Lotus Elan

Could I possibly have a 3.9 diff on my stock S4 SE (Federal)

PostPost by: 1owner69Elan » Fri Oct 21, 2016 1:26 am

So now I am confused.

I was putting my diff back in and noticed that my builder, after installing a Quaife ATB, had written 3.9 on the side of the diff. I always thought I had a 3.55. Speaking with the builder he hadn't changed the R+P so the ratio hadn't been changed from original but the 3.9 is what he had counted. He did offer that perhaps he miscounted. But, he normally is so meticulous that doesn't seem entirely probable.

I am the original owner of the 1969 S4 SE DHC (Federal) Elan that I picked up at Hethel, drove the car for 2 months in the UK/Europe and then shipped back to California. I checked my original invoice and there is no mention of any optional differential being specified. No mention of diff at all. The invoice (dated 28th June 1969) does refer to my car as a "super safety". Badging is S4 SE. Stromberg emission engine. Dual circuit brakes, Headrest seats, ...

Just wondering if anyone knows what the standard spec for the diff might have been for my car.

Back then, I was able to make what was shown as 125 mph or so (legally on the Autobahn) not exceeding redline. Maybe that's why I always thought I had a 3.55. Not sure what the speedo error might have been. Tires were the original 155x13 Dunlop SP sports as delivered.
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PostPost by: tedtaylor » Fri Oct 21, 2016 1:46 am

if i were going 125 MPH in an Elan, i'd have to change my shorts !!! :)
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PostPost by: 1owner69Elan » Fri Oct 21, 2016 4:06 am

Really not that bad on the autobahn. But I was young. Didn't seem all that nuts at the time.

Done 150+ more recently in more modern cars on the autobahn. And passed by others.
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PostPost by: Elanintheforest » Fri Oct 21, 2016 6:47 am

The brochures for the S4 show the standard car as having a 3.7:1 ratio, but no mention of the 3.5:1 ratio in the list of options provided on the S/E. The brochures for the S3 show that the S/E had the 3.5:1, with the standard car having the 3.7:1.

A lot of Brit cars did have a higher (number) differential put into their Federal cars to improve the acceleration numbers, but of course you could order whatever ratio you wanted, and if you didn't ask, you probably got whatever ratio the factory had!
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PostPost by: 1owner69Elan » Fri Oct 21, 2016 7:37 am

To add to my uncertainty now is that at the time of my order there was a backlog on orders in 1969. You had to wait to get delivery. Unlike a few years later when demand dried up.

I was fortunate to know the fellow that had developed the Stromberg emissions setup for Lotus, who was now in the US. He drove a RHD Lotus Cortina and was constantly being pulled over because the Highway Patrol couldn't figure out who was driving.

He called his friends at Hethel and they pulled a car off the assembly line for me. I guess I "jumped the queue". But it may have led to some of the confusion over exactly what is the configuration of my car to this day. I'm not sure how I had come to the conclusion that I had a 3.55 and that has been my belief for the last 47 years until someone just counted the teeth and said it's a 3.9! Still not sure I believe it.
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PostPost by: gjz30075 » Fri Oct 21, 2016 9:04 am

You can still determine the ratio with counting the wheel rotations in relation to the driveshaft rotaions. Not
really easy with our type of rear suspension but doable. In your case, I suspect both wheels will need to
be in the air to rotate.
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PostPost by: Chancer » Fri Oct 21, 2016 9:10 am

Take the plugs out, put the car in 4th gear, mark the crank pulley against the TDC mark or something else suitable, mark the rear tyres contact points, push the car forward one revolution of the wheels, look at the crank pulley and guesstimate if it has moved 0.54, 0.77, or 0.89 of a revolution (plus " r?volutions).

How will you feel if you have been wrong all these years?

Editted, if it does turn out to be a 3.89 then you should suspect that you have an incorrect speedo drive gear, bonus though, your car has done less miles than you thought :lol:
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PostPost by: elanner » Fri Oct 21, 2016 11:45 am

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PostPost by: msd1107 » Fri Oct 21, 2016 5:22 pm

RE: Ted Taylor.

I had my rev limiter adjusted to 7,000 and ran up to it on my 3.55 S4 FHC with 165-13 tyres. 130+. No sweat. The car seemed happier there than at 75-80.

I came back from Monterey to San Luis Obisbo in company with a Porsche and a Ferrari at over 6,000. No sweat.

Maybe a FHC is better for high speeds?
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PostPost by: 1owner69Elan » Sat Oct 22, 2016 10:37 pm

Well, it's definitive. I don't have a 3.55 as I had thought for 47 years. And I don't have a 3.9 either.

I do have a 3.77 which seems to be the factory default at the time of my delivery. Not sure how I went down the 3.55 path - perhaps my early experience on the autobahn.

To determine the ratio I used the obvious method of simply counting the relative rotations of the prop flange vs the diff output shafts. I Just made a mark on each, turned the flange by hand and counted. Quite accurate even if for a single output rotation - immediately obvious whether a 3.55, 3.77 or 3.9. Also did it over 4 output rotations which equated to ~15.1 prop shaft rotations (15.1 / 4 = 3.77). My diff was disconnected and easily accessible as no suspension in the way at the moment.

So I think the 120 mph that i read on the speedo while on the autobahn was undoubtedly optimistic. The 3.77 with the stock 155 tires back then should have yielded about 113.5 mph top speed at 6500 redline, I think. So speedo about 5% optimistic - not surprising.

It also means that the speedo has been even more optimistic by going to a lower profile 175/70 tire - but the speedo drive had failed many years ago so I have been using the tach to gauge speed. Speedo now repaired but no experience yet with speedo calibration as the car is being reassembled.

I guess I am happy that I have the 3.77 as a trade off between top speed and acceleration. Top speed not really usable in California. Also, probably not going to be taking frequent, long cruising trips after I get the car back together.

Last question, are the speedo drive gears available anywhere? I am thinking I may ultimately need one that is closer to the 3.9 diff version (25 teeth, blue one) but I couldn't find it anywhere.
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