acceleration in first gear before you get into the powerband. After that, assuming you don't have a ridiculously narrow powerband and assuming you have a sufficiently closely spaced gearbox to stay in the powerband, you'll accelerate pretty similarly. It will also improve controllability and torque when starting, which is good for towing or off-road use. Highway gas mileage will suffer though, since the engine will be spinning faster to maintain the same cruising speed.
     
  6. Nov 24, 2015 #5
    yes desire differential have greater gear ratio
     
  7. Nov 24, 2015 #6
     
  8. Nov 24, 2015 #7

    rcgldr

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    Acceleration is increased in every gear (if not traction limited), but the top speed in each gear is reduced. The time it takes to shift make reduce the advantage of a higher gear ratio.

    Yes if the gear ratio is increased by 10%, then the engine rpm at a specific speed in a specific gear is increased by 10%, and the top speed in each gear is reduced by 10%, meaning you have to shift gears at a lower speed.
     
  9. Nov 24, 2015 #8

    Mech_Engineer

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    But WHY do you want to?
     
  10. Nov 25, 2015 #9
    actually somebody proposed me to change differential. currently gear ratio is 4.3 and proposed one have 5.857. But I am not sure what are pros and cons of changing it.
     
  11. Nov 25, 2015 #10
    reason behind proposal is less gradeability. To enhance it new differential with higher gear ratio is proposed
     
  12. Nov 25, 2015 #11
    Thanks for your reply.
    Is there any mathematical calculation for it..??
    or It is just a rough estimation.
     
  13. Nov 25, 2015 #12

    Mech_Engineer

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    Is this an off-road vehicle like a jeep or Toyota Land cruiser ?
     
  14. Nov 25, 2015 #13

    cjl

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    Acceleration in every gear is increased at a given RPM, yes. Acceleration at every speed is unchanged though, since you'll be in a higher gear at the same speed (unless your gearbox doesn't have good ratios for your powerband).
     
  15. Nov 25, 2015 #14

    rcgldr

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    That could result in the engine running very fast if you ever drive that truck on the highway.

    With a 10% to 20% increase in differential gear ratio, the speed range for the normal and higher gear ration differential lower gears overlap, until you reach a gear where the higher gear with the higher gear ratio is the same as a lower gear with the lower gear ratio. Within the speed ranges where the lower gears overlap, when the overall gear ratio is higher, the torque multiplier is higher, and the acceleration is greater.
     
  16. Nov 25, 2015 #15

    cjl

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    Only if the torque is the same at higher RPM as it is at lower RPM. If you run most engines to redline, you'll be past the torque peak nearly 100% of the time, and therefore at higher RPM, the torque will be lower. Whether or not the acceleration will be greater before or after the diff change will then depend on how sharply the torque drops off.

    You can bypass all these concerns by simply looking at the horsepower the engine is delivering at any speed. In all cases, the acceleration will be greater where the horsepower is greater. If at a given speed, the car was running at the horsepower peak before the gear change, and after the gear change at the same speed, it's running above the power peak (but below the redline), it will accelerate slower at that speed after the change than before it. If before the gear change, the car was running below its power peak, and after the change at the same speed, it is now running at the peak horsepower RPM, it will now accelerate faster than before.
     
  17. Nov 25, 2015 #16

    Mech_Engineer

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    Changing the differential gear ratio won't necessarily increase the grade you can go up at speed since you may be limited by your engine's maximum power output; it may help with the absolute maximum grade that can be ascended very slowly.

    We need to know more about the vehicle before we can really give useful feedback. What kind of vehicle is this, and what sort of grades are you hoping to ascend? Is it 2WD or 4WD, and what sort of transmission and/or transfer case does it have?
     
  18. Nov 25, 2015 #17
    I am working with a truck modifier. Just started to work here
     
  19. Nov 25, 2015 #18
    It is a 4x2 truck with Transmission model MF06S. having 4.3 rear axle ratio.
    suggested rear axle ratiion is 5.857.
    Do you need anymore..??
     
  20. Nov 25, 2015 #19

    Mech_Engineer

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    I think a 5.85 gear ratio will be way too high for your truck, it will severely change your engine rpm on the highway and likely a detrimental effect on your mileage. I suggest looking into engine improvements like a supercharger if you need more power for going up hills.
     
  21. Nov 26, 2015 #20
    the proposal mentioned that already this engine is using with that differential in a standard vehicle of the manufacturer (not locally modified, means company is offering that differential with same engine with high price)

    So I am now confused that if the engine is same and we are going to make differential to be same then what could be affected with our vehicle.
    dose drive train (propeller shaft) have any effect on it..??

    For your suggestion, we have to work for engine side. will try to check it whether it is possible for us or not.
     
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