diff ratio?

[Elanconvert]

hi guys

being a lazy bu***r, [it's also cold and windy outside] I am wondering if there is any quick way of telling which diff is installed in my 1970 s4 without doing the jacking up/wheel turning/measuring job?

thanks in advance

fred

[Chancer]

Take the plugs out, mark the tyre contact point at 06.00, mark the crank pulley at a suitable reference point, push the car in 4th gear (direct drive) for one wheel revolution, look at the angular position of the crank pulley.

[Elanconvert]

good tip ..thanks

incidentally, were the different differentials [3.55, 3.77] fitted randomly, or was there a sequence of chassis numbers or models with one or the other?

[rgh0]

The most likely diff you have in a 70 S4 Elan is a 3.77. The 3.55 was an option as I understand it. The only car with 3.55 as standard as I understand it was the Plus 2S 130/5 but maybe the late Elan sprints also had it as standard, others may know. The earlier S1 and S2 cars had a 3.9 diff as standard I am not sure when it was change to the 3.77 as standard but I presume around 1966 or 67 with the S3. Not sure when the 3.55 became available as an option but I presume in the early 70's

cheers
Rohan

[Elanintheforest]

I believe that the S3 S/E came as standard with the 3.55 : 1 ratio,




and the S4 S/E with the 3.77 : 1, seemingly without the option of a 3.55..





But of course anything could have been fitted in the last 40 years.
Mark

[rgh0]

Hi Mark

I was not aware that the 3.55 went back as far as the S3 SE. I know it was an option on later S4's at least but wonder why it disappeared on S4 SE's as standard

Lotus was a wonderful company in that period with lots of strange things going on

cheers
Rohan

[Elanintheforest]

I started off with a S3 S/E Rohan, so I always thought that all S/E cars had the 3.55, and the standard car had the 3.77. As usual though, Lotus presented quite a confused picture on the final drive ratios!

The first reference I can find of the 3.55:1 ratio is on a launch brochure for the S3 Coupe in 1965, which has the 3.55:1 as an option.

It only seemed to be standardised when the S/E was introduced in July 1966, and I believe it remained as standard on the S/E until the end of S3 production.

The S4 sheet above is the launch sheet, and there is no mention of the 3.55:1 even as an option for the S4, and it's not mentioned on the main S4 brochure (with the yellow car on the front).

I can't find any mention of the 3.55:1 as an option on the Sprint brochures either.

But there is a Lotus road car 'range' brochure from 1968, which shows the S4 as having 3.77: 1 as standard, and 3.55:1 as an option...see below.

I'm sure you could get whatever ratio you wanted from Lotus if you knew who to ask and knew what was available in the Ford range though.

But it does look like Lotus settled on the 3.77:1 as being the best all-round ratio for the Elan and Plus 2, after a couple of years of 3.55:1 with the S3 S/E.

I would imagine that it had something to do with initial impressions and general drivability...the Elan really won customers on acceleration and handling, and not cruising comfort.



Mark

[Elanconvert]

thanks guys!

never ceases to amaze me how much you know!!!!

fred

[stugilmour]

Fred, if your car is running (and weather OK ), you could use David's (aka msd1107) excellent spreadsheet to match your tach readings to either GPS speed or your speedometer. Might be more fun driving around instead of pushing the car in the drive.

The spreadsheet accepts your present tire size, gear ratio's, & final drive. The output is speed at various RPM's. I used the sheet to dial in my tach. Here is a link to the sheet, and some instructions for downloading and opening it; scroll to the bottom of the thread for the link.

lotus-suspension-f42/updated-spreadsheets-t18445.html

HTH

[saildrive2001]

When I purchased my S4 SE in 69 in the UK I ordered the 3.55 which was a no charge option.

[Elanconvert]

yes you've guessed it.....
I'm trying to work out if my tacho/speedo are reading correctly!!! [don't want to lose my clean licence.......if the elan is anything like my g15, it will attract the attention of local pc's -- I've been pulled over three times in the ginetta by police who just wanted to find out what sort of car it was ]

not tried the GPS method....since I haven't got satnav, and I'm one of only 3 people left in the country who doesn't have a smart phone, will have to borrow a gismo I suppose.....
fred

[stugilmour]

Sounds like a quick speed comparo to a borrowed GPS is the way to go Fred.

The 'correct' way to dial in the speedo is to get the odometer reading correctly first. I used about 20 miles of US Interstate mile posts to determine what I needed and got it dialed in professionally due to final drive and transmission changes. If getting a new Turns per Mile (TPM) gear, then they dial in the speedo on the bench; not really a DIY deal in my view.

I used the spreadsheet to dial in the tach, but eventually got it professionally rebuilt due to drift and flutter. Spreadsheet works very well for accounting for all variations like tire size, final drive, etc.

Props on the Ginetta. Pretty rare over here so they get full attention!

Stu

[Elan45]

The S1-S2-Coupe partsbook lists the 3.55 : 1 ratio, used in S2 and Coupe, pre-S3.

Roger

[Elanconvert]

stu
yes think you're right about the GPS route......I'm not that bothered about the instruments reading spot on, as long as I know the relationship between 'true' and 'indicated'......
don't have any M'ways near here.......can't remember if they have 'mile posts'.....do they?
fred

[billwill]

Our motorways here in the UK have white marker posts every 100 metres, so it is easy to get a measured mile here by counting off 16 of them.