Lotus Elan

Differential gear ratios

PostPost by: prezoom » Sun Apr 26, 2020 2:39 am

No, just another way of skinning the cat. I had the close ratio, and was trying to make a decision on installing it in my S2, or see what would happen if I installed it in the Plus2. My S2 came with a mid close, which I would dearly love to convert to a close ration, funds permitting. So the Plus2 won out, as the end result, if things didn't work out with the Plus2, the gearbox would wind up in the S2. The switch started with the original 3.77 in the Plus2. I had the Burton supplied 3.54 on the shelf, actually two of those, so the first conversion went to the Plus2. The engine has enough torque to carry the taller 1st gear without undue strain on the clutch. My driveway is fairly steep getting up to the road, and my S2, with a 3.9 requires more throttle to get up to the road than the Plus2. My Sabra Coupe has a ZF gearbox with near identical ratios as the close ratio 2000E, with a 3.55 and 165-15 tires. It also climbs out with less effort than the S2. However, I counts its ability on the boat anchor weight flywheel's inertia to make it happen.

I have a T9, with its stump puller 1st gear and appropriate bell housing to mate with a TC and have considered making the shifter conversion myself. But I also have an Elva Courier Coupe that is destined to get a TC conversion, where the shifter location of the gearbox is not an issue. Time will tell.
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PostPost by: vstibbard » Sun Apr 26, 2020 8:14 am

Ive had S4SE an S3 Se both with close ratio and 3.55 diff fitted and loved them both, the S3 was however a Mike Spence car and made about 135bhp, the S4SE was UK delivered Strombergs with the D type cams and larger inlet valves standard and it had and Ed Winter head job, it made a solid 125bhp on QED's dyno, when Ken Snailham owned it. I assembled the engine, QED supplied parts and Ken ran the engine in and then set it up on the dyno, we adjusted ignition curve, springs and replaced the needles in the Strombergs. The S4 was nicer less fussy down low than the S3 but both terrific to drive and fast.
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PostPost by: Quart Meg Miles » Sun Apr 26, 2020 10:53 pm

TBG, your tyre size gives the same gearing as a standard 80/145 and with a "standard" semi-close box and your BHP I reckon you would be fine with 3.0 on the back. But you probably wouldn't like the first gear of a proper close rato box though it would provide 57 mph at 6,500 rpm!

I had a S130/5 and its power masked the wide ratios, and most people find 3.55 and close ratio boxes are fine when they have more than standard power. Bitsobrits confirmed my impressions in an early posting here.
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PostPost by: TBG » Sun May 03, 2020 9:31 am

Meg - would you be very kind and let me know the speeds per 1000 rpm in each gear with the specification I am going for. I have not a clue as to how to work it out for myself being a mathematical pygmy! :shock:
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PostPost by: HCA » Sun May 03, 2020 12:17 pm

I think you might have to remind everyone of the 'spec you are going for' first: we know the 3:1 final drive, but the gear ratios and tyre size/circumference are needed.
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PostPost by: TBG » Sun May 03, 2020 12:32 pm

3:1 diff - standard gearbox, 165-70-13 tyres
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PostPost by: HCA » Sun May 03, 2020 1:14 pm

I am not hugely au fait with what a 'standard' Elan box ratios are, but picking one out from the info here , you have, on those tyres:

1st 3.54 = 6 MPH
2nd 2.39 = 9 MPH
3rd 1.41 = 15 MPH
4th 1.00 = 22 MPH

These are plus/minus 1 MPH especially on higher gears, all dependent on the actual tyre diameter deflation etc.
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PostPost by: HCA » Sun May 03, 2020 1:19 pm

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PostPost by: TBG » Sun May 03, 2020 1:40 pm

HCA - that is most useful and hugely encouraging! I cannot wait to have it all buttoned together and give it a go. At the moment I get about 18.5 mph per 1000 rpm in top so it should all be a lot more relaxed when cruising. Thanks again. :) .
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PostPost by: Quart Meg Miles » Sun May 03, 2020 5:44 pm

A bit late to the party!
The 'standard' gearbox for an Elan is the semi-close which is rather more sporting than HCA's ratios:

Semi-close:

1st 2.97 = 7.20 MPH > 43 @ 6,000 rpm
2nd 2.01 = 10.66 MPH > 64
3rd 1.40 = 15.34 MPH > 92
4th 1.00 = 21.4 MPH > 128

