Did Tony Buy the Right Spark Plugs?

Dear Car Talk

Dear Car Talk | Nov 21, 2013

Dear Tom and Ray:

I was shopping for spark plugs for my 2002 Dodge truck from a company that has a site on the Internet. They had spark plugs for carbureted engines and fuel-injected engines. I bought plugs for injected engines. Is there a difference?

-- Tony

TOM: Every engine has its own unique requirements for spark plugs, Tony. It's not really based on whether the engine is carbureted or fuel-injected; it's based on the way the engine was designed.

RAY: For one thing, engines have different physical characteristics. For instance, some engines have thicker cylinder heads and require longer plugs with more threads to get the tip of the plug into the combustion chamber.

TOM: The amount of room on top of the cylinder head is another consideration. Older engines had only two valves per cylinder, so there was plenty of room for a nice, fat spark plug to stick its nose into the cylinder. But the majority of engines today are multivalve. You could have four or five valves cut into the top of each cylinder now, leaving much less room to fit a plug there. So most newer engines call for smaller, thinner plugs.

RAY: The other reason plugs are specific to engines is because the vehicle's engineers designed the plug as part of a system needed to create a very specific pattern of combustion. They may want a spark plug with a hotter spark, or one that sits farther down in the combustion chamber to produce the precise shape, size and duration of flame they're looking for in there. Those details affect an engine's power, gas mileage and emissions.

TOM: For those reasons, there are hundreds of different spark plugs on the market. The only way to know which one you need is either by removing an old one and reading the number, or by looking up a car's year, make, model and engine size in a parts locator, and getting the plug number that way.

RAY: Perhaps what you were seeing was a general "spark plugs for Dodge trucks" page on a website. Over time, there certainly have been both carbureted and fuel-injected engines used in those trucks.

TOM: But not in 2002. By then, every car and truck sold in the United States was fuel-injected. That means there's only one correct spark-plug type for your truck, Tony. I hope you got that one.

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