It depends on the modifications. The term “modified” refers to those engines with bolt-on components that may or may not raise the total compression ratio of the engine. These can include turbocharging, supercharging, nitrous oxide injection, smaller-chambered cylinder heads, free-flowing cylinder heads, modified pistons, change of induction components and/or the use of different fuel types and octane. These kinds of modifications may need spark plugs different from stock.

Modifications that typically will not require specialized plugs (where the factory installed plug will be adequate) include adding a free-flowing air filter, headers, mufflers, rear-end gears, etc. Basically, any modification that does not alter the overall compression ratio will not necessitate changing plug types or heat ranges. Such minor modifications will not significantly increase the amount of heat in the combustion chamber.

However, when compression is raised, along with the added power comes added heat. Increased cylinder temperatures cause increased spark plug firing end temperatures. Since spark plugs dissipate their heat to the cylinder head at a certain rate to avoid overheating the ceramic firing end, it is crucial that a suitable heat range be used. A modified engine with higher cylinder temperatures will usually need a colder heat range spark plug. Generally, for every 75-100 hp you add, you should use a one (1) step colder heat range to be safe and avoid pre-ignition. The plug gap may also need to be adjusted smaller to avoid misfire.

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