+2 diff ratios

[Baggy2]

Hello,
I am currently re-commissioning my early +2 that has been off the road for some years - its a 1968 car wiith SE engine ,four speed box and the original 3.77 diff. I have a 3.54 diff but having done the sums I'm wondering if its worth the trouble of putting it in - it seems it will only drop the engine revs at 70 or 80mph by a few hundred rpm. I'm sure there are many out there with first hand experience who could give me some guidance? The car is only for road use touring on holiday and the like.
Thank You
Baggy2

[kstrutt11]

Is it a 4 or 5 speed?

In my experince the 4 speed is very undergeared for a touring car which eventually gets annoying, If you are not looking for out and out performance I would go for the longest gearing you can.

If it's a 5 speed I would leave as is.

[Roy Gillett]

Hi Baggy

I have a +2 S130 with a 3.77 diff and my brother has a very similar +2 on a 3.54. We often drive the cars together so I can offer a direct comparison, for what it is worth. Our experience is that the revs mine is cruising at at 70 are about the same as Paul's car doing 80. That makes his car a lot less tiring when we are both doing 80!

Mine is definitely quicker off the line and out of roundabouts (where i can hold on to 3rd if I am not in a hurry - where he has to drop into 2nd.)

So it depends on how you use the car. A lot of motorway / fast touring and the 3.54 would be my choice. A lot of twisty 'B' road work and the 3.77 is more fun

Hope that helps

Roy

[david.g.chapman]

Just to make life more complicated - I have a 4 speed box and a 3.77 diff on my +2, but I also have an extra silencer "bomb" before my rear box in the exhaust airstream. I also have a home made baffle in the tailpipe - just a tube with some holes in and a closed end facing downstream.

This has the effect on my car of making the noise from the exhaust less at 80 then at 70mph. Bearable for those long motorway trips (in France of course)

I can't say I notice any drop in power, but then I don't have much to start with (120bhp if I am lucky).

Just another option you might want to consider....

Dave Chapman.

[Baggy2]

Thanks all for your opinions - on balance I think I'll probably do it. A related subject is tyre size. I have 185 x 70's on. I think the +2 was originally imtended for 165 x 80's which were probably slightly larger rolling radius - i'll look it up....
Thanks again
Baggy2

[gerrym]

Baggy, the 185/70 and the original tyres (ie what would now be a 83/165 section if you could buy) are so close on rolling diameter that in practice it's a matter of tred depth, design of tyre in terms of flex, curvature etc).

However interested to hear that you are getting these to fit. Are you using 2-1/4" ID rear springs or the standard Lotus. Do you have any rubs on the inner front mudgards. These may only happen with some steering angle and suspension compression so you may notb even know, unless you take a look and see the evidence. When I asked this question last year, some forum members posted photos of the rub coming all the way through the inner mudguard, with a hole into the engine compartment side.

Regards

Gerry

[msd1107]

For a +2 with 165-13 tires, a 3.77 diff gives about 3344 revs at 60mph, a 3.55 diff gives about 3154. At 80 mph it is 4459 and 4205.

185/70-13 has an average of 893 rev/mile as gleaned from the tire manufacturers technical literature.

165-13 has an average of 887 rev/mile - virtually identical.

Others are: 175/70-13 is 917 rev/mile. 155-13 is 912 rev/mile (Standard S4). 145-13 is 939 rev/mile (Standard S2). 185/60-13 is 956 rev/mile (common replacement, very small and high revving). 185/55-15 is 904 rev/mile (S4 replacement if you go to larger wheels). 195/55-15 is 887 rev/mile (+2 replacement if you go to larger wheels). 185/60-14 is 915 rev/mile (common Spyder replacement for their Minilite type wheels).

There is a downloadable spreadsheet at the bottom of viewtopic.php?t=16413&view=next that has two worksheets (Tires and TiresS) that contain all this informatiion for more tires than you care to think about.

At the bottom of elan-f15/updated-spreadsheets-t18445.html is another spreadsheet including correct transmission speedometer drive gears and speedometer/ odometer calibration and gearing.

If you look hard enough, there is a spreadsheet to give you confusing answers for almost any drive train question an owner may have.

David
1968 36/7988

[Baggy2]

Blimey - enough info there to totally boggle the mind - I like to keep the car reasonably original so that means 13" wheels and it looks as if the current tyre choice is pretty good on that basis.
Gerry - I am running standard Lotus rear springs - the is the slightest evidence that the tyre has touched the bottom spring platform but it really does seem to be a light touch - the evidence is on the spring platform - cant see anything on the tyre! There is no problem at the front at all.
Thanks again
Baggy2

[gerrym]

Blimey, looks as though the tolerances on your body/chassis are good enough for the 185/70s. For the rear, it has been reported that a well aimed hammer can give the smidgeon extra clearnance.

Regards

Gerry