A Close-Ratio box would give you:

1st 2.51 = 8.5 MPH > 51 @ 6,000 rpm
2nd 1.64 = 13.1 MPH > 79
3rd 1.23 = 17.4 MPH > 104
4th 1.00 = 21.4 MPH > 128

Edit: did have 1st 3.324 = 6.44 MPH > 39 for semi-close (which was reverse ratio)!
Last edited by Quart Meg Miles on Sun May 03, 2020 9:57 pm, edited 1 time in total.
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PostPost by: benymazz » Sun May 03, 2020 5:57 pm

Quart Meg Miles wrote:A bit late to the party!
The 'standard' gearbox for an Elan is the semi-close which is rather more sporting than HCA's ratios:

1st 3.324 = 6.44 MPH > 39 @ 6,000 rpm
2nd 2.01 = 10.66 MPH > 64
3rd 1.40 = 15.34 MPH > 92
4th 1.00 = 21.4 MPH > 128

The Close Ratio box would give you:

1st 2.51 = 8.5 MPH > 51 @ 6,000 rpm
2nd 1.64 = 13.1 MPH > 79
3rd 1.23 = 17.4 MPH > 104
4th 1.00 = 21.4 MPH > 128


Meg, what diff ratio and tires are this for? I believe that I calculated my S2 running on 145s with its 3.9 diff and I got about 43mph at 6,000 rpm in first (I have the close ratio box)


Also this may have been covered already but what’s OP’s plan for their speedometer? I thought that the speedo take-off gear in the transmission was different depending on if you had a 3.9, 3.77, or 3.55 (3.54?) cwp. I know the speedo has some adjustment mechanism in it that can be used to recalibrate if non stock tires or such are used but I don’t know how wide that adjustment range is. Obviously If you have a 3:1 diff the prop shaft will be spinning significantly slower in all gears than if you had a 3.55, so the speedo will be significantly under-reporting.

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PostPost by: 1owner69Elan » Sun May 03, 2020 7:23 pm

Not sure where the first gear ratios of >3 in previous posts are coming from - perhaps the five speed Lotus Maxi box -previous post has been corrected. The standard (semi-close) Elan 4-speed 1st gear ratio is 2.97, AFAIK.

Plugging into the calculator: a) the standard gearbox ratios, b) the proposed 3.0 diff and c.) the specified 165/70r13 yields the following information, presented in graphical form:

diff 3.jpg and


You can see the speed vs rpm in each gear, as well as rpm drops at a shift point of 6000. You can easily use the calculator yourself to assess the effect of changing parameters.
Last edited by 1owner69Elan on Sun May 03, 2020 10:49 pm, edited 1 time in total.
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PostPost by: TBG » Sun May 03, 2020 9:44 pm

Thank you all so much. I am flabbergasted at the knowledge you all have and your willingness to help an old man!! I Am even more excited now and think I may have stumbled on a good idea. Any one agree? :o
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PostPost by: Quart Meg Miles » Sun May 03, 2020 10:31 pm

benymazz wrote:
Meg, what diff ratio and tires are this for? I believe that I calculated my S2 running on 145s with its 3.9 diff and I got about 43mph at 6,000 rpm in first (I have the close ratio box)

Also this may have been covered already but what’s OP’s plan for their speedometer?
-Ben

Ben,
His stated plan was 3.0:1 and 165/70 13s and I computed for that.

I agree with your figure for standard tyres, 3.9 and CR box.

The speedo drives have a changeable plastic gear wheel to match the diff but I couldn't buy one for a 3.55 thirty years ago. They wouldn't have one for a 3.0 I'm sure!

Unfortunately, as pointed out by ..69elan, I mistakenly used the Reverse gear ratio for the semi-close instead of 1st gear and have corrected that in my original posting (but can't change your quote of it). Sorry!

TBG Some of us old men have old manuals and old slide rules to work it out!
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PostPost by: TBG » Mon May 04, 2020 8:48 am

Thanks Meg, brilliant stuff from all of you. Regarding the speedometer I am not too worried as outside towns and villages I rarely look at it and my telephone that I use for navigation - the simply amazing Waze - has a digital speedo.

Hopefully the Rozzers won't be able to keep up!! :D
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