Diff ratios

Discussion in 'Engine' started by Druk, Nov 10, 2011.

  1. Druk

    Druk Administrator Staff Member

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    Question for the mathematicians out there.

    If my car does 3500RPM at 70MPH with a 3.58 diff ratio what would the revs be at the same speed with a 3.07 diff ratio?

    :wallbash:
     
  2. Dieselman

    Dieselman Banned

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    Ideally you want 2.25 or lower ratio to get the revs down to 2100rpm at 70.
     
    Last edited: Nov 10, 2011
  3. OP
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    Druk

    Druk Administrator Staff Member

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    Tnx Will. Got an email saying simply '4081' which I pondered for a while before clicking the linky to the thread. Oops indeedee :D

    2.25 is going to be difficult however given that the lowest I can find is 2.7 (?) from a 500SL. It's also got to be fittable. :dk:
     
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    Druk

    Druk Administrator Staff Member

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    Me bad. :doh:

    Just found a 2.24 is fitted to most 500 S class 126's. Although, would the compromise 2.7 not be better for everyday town/country driving?
     
  5. WDB124066

    WDB124066 MB Club Veteran

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    Are you sure......??
     
  6. grober

    grober MB Club Veteran

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    In theory we could all do this and achieve much better mpgs-- however-- you then have to track back via the gearbox ratios to what RPM the engine is doing to figure out what power and torque its generating at those lower revolutions. Assuming a certain power is required to merely overcome rolling and wind resistance at a particular speed and that surplus power above that is required to accelerate the vehicle to higher speed you very rapidly reach a point where you are left with a high geared vehicle that accelerates very slowly [ if at all] To give you an idea one rough rule of thumb used to be at 100mph 1 extra mph requires10bhp which of course requires more rpms
     
    Last edited: Nov 11, 2011
  7. Troon

    Troon Hardcore MB Enthusiast

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    Don't forget you need a usable first gear for pulling away up hill, too. Changing the diff ratio affects all gears.
     
  8. Dieselman

    Dieselman Banned

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    Yes.

    What do you mean? :dk:
     
  9. Martyn_n

    Martyn_n Hardcore MB Enthusiast

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    Not sure you would notice a massive difference in fuel economy, you are still moving a given mass at a given speed with a given wind resistance. The energy required to do this is the same. Who is to say that your engine more efficiently extracts that energy from the fuel at 2100rpm than at 3500rpm? Early electronic fuel injection systems used to have a deliberately lean window in the map at a given RPM to give falsely lower fuel consuption figures (56mph in top) This resulted in a flat spot at that rev point. Modern engines are far more efficient and have closed loop systems to ensure the mixture is optimum at constant speed / throttle positions. As graeme states you could end up with an undriveable car (all be it quiet if you manage to get it up to a cruise). My view is that I trust MB to have done the correct sums when they came up with the compromise ratio, because everything is a compromise on a road car and they have been doing it longer than me.
     
  10. Dieselman

    Dieselman Banned

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    ^ Hi Martyn, a good and informative post, however MB used to under-gear their cars for quick acceleration at the expense of top speed and fuel economy.

    It annoys me that my own car is geared like this because with the correct gearing I'm sure it would achieve better mpg as it has the torque to pull a higher gear.
     
  11. OP
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    Druk

    Druk Administrator Staff Member

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    I should add that the inefficient petrol engine that seems to deliver no useful torque under 3000RPM is being replaced with a turbo diesel that does the exact opposite. Using the same direct drive 1.1 top gear ratio I'm going to need to change the diff ratio drastically to drop revs at motorway speeds. Pulling away probably isn't going to be a problem but also I don't want to be too slow off the mark.
     
  12. Martyn_n

    Martyn_n Hardcore MB Enthusiast

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    Now it make sense. Thanks for the clarification.
     
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  13. Dieselman

    Dieselman Banned

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    I'm pleased you said that....:D

    For info, the OM606TD generates maximum power at 4,400 Rpm. In the W210 E300TD the gearing is such that V-max is achieved at 4,600 rpm, thus the car is under-geared and would actually be faster and more economical with a longer ratio.

    With a diff giving 70mph at 2,200 rpm (31.8mph/1000rpm), your car will have no problem pulling away or attaining speed since the quoted maximum torque is from 1,600-3,600 rpm.
     
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  14. SL300-24

    SL300-24 Hardcore MB Enthusiast

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    Don't know if it affects anthing Derek, but 500SL 107 diffs are all LSD I believe....
     
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  15. NOMONEYBUTAMERC

    NOMONEYBUTAMERC Hardcore MB Enthusiast

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    Wondered why your 280 engine was sold the other day . Should make an interesting conversion . DSL R107 or SL R107D ?. I was thinking of fitting mine with 4 of the new Ford 1 litre engines , they seem to have generated a lot of interest..........................................;)
     
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  16. WDB124066

    WDB124066 MB Club Veteran

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    I got 3001 rpm, but I might be wrong......
     
  17. Dieselman

    Dieselman Banned

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    All I did was work out the difference in ratio as that was all that changed.

    original diff 3.58
    proposed diff 3.07.

    3.07/3.58 = 0.857
    0.857 * 3500 = 3001


    Looks like you are indeed correct. :eek:

    Just back working my maths shows I must have mis-keyed 3300 in as the original rpm. When I answered yes that was to the bit about the 2.25:1 diff.

    Thanks.. :)
     
    Last edited: Nov 11, 2011
  18. WDB124066

    WDB124066 MB Club Veteran

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    You had me wondering about myself there for a moment.....:):)
     
  19. krimat2005

    krimat2005 Hardcore MB Enthusiast

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    I think it's important what engine you've got, is it OM603 or 606 turbo version?? you can get 2,24 diff from late 500sl, but this might be to low for good fuel economy. I would try 2,47 from late 420sl or 560sl. also early 500sl had 2,72 diffs. just to compare w124 300 turbo diesel (euro version 147bhp) had 2,65 diffs and that'll give you arund 2600rpm at 70mph. to compare go DANE TECHN. - EN
     
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  20. OP
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    Druk

    Druk Administrator Staff Member

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    ^ much appreciate your answer. Although possibly 2.4 would give the car longer legs at speed I think the 2.7 from the pre-facelift 500SL is the way to go. Not least because the rear subframe mounting are the same and it should bolt straight up. The engine I'm using is the 606 turbo giving 180bhp although I'm swapping the 6mm elements from the electric pump into the earlier manual pump and rumour has it that the slightly longer internal stroke of that pump combined with the bigger elements will give nearer 200bhp. Either way the midrange torque should be fun with a 2.7.

    Next question. Anybody got a 500 2.7 diff they don't want? :D
     
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