It cost more than I'd planned and I'm still getting diff oil out of my ear but at last the new 3.55:1 diff is in. What's more, I've driven it.
Early days but already the difference is significant. For those who haven't been paying attention here's a brief resume;
After 30 years of ownership I found out that my 3.7 diff was actually a 4.2 and my speedo was basically guessing what speed I was doing and my engine sounded like a dentist drill so time to change.
For many years BMC and then British Leyland had a special tuning division that sold uprated parts to the more serious of drivers. Think of it as an early example of BMW's M-sport, sort of.
They offered a special 3.55:1 diff exclusively for the Midget 1500 as the 1275 version lacked the torque to handle the longer legs the diff gave. My car should have the rare 3.7 but actually had a 4.2 so I decided to re-create these 'uber rare' 3.55 equipped midgets.
So...what's it like?
Fitting is easy. (see previous posts) Filling it with the right oil without a pump isn't but the difference to the driving is significant. To help explain it I have created a graph of the rpm vs speed for different ratios.
First of all let's not kid ourselves. This is not a car for cruising in no matter what diff or gearbox you put in it. It's inherent 'midgetness' is it's ability to be chucked around at low speeds and low gears. But on a dual carriageway it sounds like three cats fighting in a bag. Going to a 3.55 is like taking out one cat. You will never be cruising in the outside lane relaxing to a Radio 4 afternoon play or the quiet bits in some German opera but a 3.55 makes long journeys a little less like dental surgery.
I am comparing the two extremes of 4.2 to 3.55 so I really can see the difference but I'm not sure that moving from a 3.7 to a 3.55 would be worth the money. With my 4.2 I hardly ever used first gear. Pulling off in 2nd was normal. Now I think it would stall. Acceleration is marginally quicker as I am using first gear range properly and gear changes are slightly longer but it still feels extremely lively. As you can see from the graph by comparing the yellow line to the blue the rev difference at low speeds is very small and rapid gear changes feel to be better spaced than before with the torque of the engine making it very satisfying.
The top end of 3rd, when accelerating hard, feels like there is still room for more power and the diff clearly can handle it (next year's project?). Finally I can use the full range of third as the 4.2 ratio meant that the poor engine was begging for mercy long before the speed indicated time for a change. 4th hits the wall at 4500 rpm and above this you could expect to be picking up bits of engine soon but you were still being overtaken by nuns on bikes. The 3.55 turns fourth into more like a fifth, leaving the first three gears just for a bit of fun.
You will see on the graph that the rev saving increases with speed and now 50 mph is about 500 revs lower than before and at 70 the difference has increased to 700. This is significant and makes the car feel much calmer albeit still noisy compared to a more modern car. This should give the engine a much longer life and keep the oil pressure at much more sensible levels.
The downside is that my speedo lies like a politician and will need to be re-calibrated ("I'm sure I can do it!"; cue months of research and boring Drivetribe posts)
So has it changed the 'midgetness'? Yes, but in a good way. Its still more responsive than most modern cars with a delightful need to flick from gear to gear but now speeds above 50 are actually possible without going deaf.