/ am a lecturer in a technical college and read
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with interest a recent Q and A dealing with engine run-on. The practice was mentioned of stopping an engine which tended to run-on by engaging a lower gear and letting in the clutch at the instant of switching off In tests at the college it has been shown that opening the throttle at the instant of switching off is effective in preventing run-on. Would you not agree that this is a preferable method? And do you consider that operating an engine on a fuel of lower octane rating is preferable to running on a higher octane fuel so long as the engine does not pink?
AOpening the throttle at the instant of
switching off cools the combustion spaces including the hotter parts that may ignite the mixture and cause the engine to run-on. While it is an acceptable method of preventing run-on in some cases, opening the throttle can easily be m is-timed particularly if the engine is running at a speed considerably higher than a tick-over, Moreover, the nonignition of the wet mixture that is drawn into the cylinders will dilute the oil on the bores: and this could result in serious dilution of the sump oil and more rapid piston/bore wear if stops and starts are frequently made.
Given that the engine does not pink, operating on a lower-octane fuel is more favourable to engine life. The lower lead content of lower-octane fuels tends to promote longer engine life, notably of the exhaust valves.