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Spark plugs

Car culture term

Spark plugs
Photo: Wikimedia contributor / Wikimedia Commons · CC BY-SA 4.0

What is Spark plugs?

Devices that ignite the air-fuel mixture in gasoline engine cylinders. Different heat ranges suit different engine applications.

Spark plugs are devices that ignite the air-fuel mixture in gasoline engine cylinders. The plug delivers high-voltage electricity (typically 30,000-50,000 volts in modern ignition systems) across an air gap, creating a spark that ignites the mixture. Different spark plugs have different heat ranges, electrode materials, and gap specifications suited to different engines and operating conditions.

Heat range is a critical spark plug specification. Hotter heat range plugs run warmer in operation, which helps prevent fouling but can cause pre-ignition under high load. Colder heat range plugs run cooler, suitable for high-load operation but prone to fouling at idle. Most cars use a moderate heat range; modified or forced induction cars typically benefit from colder heat range plugs to prevent pre-ignition. Heat ranges are typically labeled with numbers (NGK uses higher numbers for colder; Champion uses lower numbers for colder; specific scales vary by manufacturer).

Modern spark plugs use various electrode materials. Iridium-tipped plugs (common in OE applications) are durable and last 100,000+ miles. Platinum-tipped plugs are also durable. Standard copper-core plugs are cheaper but require more frequent replacement (typically 30,000 miles). Some applications use specialized plugs (NGK BPR, Denso ITV, etc.). Spark plug gap specifications vary by engine; the gap is typically 0.030 to 0.044 inches for most production engines. Gap can be checked with a wire gauge during installation.

Related terms

Compression ratioOctaneKnock

Frequently asked questions

How often do I need to replace spark plugs?
Per the manufacturer specification. Modern iridium plugs typically last 60,000-120,000 miles. Older copper-core plugs require more frequent replacement (30,000-50,000 miles). Modified or forced induction engines often need shorter intervals due to higher cylinder temperatures. Check your specific engine documentation.
What is heat range?
The thermal characteristics of the spark plug. Hotter plugs run warmer (good for low-load operation, prevents fouling). Colder plugs run cooler (good for high-load operation, prevents pre-ignition). Modified and forced induction engines often need colder plugs than stock to prevent pre-ignition. Numbers and grades vary by manufacturer; consult your engine builder or tuner for the appropriate heat range.
Can the wrong spark plug damage my engine?
Yes. Too-hot plugs can cause pre-ignition, leading to detonation and potential piston damage. Too-cold plugs can cause fouling at idle, leading to misfires and possibly emissions concerns. Wrong gap can cause misfires under high load. Using the correct plug specification (heat range, gap, threading) is important for engine reliability.
Are aftermarket spark plugs better?
Sometimes, sometimes not. Premium iridium plugs (NGK BPR, Denso IK20, etc.) are often equivalent to or better than OE plugs. Specialty performance plugs (like NGK 1ZFR or Denso IK20-SR8) are designed for high-performance applications. For most stock applications, OEM-spec plugs (or equivalent quality) are appropriate. Premium plugs offer marginal benefits over good OE plugs in most cases.
How do I check spark plug gap?
Wire gap gauge or feeler gauge. Insert the appropriate-size gauge through the gap; it should pass through with light resistance. Adjust the gap by gentle bending of the ground electrode if needed (typically 0.030 to 0.044 inches for most engines). Most quality plugs come pre-gapped and only require verification rather than adjustment. Gap consistency across all plugs in an engine is important.
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