This link:- http://www.austin7racing.com/charles-goodacres-mille-miglia/ includes:-

The engine had a pressure fed roller crank, 6:1 compression ratio, a Solex carburettor supplied the mixture to a No 4 Cozette supercharger (built by Austins) blowing at 9 psi, and with magneto ignition by BLIC, 46 bhp was produced at 5,000 rpm. The maximum revs allowed were 5,800 and the engine ran on KLG 356 mica plugs

Location: Oxfordshire

How silly of me. I should have re read the whole article instead of just the bit I remembered. Thank you, Austin.
It's interesting that Austins (re)built the blower. I wouldn't have thought you could increase its efficiency, just the reliability - and they took a spare. They must have lacked confidence in M. Cozette!
I used to see 9 psi on one of my cars with the throttle well open but I can say with reasonable confidence that the camshaft design was partly responsible.

Regards,
Stuart

That was a works car built for one event, it wouldn't have been standard!

Even VE 9594, (factory supercharged car) had some works support in that they sent a "*******" gear to drive the supercharger faster. I'm sure when dad / current owner see this they may say more.

Location: United Kingdom

Standard 15 tooth drive gear,1:1. 14 tooth corrected tooth form,used by works 1.07 x engine speed I believe.

From a Massachusetts Institute of Technology lab test of a Cozette done in 1927 by a chap called Charles W Snow I can quote:

The results of this test tend to show that the efficiencies of this supercharger are rather low, that for best overall efficiency a discharge pressure of 6# / sq.in is best, but that, as may be expected,volumetric efficiencies are better at lower pressures. From the stand point of heating, high pressures are prohibitive, particularly when accompanied by high speed, although the fact is apparent from the data that the temperature is a function of the pressure almost solely, heat from friction being practically negligible.

Anyone who wants a copy can email me

Best wishes

Charles