Diff Ratio for Beauford

[chris gregory]

My Beauford has 2.4 litre nissan engine with auto box,205/75/15 tyres
and cortina rear axle with3.75:1 diff ratio.
It seems rather high geared,wanting to keep changing down at around
55mph in top.
Have members any suggestions regarding suitable gearing,thanks

[ROBIN...@aol.com]

Hi there.

 

I ran the same set up for many years with no problems.

 

Are you sure that the gearbox is sensing the throttle position correctly.

 

Are you sure that the kick-down sensor is not being triggered.

 

Regards 

 

ROBIN THORN
46, NORTH AVENUE
LETCHWORTH GARDEN CITY
HERTFORDSHIRE
SG6 1DJ
01462487188
07721642620
ro...@robinthorn.com

 

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[Polishingpete]

Hi Chris,
I have the same problem, used the factory build book recommended tyres
(same as yours) and diff (ditto) and I rarely get the car in fifth,
and it doesn't like being in fourth sometimes but the amount of work
to change the rear axle diff makes it not worthwhile. At least being
manual it is more tolerable.
Kind Regards
Peter

[Hamish Freeman]

Hi,

I am not familiar with the gearing on the original Nissan to provide a
direct comment but can advise that you should be trying to achieve a "road
speed to engine speed" ratio on your Beauford as near to that of the
original car from which the engine came. Given this relationship, your gear
ratios will then be correct for the engine and its calibration and torque
curve whence the change points on the auto will once again be sensible.

To calculate this, you need to know what was in the original car, its final
drive ratio and the tyre specification. For your Beauford you can then work
out what drive ratio gives you the same speed ratio.

Whilst there are plenty of spreadsheets available on the web to make these
calculations, they seem to be incestuously based on one particular example
that contains an error in the conversion of metres/min to miles/hour. I
have attached my simple spreadsheet to undertake this task. The
calculations are listed beneath the two columns. The input on the LHS is to
ascertain the original car "speed" ratio whilst the RHS can be used to
investigate what you will get with the available final drive ratios. It is
possible to do a goal seek on the speed by varying the drive ratio, F10 and
F8 respectively. This spreadsheet could be rearranged to automatically work
out the ideal final drive ratio but unfortunately the need for an integer
number of gear teeth means that only certain ratios are available, of which
only a few were ever produced commercially.

Using this approach I arrived at the values already in the spreadsheet from
which I was able to purchase a 4.42 final drive to use with the large
profile 15" wheels. This ratio was produced by a specialist manufacturer
who catered for the motor cycle engine in Lotus 7 look alike market about 8
years ago. They seemed to disappear off the face of the earth soon after I
purchased my Diff. The last fine tuning is possible to a small extent with
tyre pressures for the sake of IVA where I was complemented for the accuracy
of the speedo - about the only thing that did get a favourable comment as
the first homebuilt B to undergo IVA!

Hope that this helps.

Hamish