Lotus Elan and Lotus Elan Plus 2 Forums

I've just bought a 3.54 diff on eBay rebuilt with new bearings, so I haven't seen it yet. It's the Ford RS2000 item. Is it a straight swap for the existing Elan diff (mine is a 3.77) or do I need to change the pinion flange? If so, does that mean the pinion bearing preload has to be reset? Thanks for any insights.

John

Christian,

No Elan is going to be setting any speed records, due to being pathetically under geared.

Consider, the highest gearing is achieved with 165X13 tires with the 3.54 differential. This gives a little over 130 mph at 7,000, a speed that a well tuned stock engine will do.

Consider, during the life of the Elan, road test reports either ran up against the rev limiter at 6,500 rpm, no matter what the gearing was, or, if the rev limiter was removed, usually got to 7,000. With 155X13 tires and 3.55 diff, this was just under 130 mph, and several road tests got this. It was probably a calculated gamble by the factory that the engine would live long enough above 6,500 to survive the road test.

Keep in mind that the SE engine, rated at 115 hp at 6,000 was probably down to 100 hp at 7,000 rpm.

With many owners tuning their twinks to the 135-160 hp range, prospective top speeds are even higher. Those owners who have adapted a 5-speed into their car can probably pull peak power rpm (6,500) in 5th gear with the more normal 3.77 diff, and possibly even with the 3.55 diff.

David
1968 36/7988

John
I did exactly the same swap last year.
To answer your questions, the Lotus pinion flange is smaller than the Ford one. I bought a spare flange (at surprisingly large expense) and got the reconditioning firm to fit it with the correct preload, although I have distant memories of doing it myself on escort diffs many moons ago.
Getting the diff out is a bit of a wrestle - after several hours and much colourful language I ended up shaving a bit of metal off the underside of the diff casing nose and that made all the difference. After doing the same mod on the new diff, the new one went straight in first time.
Overall I regard the taller diff as a big improvement

Good luck

Kevin

John,
Type "Differential" in the search box and it will come up with lots of threads giving useful information on 3.54 difs. I even put some stuff in myself at various times as I did mine a couple of years ago with a dif from an Escort Mexico which required the smaller input flange.

Gordon

To consider as part of the swop to 3.54 CWP ratio: You will need to retain the smaller Elan diff input flange. This has pretty small fasteners (4 off 5/16") for the torque being transmitted, compared to Ford's later designs for the RS2000. With an uprated engine, it might be worth considering using quality higher strength fasteners (eg NAS bolts and K nuts) and torquing them up to a level where the drive is being transmitted by friction between the flanges.
Regards

Gerry

Thanks to all for your helpful replies.

John

I am doing the same swap as part of my rebuild. I had the 3.54 diff refurbed with new bearings and crush spacer by specialists. They also swapped over the flange from the 3.77 diff for me. I then sold the 3.77 on ebay to recoup the cost of the 3.54, so all I have paid for is a rebuild of the diff, which cost ?100, including VAT (seemed reasonable to me, any comparisons?).

Jeremy

This is probably a daft question, but couldn't the end bit of an Escort propshaft with the right flange size be built into an Elan propshaft? I have a spare new 3.5 diff and an empty ally Elan casing...and a spare smaller flange. Just thinking about a simple alternative...but I'm sure there must be a good reason not to do it!

Mark

Extra weight...

Matthew

Mark,
I have fitted the big flange diff to my standard type chassis - Don't do it!! I had to remove loads of metal from the diff it just made a difficult job into an horrendous job and that was with out the body fitted.

Also the casting of the Escort diff was bigger than the standard 3.7 that was in my car.

Cheers
Steve

3.54 diff now fitted - done while rear suspension in pieces so several birds killed with a single stone. Re the pinion flange, I reasoned that the collapsible spacer and suitable shim had already been set up correctly so if I re-torqued the flange nut to 30lb ft (the lower end of the 30-35 range given in the manual) after fitting the smaller flange, the preload etc would surely be about right. And the resistance to turning indeed felt almost identical to how it had felt before. I Loctited the nut thread as well as staking it, just to make sure.

The diff is silent and all seems to be fine. The longer gearing suits the Elan very well, and amazingly the speedometer is dead accurate right the way up to 80mph (haven't ventured further yet) judging by comparison with a Road Angel GPS system's speed readout.

John

John,

Were you previously on 3.9 or 3.77?

I have a 3.9 fitted and 3.54 sitting on the shelf and I can't decide if I want the longer legs of the 3.54 or the all-out acceleration of the 3.9.

Paddy

Paddy

It was 3.77 before, but second-gear roundabouts at one extreme and open-road cruising at the other were just a bit too frantic. With the 3.54 (six per cent longer-geared), third becomes a brilliant country lane gear and very 'rangey', second does all tighter corners without hitting huge revs all the time, and it's more relaxed at speed. Outright acceleration is similar, but achieved differently; I just hang on to the lower gears longer when needed. Contemporary road tests bear this out. I'm very glad I made the change even if the Elan isn't subjectively quite as bombastic from rest now.

John

PS - anyone want a 3.77 diff in excellent, silent-running condition? Otherwise it will probably go on eBay.

Hi
Found this old thread very useful as it answered one of my questions about the diff swap - ie size of flange - thank you everybody.
I do have a second question on the same subject. I have been told that the drillings fixing the diff into the Lotus diff housing don't match the Ford diff . Can anybody confirm this or is it just a straight swap?
Thanks again
Baggy