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Thread: Sportster gearing revs and differentials.

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  1. #1 Sportster gearing revs and differentials. 
    frequent forum contributor Club Member stevepritch's Avatar
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    I think generally we all have thoughts of a magic gear box that moves 2nd to first and a new ratio in the 5 speed Getrag.
    I have been looking at alternatives and think a new the diff might be an answer.
    The curb weight being around 850 kg its significantly lighter than the original car, and I do believe the change will significantly increase cruising mpg.
    Currently 3k rpm is 70mph according to the clock.
    My current diff is a 3.73 ratio so I was thinking moving to a lower ratio maybe closer to 3.2, or maybe high 2 ratio.
    I do not think it will greatly affect 0-60 as first gear will actually be of use again, but wondering how low...or high do you go?

    Looking for thoughts and any advice on what others have done.

    Thanks to all..............Steve
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  2. #2 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member
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    Hi Steve
    I changed the diff in my Cabrio to a 3.38 [again BMW 2.5 engine]and thought it suited the engine/vehicle weight better.
    Mike Garner I think tried a 3.14 in his Cabrio but found it too tall and went back to 3.38. Take a look at his build record in the members links as to the why's and wherefores.
    David Siddall I believe is also using 3.38 in his Sportster. No doubt he'll be on here to confirm/deny!
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  3. #3 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member
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    Hi Steve . As Alan says, mine is a 3.38 and I still find 1st gear all but redundant.I hasten to add we are talking about Ford units, if yours is a BMW item, there is probably a greater choice. You could explore a later gearbox, even 6 speed , bearing in mind that more recent units are at 20 degree incline rather than the 10 degree of the M20. Interested to see what you come up with.
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  4. #4 Re: Sportster gearing revs and differentials. 
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    Hi Steve
    As David says 1st gear is a bit of waste regardless of diff ratios. I have read somewhere that a close ratio box was offered on some of the 325i models but I've never researched that information to confirm its validity - maybe an avenue for you to explore.
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  5. #5 Re: Sportster gearing revs and differentials. 
    Club Member Mike's Avatar
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    Hi Steve

    As has been said above, I tried a 3.14 diff (after a 3.38 diff) and found it too tall for cruising on a motorway inclines - I felt it necessary to change down a gear. I find the 3.38 is a better drive overall.

    That said, I have the later M50B25 engine, which delivers its power higher up the rev range, so you may find a 3.14 or BMW's nearest equivalent, will work for you with the M20B25 engine?

    With regard to gearboxes, be careful to check all the gear ratios: I have a 6 speed manual gearbox lined up for my Cabrio from a Z4 (to go with the more modern M54B30 engine), but it's top gear is the same as my current 5 speed.

    I have had it suggested to me that a 5 speed box is better than the 6 speed to drive as you don't need to change gear as often due to the close ratios in the lower gears in a 6 speed box.

    A possible solution for your scenario would be to go for a 6 speed and a low ratio diff, and then regard 6th gear as an overdrive for high speed cruising?
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  6. #6 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member Patrick's Avatar
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    I have the M20 rebuilt with an M50B28 crank so it's a 2.8, and a 325 Getrag 260 box and a 3.91 LSD. 1st is pretty much a waste of time, only use it to pull away a little and switch to 2nd.

    The box has these ratios (all 325 models, except some of the sport ones with a Getrag 260/5 (more info: https://www.e30zone.net/e30wiki/inde...rain#Manual)):

    1) 3.83:1
    2) 2.20:1
    3) 1.40:1
    4) 1.00:1
    5) 0.81:1

    I do have a 6 speed box in storage from a 320D (E46) - same bolt pattern and the right angle for the M20 (the ZF N/M5X gearboxes all end up the wrong angle on an M20). It is longer so I'd need to shorten the drive shaft if I do ever use it.

    I'd still need to change the diff ratio, 3.15 maybe but they're getting harder to find (at a sensible price) with an LSD or Torsen.

