Lotus Elan

Must be nuts

PostPost by: gav » Thu Dec 26, 2019 1:57 pm

I am running a Sierra diff in a Spyder spaceframe chassis and it is a pig to fit with the body on. When I fitted the current diff it was with the body off and relatively straightforward to fit

Annoyingly, the current diff ratio I am running (around 3.92) meant that at 70 mph the engine is running at 3,500 which isn't great for cruising. I have just started the strip down to replace it with a 3.62 and predictably the car is fighting me at every stage.

Just finished the first of two sessions removing exhaust, drive shafts and prop shaft. Next session is to remove the lower wishbones, the removable bars under the diff, the front diff mounts and the diff itself. Then the replacement diff needs to be installed.

The diff weighs a lot and manoeuvring it is trick so overall, a horrible job - must be nuts -all for 500 RPM

Still - gets me away from the gogglebox....
One day I'll actually finish - completely - one day....
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PostPost by: Foxie » Thu Dec 26, 2019 8:59 pm

My experience with diffs.

If you are going for a 3.62 to replace a 3.92 Zetec diff, you are only saving 300 revs at your cruising speed of 70 mph.

I think one of the great advantages of the original diff is the range of ratios available. Burtonpower list 9 different ratios, from 3.54 to 5.29.

Several year ago, I fitted the complete Zetec Sierra brake system to my Spyder chassied +2. However I retained the original diff as my chassis was a Mark I and would not accept the Zetec diff. Spyder made up a set of custom drive shafts to mate the original diff to the Sierra hubs.

The original diff in the Plus 2 was a 3.9, with the 4 speed gearbox. Over the years it went to 3.54 with a T9 5-speed gearbox, which then acquired a long 1st, then recently to a 4.55 for competition use. The engine is much modified and is presently at 1860 cc / 170 hp.

I made up a standard diff brace very early on, and recently modified it to attach to 3 ring bolts.

As the car is road legal and I drive to all events, the 4.55 was very limiting for road use even with the T9 5 speed so I am at present replacing it with a 3.9.

(Great recent mod I learned here was to cut of excess casting metal under the diff nose, makes diff swops a doddle ! )

One of the original diff output shafts died quietly from fatigue failure while returning on a 265 mile trip from a hillclimb last year. Both output shafts were replaced by Tony Thompson billet shafts. I estimate the original shafts survived about 700 racing starts.

I fitted a Quaife ATB diff quite a few years ago, which is changed over with the different ratios.

As I have just won the 2019 Irish Hillclimb Championship (Historic Class) for the second year running, at my age I look forward to taking it a bit easier next year !

:)
68 Elan +2, 70 Elan +2s
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PostPost by: gav » Fri Dec 27, 2019 11:39 am

Well done Foxie - two years on the trot - very impressive.

You are right about the drop in revs - it is around 300 rpm and if it weren't for a leak in the oil seal to the back plate I'd live with the current diff but I think it needs to come out anyway for that to be resolved (the bolts are inaccessible in situ - grr).

I'm running a 4.26 in the competition car which I think will be a good compromise so not far off your set up.

In terms of gearbox, I'd love a 6 speed with a first gear around 2.9 and a double over drive. I have wondered about an overdrive set up but now the car is running and has loads of torque, the plan is to put some miles on it so this is the last of the large tasks in preparation for the spring.

I can't see you putting your feet up next year so whats the plan?

All the best
Gavin
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PostPost by: Foxie » Fri Dec 27, 2019 2:45 pm

Well I was looking at fitting a 6-speed in my regular Plus 2.

Spyder have a project to adapt the Mazda 6-speed, but it has been put on the long finger indefinitely due to present work load.

Now that Foxie seems to be fully sorted, maybe I can get back to finishing some other projects, like putting a new chassis, and other work, in my other Plus 2, ( which hasn't run in 18 years)

Also getting a couple of +30 year old 3-series BMWs up and running, and SOLD.

Then there's the + + 30 y.o. Nissan Patrols......

:shock:
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PostPost by: Foxie » Fri Dec 27, 2019 2:49 pm

A bit like yourself, actually !! :D

gav wrote:One day I'll actually finish - completely - one day.... :roll:
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PostPost by: gav » Sat Dec 28, 2019 10:25 am

As my daughter quotes to me all the time - that's my muchness....(Alice through the looking glass)

I think it is rather appropriate
One day I'll actually finish - completely - one day....
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PostPost by: gav » Sun Jan 05, 2020 2:01 pm

What an awful job - I forgot that it is really difficult to replace the Sierra diff with the prop shaft in place - and to make it more difficult, I did it with the rear mount in place otherwise I'd have to strip out the fuel injection system in the boot because it sits over the diff mounting bolts.

Just for a challenge, I decided to fit a 7.5" version which is slightly heavier but stronger because it seems that I blew up the diff in the 7" version on my last event (and I had one to hand already refurbished). That made it more exciting still. Many hours later it's in and I've only got the driveshafts and exhaust to go before I'm done.

It wont be coming out now until the body is next off - and I'm hoping that will be many years away
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