Q I operate a 1962 Standard 7cwt van

10th November 1967, Page 207

10th November 1967

Page 207

Page 207, 10th November 1967 — Q I operate a 1962 Standard 7cwt van

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fitted with a petrol engine having a compression ratio of 6.8 to 1. What is the lowest octane fuel rating acceptable for the engine that would not cause damage to the unit?

AAn engine with a compression ratio as low as this should operate satisfactorily on petrol with an octane rating of 85.

But remember that the octane requirement of nominally identical engines varies considerably because the effective compression ratio may be as much as -1/1 ratio greater or below the specified ratio on account of machining tolerances.

The octane requirement can also be affected by mixture distribution and by carbon deposits in the combustion chamber and normally an old engine requires a fuel of higher octane than a "clean" engine in good condition.

If an engine does not pink on the standard ignition timing and does not run-on when the ignition is switched off, it can be assumed that the fuel is satisfactory and that no harm will result from its use. Slight pinking can often be cured by retarding the ignition a degree or two, and this can be approved if it does not materially reduce engine performance.