Tall 3.0 diff ratio

[EdHolly]

These days to get from A to B we mostly use highways and freeways that simply didn't exist all those years ago when Elans were new. Modern cars have very high gearing, my Ford Falcon cruises at 110kph at 1700rpm, well over 1.6 tonnes and under these conditions returns around 7.5 litres per 100km, or 36mpg.

My Elan gives exactly the same consumption at this speed, yet improves to over 42mpg when cruising about 10-15kph less. The Elan is doing 3900rpm (3.9 diff) at 110- a bit different to the 1700 of the modern car.

I used to have a MGA Coupe 1600 and I had a 3.9 diff in it which gave almost 20 mph per thousand revs (15inch wheels) rather than the 4.3 diff as normal. It also had a close ratio gearbox with a very tall 1st gear - YET - getting off the line from say a set of lights was no more difficult with the 3.9 as it was with the 4.3. Oh and the MGA had a 3/4 race engine so it didn't have a lot of torque down low.

Guess what I am saying is that in a light car like the Elan, when used for the road, the diff ratios we run say 3.9, 3.7 etc ... then we are paying a penalty in making the engine rev a lot higher than it needs to. Sure we can go the 5 speed box route, but the standard Elan box is a delightful gearbox and the gearshift is just in the right place.

Now what if we were to drop the 3900 rpm at 110kph down to 3000 - sure would make legal cruising a lot less busy and a lot less noisey.

So I am contemplating have a 3.0 crownwheel and pinion made to investigate what this does to the driveablity of the car.

Would appreciate any comments ...

(posted this in error in maintnance gearbox final drive initially)

Ed

[rgh0]

Hi Ed
Should work fine I would think. it depends a little on the 4 speed box - close or semi close or standard cortina ratios, the engine tune and the tyre diameter but a 3.5 diff works fine in a heavier plus 2 so apart from being a little slow off the line in comparision a 3.0 should be OK in an Elan.

cheers
Rohan

[elansprint71]

I have a 3.5 in my Sprint and do now have to slip the clutch just a little to get off the line (I don't know what my gear-box ratios are!). 3.0 might be just a little difficult.

[Lyn7]

Hi Ed, I agree with Rohan, I think this should work fine. I have a 3.5 diff in my Sprint and a 5 speed box. If a taller diff was also available I would be prepared to give it a try. Good luck getting a crown wheel and pinion made.
Lyn...

[elan66]

Hi Ed
i spoke to someone the other day who has had drawings drawn up and is having 3.32:1 crownwheel and pinions for the Escort boys made up.Should be available in the next 6 months at around the ?300/?350 mark
regards
Paul

[vstibbard]

Ed,
I've run two SE Elans both with close ratio box (2.5 first gear) and 3.55 diff, they were sensational both off the lights due to the long first and at cruising speeds.

If a 3.3 is a reality I'd stop at that as pulling out of uphill junctions did require some clutch work with the 3.55!

My next Elan has a1760 engine with very flat torque curve and healthy HP, I was planing on fitting a Lotus 5 speed but do agree with you about the quality the 2000E gear change.

BTW I emailed you re Group M inspection for the S1 I'm about to finish, did you get the email?

Cheers

Vaughan 0412 975 932

[promotor]

If you wanted to fit a 3.0 diff then I reckon you may want to change the gearbox to a 3.5:1 first gear to get you off the line if you didn't do anything to improve the torque/power of the engine. That way you could keep the quality of the gear change and still get off the line.
The problem with a 3.5:1 1st gear type box is that you change out of 1st gear very soon after setting off because it is so short, and 2nd and 3rd have quite a large drop and that becomes very noticeable on a not so torquey engine so may ruin your experience of driving the car. Bad ratios can mean worse MPG if the engine isn't keep within it's most efficient range for as much time as possible.

IMHO the best way around it (if you really want good MPG) is to fit a proper Lotus 5 speed box or a shortened Type 9, and fit a big capacity Lotus Twincam (1700+) with modern spec fast road cams on either very well setup Dellortos (better for fine tuning the fuelling requirements for good MPG than Webers) with mapped ignition, or full on fuel injection. Then you should be able to fit any ratio of diff and still manage to get away from the traffic lights