Differential change for Series 1 V12

[Meonstoke]

Hi,

I have a series 1 V12 LWB Daimler with the BW-12 3 speed automatic gearbox. I am considering to swap the differential from its current 3.31 ratio to a 2.88 ratio as used on the later series 3 V12s. I figure it will drop the revs by about 500 rpm at cruising speed to about 2500 rpm. What are your thoughts on this? It will improve the mpg but it will blunt the acceleration a bit obviously. However I figure that as I use the car mainly on the motorway, the kickdown at anything above 50 mph should be sufficient. Note: I may need to change the differential bearings anyway as they are starting to cause some noise above 2500 rpm. Hence, a cheaper more efficient option is to change the whole differential unit in one go.

I'm interested in people's thoughts and suggestions.

Thanks.

M

[mike020150]

I was bugged by a similar issue a few years ago. My series 2 V12 had a 3.07:1 rear axle which meant 70mph cruising was done at a busy 2700 revs to the detriment of fuel consumption etc. Your car's engine must be turning at close to 3000rpm at the same speed which would drop to about 2600rpm using a 2.88:1 rear axle. Naturally, acceleration would suffer and generally, the performance would feel slightly blunted. Series 1 V12s always feel very lively which is part of their charm. However, my fuel injected car (with not a lot more bhp) is now geared in top gear to cruise at 2300rpm at 70mph and it copes quite happily with this arrangement. In my case, initially, I changed the gearbox for a four speed auto containing an overdrive top gear. That allowed 70mph cruising at around 2000 rpm, just like many modern cars but it devastated the performance. The car just didn't drive like a Jaguar! On the plus side, the fuel consumption was good though definitely not worth the performance sacrifice. I went on to change the differential to achieve the car's current gearing, which with a four speed box has given me the best of both worlds, brisk performance from rest plus more restful cruising at speed. In summary, if you already have a noisy diff, you haven't much to lose, apart from a few revs! Mike.

[Meonstoke]

Thanks Mike for sharing your experience, appreciated. I'll stick to the current 3.31 diff for as long as possible. It's not that bad and only I really notice it. I wouldn't want to swap out the gearbox as to be frank it would be more cost and effort than the car's worth. With the current diff ratio it is quite amusing to see the looks on folks faces when at 65 mph I hit kickdown and off she goes like a rocket ....

I'm interested in hearing other enthusiasts thoughts on any alternative options ?

M.