In short, yes, in some situations spark plugs can increase horsepower. These ‘massive’ gains of one or two per cent generally won’t be exceeded, even if you’re replacing really old and worn spark plugs for new ones. In this case, your essentially just restoring your car back to its peak performance.

Typically, spark plugs in four-cylinder engines are located on the top or side of the engine in a row. Spark plugs in inline six-cylinder engines are usually located on the top or side of the engine head.

What symptoms may indicate my vehicle’s spark plugs need replacing?

1. Rattling, pinging or “knock”-like noises. When spark plugs begin to misfire, you may notice unusual noises from the force of the pistons and combustion not working properly.
2. Hard vehicle start.
3. Reduced performance.
4. Poor fuel economy.

The torque specification for a spark plug is for when the engine is cold. Installing a spark plug with the engine hot can change the torque specification and how the threads on the engine and spark plug interact. This can lead to a plug that is too loose or too tight depending on the application.

I would only change the plugs when the engine is cold. Most of the time the reason that a sparkplug gives people problems is because of not properly maintianing the vehicle. If a sparkplug is not changed for a while then the carbon builds up on the threads of the plug and that is when you run into problems.

Luckily, spark plugs don’t wear out very quickly. You can typically get 80,000 miles on them before they need replacing. But if you notice any of these symptoms, it’s time to get your spark plugs checked out with an engine tune-up.

Conventional spark plugs need to be replaced every 30,000-50,000 miles. Long-life (iridium- or platinum-tipped) spark plugs need to be changed between 60,000 and 150,000 miles, depending on the vehicle.