Supercharger pressure gauge                  

It seems likely that Alvis did not fit any supercharger pressure gauge on their production blown 4 cylinder cars.  Most of the cars surviving today have blowers, but several of them were originally unblown, so the pressure gauges would have been added later.  The evidence below comes from a few of the surviving cars. 

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Jaeger boost gauge, 60mm dia. (reproduction by Patrick Henry).

This was copied from an original of the correct period. (Chassis 7308, Pete Livesey)

Jaeger 52mm supercharger gauge.

This is not known to have been fitted to any Alvis, but is a match for the other standard instruments on the FWD.

Replica gauge with Alvis triangle (how did that get there?).

From Chassis 7256.

A larger boost gauge, about 75mm dia. at the bezel.

These gauges read from -12 to +25, so suit the take-off boost of a Rolls-Royce Merlin (18psi). It is not certain whether this type of gauge was for aeronautical use. (Chassis 7191).

Another large diameter boost gauge.

From Chassis 7275, a four-seat tourer with many special features of early date, restored post-war by Colin Crabbe (Alan Stote).

An aeronautical boost gauge, not (yet) fitted to a FWD.

This is calibrated for 8psi boost, which would suit the +5psi capability of the FWD blower. However, there is probably too little range on the negative side.

A suitably calibrated automotive gauge of later period.

Chassis 7129, when owned by David Webster.