Quote:
Originally Posted by Othen
Thank you for that John,
All I can conclude at the moment is that I have to do some more research and think about this one a bit. The engine speed at 70MPH I got from the chap selling the wheels (who had also just sold the AW71 gearbox) - I should be ale to find out how the AW71 and BW55 are geared internally from the Volvo green book and compare them.
My feeling is that if the internal gearing really is 56% higher then the humble B21a motor will not cope with that, and would probably be running in third to achieve 70MPH (so there would be no gain). If that is the case you are right, I would either need more torque or lower final gearing. On a bike that is trivial of course, us bikers change gearbox and rear wheel sprockets to fine tune the overall gearing frequently, but I'm not sure I'd see the value in changing the rear axle (which works just as it should) to make this work.
As you know, I'm planning to keep the RB as a long term project car, so I'm certainly not ruling out the AW71 gearbox swap, but it may need to be part of a package of changes to make it work. I was certainly not planning to make the gearbox change until after 1 April when the RB will be safely registered as an historic car, so I have plenty of time to research and think about this issue.
The good news is that I was really impressed with the RB as it is (I'm even getting used to the buzzy 3 speeder gearbox). I thought the performance and fuel consumption were both excellent for a 40 year old car. It is a bit underpowered for its weight (nothing much wrong, even when it was new 100 HP to shift a ton and a half was not much by today's standards) - so I would not be agin some improvements to the engine.
Bob and I did have a really nice day. Thank you.
Alan
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I'd hazard a guess you have the 3.54:1 diff in the RB Alan because in 3rd in mine, it's also doing ~3750rpm and that's on 195/65/15 tyres and a 3.54:1 diff. Originally it was a 3.91:1 diff but when that went, i had to change the back axle and luckily found a 3.54:1 back axle.
I'd question the up-change speed as it's difficult to judge 3/4 throttle, i know 2nd is good for 85-90mph and never fully explored 3rd or 4th yet.
However, at 70mph which is where you seem most concerned with, it's more about keeping it rolling than getting it rolling. If you treat 4th in the AW71 as an overdrive (you can let the box do it all or use the button on the side of the gear lever) then it shouldn't be a problem. Even if you have to knock the overdrive off on a long motorway hill then back on after, you've not really lost anything and youll be gaining on the flat and downhill.
Might still be a good idea to boost the output slightly with a twin choke Weber and 4 branch manifold etc but i'd say try the AW71 box first before you start on the engine.
Glad you had a good trip and the economy is pretty good too!
