Overdrive - Triumph Spitfire MK3 Handbook

OVERDRIVE

OVERDRIVE (optional)
An overdrive unit serves as a convenient method of providing, at
will, a numerically lower overall gear ratio to reduce engine speed
and to improve fuel economy.
The Laycock de Normanville overdrive unit incorporates an
epicyclic gear train which is engaged, to give overdrive condition,
by a cone clutch moving under the influence of hydraulic pressure
generated by a small piston pump. When pressure is released via a
control valve, the clutch is returned and held in direct drive by
compression springs. A uni-directional roller clutch enables
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the change into, or out of, overdrive to he made when transmitting
full power without loss of road speed.
The hydraulic control valve is linked to an electro-magnetic
solenoid, which is operated via a relay by a two-position switch
mounted on the steering column (Figs. 9 and 10).
Greatest benefit will accrue from judicious use of the overdrive,
the governing factor being that the vehicle continues to run easily
without sign of engine labouring, combined with the minimum
amount of throttle opening necessary to maintain this condition.
Maximum disengagement speed: 4,800 r.p.m.
The preceding disengagement speed corresponds approximately
to peak revolutions in normal gear.
Disengagement of the overdrive at a speed higher than stated
may cause damage from 'over-revving".
Operation
Move the selector switch down to engage overdrive and up to
release it.
Lubrication
The same oil is used for both the overdrive unit and the gearbox,
an internal oil transfer hole allowing the flow from
into the overdrive unit until a common level is attained.
Periodically check and if necessary top up the gearbox-
overdrive unit oil level via the gearbox filler orifice. (Refer to
page 51).
the gearbox