Hi!

A Quick question. Is it ok to mix different spark plugs if its the same brand? And if not what could happen? In my XJR - 98 I have right now since a couple of days back six NGK - I R and two NGK - R. So far Everything seems fine..

 

It's not advisable to mix plugs as they have different heat ratings, depending on the length of the inner ceramic insulator between the plug body & firing end of the central electrode.

A hot-running engine needs a 'cold' plug to avoid plug overheating-this will have a short insulator length to allow the heat from the very hot firing end of the plug to reach the cooler cylinder head metal via the shortest thermal route.

If you fit a 'hot' plug to a hot-running engine, there's a risk of detonation & pre-ignition under high engine loads-which in extreme cases could blow holes in piston crowns with sustained pre-ignition.

Conversely, a cold engine needs a 'hot' plug with a longer insulator to avoid the heat being conducted away from the plug too quickly. If you fit a 'cold' plug to a cold-running engine, then the plug may foul up with carbon as it won't get hot enough for the soft carbon to burn off.

Also, plugs come in 'resistive' & 'non-resistive' types. If an ignition system requires a resistive plug & you don't fit one, then you can get greater electrical interference due to the stronger electrical pulse through a non-resistive plug.

Conversely, if you fit a resistive plug to an ignition system where a non-resistive plug is specified, then the extra plug resistance can reduce the power of the spark & may cause misfires.

The actual spark plug numbers contain letters & numbers which identify their heat ratings & electrical properties.

You probably won't see any immediate catastrophic effects by mixing plug types-unless the threaded section is obviously longer than normal & touches a piston crown-but you may get problems later during certain types of driving conditions.

 

So "I R" and "R" stands for 'resistive' & 'non-resistive'? Which one should the 1998 XJR have?