Sportster gearing revs and differentials.

[stevepritch]

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

[A_Hogg]

Re: Sportster gearing revs and differentials.

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!

[copperleaves11]

Re: Sportster gearing revs and differentials.

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.

[A_Hogg]

Re: Sportster gearing revs and differentials.

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.

[Mike]

Re: Sportster gearing revs and differentials.

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?

[Patrick]

Re: Sportster gearing revs and differentials.

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.

[Patrick]

Re: Sportster gearing revs and differentials.

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

[stevepritch]

Re: Sportster gearing revs and differentials.

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

[Patrick]

Re: Sportster gearing revs and differentials.

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

[Skittler]

Re: Sportster gearing revs and differentials.

Members may be interested in this website - https://tiresize.com/
It contains many pages of comparative data

[stevepritch]

Re: Sportster gearing revs and differentials.

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.

Hi Dave, thanks, I had a look at 6 speed but the final ratio is no different to the 5, so the 1st 2nd, 3rd will be lower ratio. I cant help thinking that it will be a diff change, as it will give the lower gears longer legs.

[stevepritch]

Re: Sportster gearing revs and differentials.

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?

Thanks Mike