Overdrive S3

[Jiminsd]

Colin, Thank you for that graph. I’m a numbers and visual guy so I really appreciate it. Now it leads me to a few more questions. As I think I understand but can argue myself in a circle since the final drive is one thing but you can change gears when in 2nd or 3rd.

If I take out my 3.54 and install a 2.88 then I will accelerate slower but will gain fuel economy via a lower rpm at freeway speeds? Or will this only be a phenomenon of first gear since I can down sift in any other gear?

So will the increase in bHp up to 400 be negated by going to the 2.88?
Or should a 400 bHp 4 speed 2.88 still be a bunch quicker (30-60 mph) than a 275 bHP 4 speed with a 3.54?

I don’t remember every reading that the European cars were slower (0-60) than the US cars, but it’d have to be if the US cars were 3.54 and the European cars 2.88, right? And the Euro cars would have a higher top speed assuming one had the power to overcome the wind resistance?

This won’t be a stop light racer, but I’m just looking for more spirit on back country twisties which, if I understand, is more torque related to bHp related. This is making me think about leaving the 3.54 and install the OD as the OD would give me quick acceleration but give me OD when cruising on the freeway. The OD would never “see” the 400 bHp as if I need to accelerate, I’d take it out of OD or shift down to 3rd.

Finally, I’m curious now why the European owners go with the 2.88 and the 5 speed? Could your program figure the rpm at 75 mph with a 4 speed 3.54, 2.88, and 5 speed 2.88?

[PeterCrespin]

2.88 plus an MOD 4-speed, or 5-speed with overdrive 5th, is not recommended. Yes, a V12 will survive, but 5-speed sellers do not advocate those combinations. Some people ignore the advice. A donkey has lower vet bills than an Arab stallion, but you wouldn't enter a donkey in the Derby, or your thoroughbred in a dog & pony show. Horses for courses.

[lowact]

Imo 2.88 is a better match for an E with monster torque. Two real world considerations, wheelspin and gear change lag. The torque you can apply is limited by wheelspin. For the same car (mass, gears, tyres) wheelspin depends on torque and diff ratio. With a 3.54 diff, 272 Nm will cause an S3E to wheelspin in 1st gear. With a 2.88 diff it is 335 Nm. Means that when drag racing with a 3.54 you cannot initially use all your torque, you need to ride the clutch more and almost immediately change gear, by which time the 2.88 would have disappeared.
E-types are the epitome of coolness. Drag racing is the antithesis of coolness, so we must be subtle. There is nothing subtle about a 3.54, higher revving and early gear changing. My rule of thumb for sensible diff ratios, you should just be able to wheel spin in 1st, and not in 2nd. Both 3.54 and 2.88 will do this, with 3.54 in an E you can wheelspin stupidly early.
At 75 mph, 3.54 = 3374 rpm, 2.88 = 2745 rpm, 2.88 with OD = 2636 rpm, these are theoretical, indicated could be ~3% higher due to tyre grip efficiency.
Using an OD to extend a 2.88, I agree with Peter, not good, two reasons: You are moving the usual operating range away from the most efficient operating range, means that you will not be getting the fuel savings that you might imagine considering rpm alone. And you would be stressing your drive chain more (lower speed = higher torque). This latter could be the reason Peter’s 5 speed sellers do not advocate this combination.
The OD is not as strong as the gearbox,(shafts are smaller diameter). Not designed for monster torques, is the reason why with powerful engines it is only used when in top gear, but even when not being used it is still transferring torque …
I’m fitting an OD but I’m ensuring this is reversible, just in case I break it.

[Jiminsd]

That’s great information. Thank you all. I will drive it for a few months and decide to exchange the 3.54 for a 2.88. Hopefully I’ll have it back in a month or so.

[Meonstoke]

That’s great information. Thank you all. I will drive it for a few months and decide to exchange the 3.54 for a 2.88. Hopefully I’ll have it back in a month or so.

From March 1973, US and Canada cars received the 3.31 drive ratio. Page 92, on section Production Changes, "Original Jaguar E-Type - Restorer's Guide" by Philip Porter

[jagwit]

wheelspin depends on torque and diff ratio.

It also depends - very much - on flywheel mass (in case of a manual).

I drove my previous S3 in two configurations (both fuel injected) :
1) low compression pre-HE with std (heavy) flywheel ;
2) 11:1 high compression HE with Fidanza light weight flywheel. This config was significantly more powerful than #1. The car could stay with a 4.2L XKR.

The car had std 4sp transmission and 3.07 diff and 215/70/15 tyres in both configs.

In config 1 it was very easy to spin the tyres.
In config 2 it was NOT. It was very easy to stall the engine. With very deliberate clutch and throttle control, tyres could spin but in config 1 it was a doddle.

[Neil Duke]

Gearbox is back from Overdrive Services in Sheffield,layshaft repaired.Very clean,& tidy job.Looking Forward to trying it out.