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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
20/25 ignition system settings, including spark plug gaps, distributor modifications, and coil insulation.

Identifier  ExFiles\Box 178\3\  img142
Date  14th April 1932
  
HS{Lord Ernest Hives - Chair}/WST.) FROM R.{Sir Henry Royce}
E.{Mr Elliott - Chief Engineer} ) (At Le CanadelHenry Royce's French residence.)

C. to SG.{Arthur F. Sidgreaves - MD} WOR.{Arthur Wormald - General Works Manager} FN. 20/25 IGNITION.

R2/M14.4.32.
Sent from WW. 18.4.32.

X6095.

My further inspection of the above says that the plugs, which are particularly bad, are:
(1) Single point ones which we should not attempt to use.
(2) As stated they are set at .025", which I say is too great for the compression ratio and insulation of the HT. distributor, which however does not fail even with this coil of more turns of secondary.

You say I am wrong in suggesting less spark gap on the plug. I say our slow running is very good indeed with .020", and it is the only way we can avoid this distressing missing at slow speed, full throttle. Even then it is necessary to have the points fairly free from oxide.

I was pleased to see that the feeler gauges sent out with the car are .020" for plug setting, and .018" for the contact maker. Between these two - i.e. to go, and not to go - makes the best setting possible to my mind and experience.

We are cutting off the tail of the distributor, but do not think any good will come from this.

It seems to only leave in doubt the insulation of the coil - i.e. bakelite - its surprisingly curious property of losing energy when hot, and the heating up of the ballast resistance.

Why the Americans (if they do) run with 25 and 30 thous. I know not, but their carburation is generally so often rich that they probably de-oxidise their plugs - i.e. they do not run with an oxidising (weak) charge, as will be gathered from their consumption of fuel.

R.{Sir Henry Royce}
  
  


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