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Post Number: 1
|Posted on Monday, 28 May, 2012 - 04:29 am: |
my name is Jan Hirsch, I am originally from Germany, but have been living in the US for quite a while now. Thanks for having me at your forum; I had been lurking for a while now...
Well, of course I have questions, too...I recently acquired my first Bentley and would be interested in an overdrive for this 1951 coachbuilt Bentley Mk VI B24MD with 4.5 L engine and dual exhaust.
The car is actually running great and does constantly 70+ miles without a problem, it is just revving a bit higher than I would wish. On the other hand, I live in very hilly San Francisco, and really appreciate the short gears.
Does anybody have experience with that model or the R type, especially the fitting with the cruciform and the dual exhaust? I have heard reports of vibration and/or damage due to a missing of the piece of driveshaft with the sprung loaded mounting, has anybody made that experience?
Any input would be appreciated!
Post Number: 2523
|Posted on Monday, 28 May, 2012 - 10:15 am: |
I looked into an overdrive for my R-Type in the 1980s, and decided against it. At least my car has the tall export 12:41 rear axle, and automatic transmission, so the engine speed is probably more preferable to the more usual 11:41. An option for you is the 12:41, or the 12:31 Continental ratio. The BDC in the UK also does a special. The main drawback of the 12:31 and BDC units is that the intermediate gears are then used and loaded more often and more heavily leading to accelerated wear of the expensive manual transmissions where fitted. The automatics are far more durable however.
The centre support bearing, spring and damper, including a metalastic mount, are vital with the two-piece propshaft. If they are in bad shape all hell breaks loose.
To fit an overdrive (like a Laycock), a special one-piece propshaft would need to be made, along with extensive chassis modifications including the rear engine/transmission mounts and cutting away of part of the vital chassis parts. I considered it too extensive and risky at the time, especially as the major structure of the chassis at the x-frame would likely have to haver been chopped up. Mounting the Laycock and making up a special housing with sealing and shafts would also have been a chore.
Post Number: 846
|Posted on Monday, 28 May, 2012 - 05:14 pm: |
Overdrives do indeed cause big problems on these cars.
Had you lived in a flatter area, a different diff would be the obvious option and custom ratios are now available in the UK to suit terrain. Unfortunately SF is an extreme.
Luckily for you the solution is fairly easy thankfully, and money saving to.
Only fairly easy I'm afraid . . . .
Add 10 minutes for every hours travelling time you expect to do and travel at 60 instead of 70MPH.
Better for the car, you, your pocket and the environment.
Post Number: 2
|Posted on Tuesday, 05 June, 2012 - 03:59 pm: |
Thanks a lot for your comments!
I guess that I am giving up on the OD idea, maybe I will go 12/41 at some point. Until then I will just be patient.
Norman Gleeson wrote some insightful comments on my question at the RRBEW forum, and the fact alone that there are hardly any cars with OD seems worrisome. Happy owners would write about it, I would think. I saw a blue Mk VI in Myocum, NSW with OD for sale, but that is a bit far for me for a test drive.
Post Number: 29
|Posted on Tuesday, 05 June, 2012 - 05:46 pm: |
I would be very interested to meet a Mk VI in Myocum. It's just over the hill from me.
Do you have any contact details?
Post Number: 3
|Posted on Wednesday, 06 June, 2012 - 12:54 am: |
Hi Carl, the URL is this:
This is all I know, of course. If you should end up to test drive it, I would be interested in the OD experience...
Post Number: 30
|Posted on Wednesday, 06 June, 2012 - 07:30 pm: |
I'll try to catch up with it. I would be very interested to try it and I will let you know my thoughts.
I have considered one for B14BH but most of my driving is winding, hilly country roads with a 80k limit with maybe once a year a couple of hours at 110k.
NBS services in Brisbane had installed Volvo overdrives, mounted at the diff with a shortened tailshaft. Beautiful workmanship but I don't know how they worked on the road.
All the best,
Post Number: 376
|Posted on Tuesday, 07 August, 2012 - 08:15 pm: |
Hi. Another Jan. Who'd a thunk it?
Anyway: I was considering having an OD unit fitted to my Shadow a couple of years ago. There's a specialist not too far from me who manufactures and repairs them. How much the full cost would work out at I couldn't say, but the unit he recommends is GBP 950 on its own. Even replacement units for cars originally fitted with ODs can cost close to that kind of mark if there is no exchange unit to mitigate the costs. As the Bentley engine is less powerful than the later 6.75 litre V8 one I should think that a lighter and cheaper unit could do the job quite adequately, although the rest of the work would cost much the same.
Assuming that there should be an override switch there shouldn't be much, if any, in the way of loss in hill climbing ability if you should decide to bite the bullet and dig deep into your wallet. Oops: bit of a mixed metaphor there ...