03-10-2018, 08:47 AM
A B5hs plug (equivalent to Champion L86C) is 'correct' in as far as it ought to go alright on most Austin Sevens.
'Correct' plug heat range though is determined by a combination of factors which include (you guessed it) compression ratio, ignition advance and mixture strength. In an engine running high compression, excessive ignition advance and/ or weak mixture a colder plug may be needed to protect against detonation (simply put, the 'standard' plug may overheat and ignite the mixture before the plug sparks).
An exhaust valve stuck open (even poorly adjusted tappets can cause this) will sit in the gas flow and overheat, which likewise can pre-ignite the mixture.
'Correct' plug heat range though is determined by a combination of factors which include (you guessed it) compression ratio, ignition advance and mixture strength. In an engine running high compression, excessive ignition advance and/ or weak mixture a colder plug may be needed to protect against detonation (simply put, the 'standard' plug may overheat and ignite the mixture before the plug sparks).
An exhaust valve stuck open (even poorly adjusted tappets can cause this) will sit in the gas flow and overheat, which likewise can pre-ignite the mixture.