I'm sure this has been discussed at length before but please could someone just clarify something for me. Here's my situation:
I have a Visa 652 engine ready (nearly) to go in an AK350.
I want to keep it on drum brakes, (as it has the chassis mounted pedal box etc).
I mainly use the van fully laden on longer trips abroad etc, (not towing).
The reason I'm putting the Visa engine in, is partly to help keep up momentum on hills and partly because I've got it, so I might as well use it! At the moment the van has an M4 top end on M28 bottom end, with a Dyane gearbox, (so you can see why hills are an issue in 4th gear!).
I've got light and heavy Visa flywheels to choose from, so I'm thinking I'll use the heavy one.

I've been advised by three knowledgable people that a good gearbox ratio combination can be acheived by using 2CV gear ratios with a Dyane final drive. I've got a drum brake Dyane gearbox so I'm thinking that I'll use that, with the 2CV gears from a disc box that I also have.
So far, so good (unless you tell me otherwise!).

However I also have the original AK350 gearbox. This is the very rare Oct67 to May68 gearbox with different ratios to virtually everything else.
According to Jeroen Cats website, the final drives are as follows:
2CV6: 4.125 (8/33)
Dyane6: 3.875 (8/31)
AK350: 3.625 (8/29)

Here are my questions:
Assuming that these figures are correct, would it be better to use the AK350 final drive, rather than the Dyane?
I beleive the Visa engine has about 40% more torque, so I'm assuming it will be able to cope with the even higher gearing ok, plus it won't be revving as high at speed. Or would this be going too far for it to cope?
How much difference would each option make? (Jeroen's site has useful kph/1000rpm figures but I can't get my head round how to adapt those figures for the combination options that I've got!).
Of course this is assuming the innards of each box are interchangable. The AK350 box is the very first of the late type box, so is it safe to assume that it's compatible with the late 2cv box?
Thanks

1st / 2nd / 3rd / 4th / final drive / kph@1000rpm (tyre size)
2cv6 2/70-
5.203 / 2.657 /1.786 / 1.316 / 4.125 (8/33) / 19.89 (125) / 20.34 (135)
Dyane6 2/70-
5.749 / 2.935 / 1.923 / 1.350 / 3.875 (8/31) / 20.65 (125) / 21.10 (135)
AK 10/67-5/68
6.051 / 3.089 / 1.923 / 1.421 / 3.625 (8/29) / 20.97 (125) / 21.43 (135)

http://www.cats-citroen.net/citroen_aty ... tions.html

Have you tried this - sorry if you have - you can fill in each individual part for your box etc overiding the standard formula.

It's Rick o'clock.

A van, fully laden, needs lower overall gear ratios, not higher.

Were these knowledgeable folk Lomax or Pembleton owners?

Final drive ratios are correct.

Closer to 15%, which would work well with standard 2CV ratios, but that's just my opinion...
No point in over-gearing, as you'll only be dropping back into 3rd gear more frequently.

Ken

That's brilliant. Thanks Chris. I've used Jeroen's site loads of times but I never knew that page was there!

Yes. If I was keeping the M4/M28 hybrid engine in it I would swap the Dyane box for a 2CV one but I'm trying to establish which ratios will best suit the Visa engine that's going in.......


Only if Roy or Rick own Lomaxes or Pembletons! I can't remember who the other person was that I asked, (I thought it was you!). I meant what I said when I said 'three knowledgeable people'.


Yes. It's often Rick o'clock! When I asked Rick (& Roy) at Registers weekend, I forgot to mention that I had the AK gearbox, (and didn't realise that it had a different final drive ratio), so that option didn't come up in the conversation. I didn't really want to phone up and pester either of them again, when I'm sure they get asked stuff like that all the time and they have much better things to do, so I thought I would put the question on here!

Hi Daras,
from Rick's 'Guru of Gearbox' site...

1). Short box - this is a mix of dyane gears & 2CV diff. In effect a dyane but 6.5% lower in every gear. Suits vans and other vehicles that are regularly heavily laden or tow a trailer.

2). Long box - this is a mix of 2CV gears & dyane diff. In effect a 2CV6 but 6.5% taller in every gear. Suits 2CVs with very strong motors and kit cars.


I'm sure I recall Roy mentioning that the best thing he ever did with his Acadiane was swap the original (Dyane ratio) gearbox for a 2CV gearbox.
It will only be pulling a few hundred rpm more at cruising speed in 4th and even if it is necessary to drop back to 3rd, the gap between those two gears isn't as wide as it is on a Dyane 'box.

ken

( Back to the tennis now... )

oh ken you do dissapoint me

Hi Ken.

I understand why the 'short' and 'long' gearboxes are good for those vehicles and why an Acad might be better with a 2cv box but I'm not really sure what you are suggesting for my situation, being different to both those.

If Rick suggests a long box for a 2cv with a 'strong' engine, then presumably that would be good for a low-roof van with a Visa engine? (This is in fact what both Roy and Rick suggested when I asked them at Register's w/end). So, accepting their opinion that the more evenly spaced 2cv ratios, made 6.5% taller by the Dyane 8/31 diff would be desirable, would an additional 6.5%, by using the AK 8/29 diff, be even better or a step too far?

I had been assuming that if a 'strong' 2CV engine would benefit from 6.5% taller gears, then a Visa engine with 40% more torque (and maximum torque at lower revs than an M28) would benefit from 13%.

Do you disagree with this? Do you think that the AK350 aerodynamics are so much worse than a 2cv that despite the extra torque, the Visa engine wouldn't be able to cope with the taller gearing?

Thanks,
Daras

NB: At the moment, with a Dyane box, M28 bottom end, M4 heads, manifold, 30 carb and the original (extra narrow) exhaust, my van will happily do over 60mph now on the flat. When I fit the Visa engine, I will replace the AK exhaust with a 2CV one, (adapted to go out the right-hand side).

Daras,
no idea where you got the idea that a Visa engine has 40% more torque than a 602 engine, but I'd have serious doubts about it, considering that the increase in cubic capacity is only 8%.
It may have slightly larger valves and a different camshaft profile, but that won't account for the difference.

Also, headwinds and (up)hill sections of road do seem to affect vans much more than saloons, in my experience...

ken