When due to too low a rear axle ratio, the engine rpm is too low,
then the carb throttles MUST be opened much more in order to allow
the engine to develop the needed hp. This greatly lowers manifold
vacuum and the net result would be a drop in fuel economy. This sort
of driving condition is also hard on internal engine parts and if
done continuouly over years, it would contribute to early engine
failure or overhaul being needed way too soon.
Rootes used the 3.89:1 axle ratio for non OD cars and a 4.22:1 for OD
cars because the 4.22 gave a really fast accel. through the gears and
when OD was engaged, the RPM in highway driving was quite reasonable
and conducive to good engine life and economy.
The series three came here with the 3.89 in all cars. This soon
proved to be a poor choice. The III was indeed a nice car in OD when
running at engine speeds over 3000-3500rpm but of course so much of
common driving has to be done at much lower mph, it was found that
drivers were simply staying in OD when they slowed way down...
leading to engine lugging and general rotten performance. For the
series IV they reverted back to the 4.22 gears when fitting OD and
continued this ratio with the series V.
I trust this makes clear why we warn about using the 3.89 gears with
OD. You can avoid all the above troubles and predictions of troubles
by doing one of the following:
Don't use OD
Don't use OD below 3000 engine rpm
Install the factory 4.22 gear set in your rear axle.
Dick T.
This archive was generated by hypermail 2b30 : Tue Sep 05 2000 - 09:56:34 CDT