3.9 or 3.77 ?

[AlfaLofa]

My car is a 64 S1 with the close ratio box and 3.9 differential.

I have a restored spare aluminium diff casing (new torque rod mounting sleeves).

I also have a new double drilled flange.

I also have a good 3.77 English diff and a good 3.9 diff.

Which unit should I have overhauled (and fitted with the double drilled flange)?

Logic says the 3.9.

(I shall be fitting the later output shafts with the smaller PCD)

[rgh0]

Hi Steve
It really depends on how fast you want to go and where and acceleration versus top speed

I run a 3.77 in my Elan and can hit 8500 rpm along a long straight on the tracks I run on and it gives me the best lap times on a mix of circuits without changing diffs between meetings which in an Elan is a real PIA

A friend runs a S4 seven with a 3.9 or 4.1 as with the same engine but worse aerodynamics he needs a lower diff to hit his rev limit which is a little higher than mine as he has more time to rebuild engines. For him a lower diff gives him quicker lap times.

For a road car its about driveability, stop light racing and how much highway cruising you do

No simple answer
Cheers
Rohan

[AlfaLofa]

Thanks Rohan,

I am in a bit of a quandary - like the old saying - "I used to be indecisive, but now I'm not so sure".

I don't want to fork out hard-earned to have both units refurbished - so which one?

If I stick with the 3.9 at least it will still be 'standard' - albeit with the later output shafts (which if need be, I will be able to mate to a pair of Issue18 uprights I got hold of recently).

However, the 3.77 could lead to a more comfortable road driving - although the nearest "highway driving" in my neck of the woods is about a 90 minute drive from home.

Acceleration is far more important than top speed for road driving in these parts - but there again I shouldn't be driving with my boot to the floor at my age!

I suppose the 3.9 has the advantage of being standard whilst the 3.77 is different to what I have already in the car and therefore offers choice.

Mmmmmmmm.......

[ceejay]

Before the rebuild of my S2, the diff ratio was 3.55, along with ultra close ratio gearbox which has a very tall first gear, it was always a drama to get the car going if stopped on an on an incline in traffic, but the long legs were absolutely awesome out on the freeway.
Now I am running the 3.9 diff and I am quite happy with it, gives great acceleration, and cruises OK at 100KPH,
but revs too high above 100KPH, it's OK in short bursts, but not for long. .
I also have a 4.1 ratio diff which I will put in one day and go into the alps next summer for a bit of fun!

Rohan, I would love to have a twink that would spin to 8500 without any drama.

Speaking of revs, I have just installed an accuspark rev limiter, there will be no more over reving now. The unit soft cuts at the chosen rev limit setting but still maintains the engine speed, they work extremely well, and for $45.00, you cant go wrong, and they are dead simple to fit. (No commercial connection).

[Jeff@Jae]

Go with the 3.77 for the best combo of highway RPM relief and off the line ease. A 3.55 is too tall for the close ratio gearbox. I have a 3.9 in my S2 with the close ration gearbox and I'll be switching to a 3.77 at the next opportunity even though the Twin Cam is quite happy at 80MPH I would like a little RPM relief !

[Ross Robbins]

I had the 3.9 in my S2 for a number of years and many thousands of miles. I changed to a 3.55 last fall and I think it is without a doubt the best thing I have done. While Jeff says, it is too tall for the close ratio box, and I normally rely on his excellent advise, this time I disagree.

I didn't think there would be enough difference in the 3.77 a decrease of only 4% while the 3.55 is close to 9%. This is minimally problematic in first gear but not really too bad. Starts require a decisive application of throttle and a rapid clutch engagement and from then on it is heaven to have longer legs in each gear, especially in the canyons where second and third are perfect for most twisty bits.

In your case though, I think you have stated a preference for quickness rather than cruising speed or canyon carving so I suggest you stay with the 3.9.

[Quart Meg Miles]

I, too, ran my S2 with 3.9 and close-ratio box and hankered after a 3.55 which I fitted after a dozen years. Hated it! Although it was great to boast 100 mph in 3rd the car felt bogged down all the time, through lack of power at the lower engine speeds, and the revs no longer seemed to swoop up in the lower gears. It was a relief to replace the 3.9 after ten years even though it means 5,000 rpm cruising on big roads. A 3.77 wouldn't be as bad but if your S1 is standard tune I wouldn't bother.

[robertverhey]

I run a 3.54 in the lotus cortina.....yes hill starts take a bit more slipping, but the payoff in quieter cruising on the freeway makes it worthwhile.

I'm gonna fit a 3.54 into the ?lan as well. I figure with solid driveshafts and the lighter weight of the ?lan, it will be okay. Would it be fair to say 3.54 and rotoflex couplings is a less than optimum solution, but 3.54 is okay with solid driveshafts? Anyway will keep the 3.77 on the shelf just in case

Depends on your needs I guess

Robert

[Mazzini]

Go with the 3.77 for the best combo of highway RPM relief and off the line ease. A 3.55 is too tall for the close ratio gearbox. I have a 3.9 in my S2 with the close ration gearbox and I'll be switching to a 3.77 at the next opportunity even though the Twin Cam is quite happy at 80MPH I would like a little RPM relief !

Hi Jeff,

Are you running a standard or tuned engine?

All the best,

[YellowS4DHC]

If you expect to do a fair amount of in-town driving, I'd opt for the 3.90 with your close ratio box. Without the lower first gear in the semi-close ratio box, congested traffic could be a real pain.

I have a 3.77 diff in my S4 and a 3.55 sitting in a cabinet in the garage ready to go. Every time I think I'll swap out the 3.77, I get a painful reminder of driving in stop n' go traffic - and I have a semi-close ratio box.

regards
Rick


'72 Europa TC
'69 Elan S4 DHC
'67 S800 Coupe
'57 T1 Speedster

[Mazzini]

I'm going to run a close ratio with a 3.5 diff in my S3, the engine will be to Q360 spec and the car has solid drive shafts, I'll let you know how I get on.

[Mazzini]

The 3.55 axle with a CR 2.5/1 box, CV driveshafts and a Q360 engine with a flat torque curve works beautifully.

[msd1107]

I have run the 3.55 differential with the C/R box for years, and would not have it anyway else.

Even on SoCals mobile parking lots, startup and low speed stop and go is no problem even with a passenger. And starting up the hills is OK also.

The revs are still too high, though. And the car will pull 7,000 RPM in top (no rev limiter).

Just my 2 pence.

David
1968 36/7988

[Mazzini]

Yes...at an improper speed (slightly above the national speed limit) I too reached for a non-existent fifth gear.

[lotocone]

Mazzini, it's good to know you're driving the S3. Hope you like the engine rebuild. Have fun !

Bob