    I changed the tyre size during the build from the E30 size to E36 size - 205/50/R15 to 205/60/R15 - about 6.5% difference which is one way to drop the RPM a bit.
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  7. #7 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member Patrick's Avatar
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    Did some maths, assuming a 6500 RPM redline (mines up a bit from stock at 6750)

    Geared speed in mph at 6500RPM with 205/60/R15 and a 3.91 diff on stock E30 260 5 speed :

    1) 31.9
    2) 55.5
    3) 87.2
    4) 122.1
    5) 150.7

    Changing to a 3.15 diff:

    1) 39.6
    2) 68.9
    3) 108.2
    4) 151.5
    5) 187.1

    3.91 diff with standard E30 size tyres: 205/55/15

    1) 30.8
    2) 53.7
    3) 84.4
    4) 118.1
    5) 145.8
    Last edited by Patrick; 23-08-2019 at 08:22 PM. Reason: wrong numbers on 3.91 with 205/55/15
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  8. #8 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member stevepritch's Avatar
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    Quote Originally Posted by Patrick View Post
    Did some maths, assuming a 6500 RPM redline (mines up a bit from stock at 6750)

    Geared speed in mph at 6500RPM with 205/60/R15 and a 3.91 diff on stock E30 260 5 speed :

    1) 31.9
    2) 55.5
    3) 87.2
    4) 122.1
    5) 150.7

    Changing to a 3.15 diff:

    1) 39.6
    2) 68.9
    3) 108.2
    4) 151.5
    5) 187.1

    3.91 diff with standard E30 size tyres: 205/55/15

    1) 38.3
    2) 66.6
    3) 104.7
    4) 146.6
    5) 181
    Patrick Wow.... need to find that formula. With a 3.15 looks like minimum 20% reduction in revs, which will make cruising more comfortable.
    Think I have 205 55 16 , need to check for certain, but think that's a slightly larger overall all dia, might contribute to slightly lower engine revs.

    Thanks..............Steven
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  9. #9 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member Patrick's Avatar
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    Quote Originally Posted by stevepritch View Post
    Patrick Wow.... need to find that formula. With a 3.15 looks like minimum 20% reduction in revs, which will make cruising more comfortable.
    Think I have 205 55 16 , need to check for certain, but think that's a slightly larger overall all dia, might contribute to slightly lower engine revs.

    Thanks..............Steven
    I got the numbers wrong for 3.91 + 205/55/15, I've edited it (sorry!) I was looking at it thinking, the 55 height should br lower than the 60.

    This is the math:

    205/60/15 - 1st Gear 3.83 - Final Drive 3.91:

    Get the diameter:

    Get 60% of: 205x0.6 = 123 * 2 = 246

    + Wheel diameter 15" x 25.4 = 627

    Get the circumference:

    2xPixRadius = 2x3.14159265359x(627/2) = 1969.78 (rounded to 2 decimal places)


    Engine RPM 6500 / Gear 1 3.83 / Final Drive 3.91 = 434.04806582 rotations per minute at the wheels * circumference 1969.78 = 854978.59 mm per minute

    1mm per minute = 0.000037 mph

    So

    854978.59 * 0.000037 = 31.88 mph

    I didn't round down any of the number during the calculation so 31.88 is a more accurate number than the ones above.

    This is a handy tool so you don't need to do the math by hand: http://www.cargister.com/calculator-gear-ratio
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  10. #10 Re: Sportster gearing revs and differentials. 
    frequent forum contributor Club Member Skittler's Avatar
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    Members may be interested in this website - https://tiresize.com/
    It contains many pages of comparative data
    Brian

    Learning more about:
    June '73 Marina based Mk2 Roadster
    1800 TC; 4 speed gearbox; 4.55:1 differential with 4.5pcd (the rare one!)

    Another interest -www.somersetschoolsathletics.org.uk
    And another - 'Tractor Doctors' skittle team

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