6 replies to this topic

[Henry8601]

I'm servicing my (totally standard) 1964 850cc mini at the moment, and I'm not sure which spark plugs I should buy - NGK BP5ES or BP6ES?

Somerford mini say the BP6ES is standard, whereas mini spares say the BP5ES is standard. I understand that the BP5ES is a hotter plug, but I don't know which I should use. I use lead replacement additive if that makes a difference.

[jaydee]

Todays fuel need a hotter combustion, so you'd better with the BP6, hotter thermostat (like 82°) and possibly the 15lbs rad cap

[Henry8601]

Thanks for the advice jaydee, but if the fuel needs hotter combustion, doesn't that mean you would need to run a hotter plug to keep the heat in the combustion chamber i.e. BP5?

[jaydee]

No its the other way round, you need an higher range plug, meaning you need a slightly colder plug

[jaydee]

The difference between them is very marginal though

[Henry8601]

Many thanks! Also, I've just got the thing about spark plug temperature. I will post here just to check that the logic is correct:

First assume that the engine is running at a set temperature. A cold plug will cool itself better than a hot plug by conducting more heat into the head (due to the shorter heat path), and thus, for the same engine combustion chamber temperature, a cold plug will be colder.

If you raise the temperature in the combustion chamber (i.e. modern fuel) the spark plug will have to cool itself more to maintain the same temperature as before. Therefore hotter fuel needs a plug with a better self-cooling capability = cooler plug.

[jaydee]

Yep thats correct
I have to say that unless you'r running a pretty high CR and long duration timing the BP6 covers pretty much all the situations for small bore engines