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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 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 U.S. 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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Dear Tom and Ray: General Motors recently unveiled the 2015 Chevy Tahoe, Suburban and GMC Yukon. The three SUVs continue to have the gear shifter positioned on the steering column instead of the floor-mounted center console, like the Traverse, Equinox and Acadia. Is this solely a cosmetic design decision, or is there an engineering reason for this choice? – Bob

Tom: It’s partially cosmetic, partially historic and partially practical.

Ray: The practical reason is that putting the shifter on the steering column leaves more room for a large center console between the seats. People like to use center consoles to store all their stuff.

Tom: Not just cups of coffee and quarters for parking, but, increasingly, purses, bags of Cheetos, laptop computers and the occasional medium-size household pet.

Ray: And some of these center consoles – like the Suburban’s – have gotten so big that you can rest your lunch tray on top of it and stash a backup Chevy Spark inside for when you’re tired of getting 16 miles to the gallon.

Tom: Putting the shifter on the steering column also gives Chevy the option of offering a bench seat up front instead of two bucket seats. A bench seat – with the proper seat belts and all – allows three people to sit across, instead of two. So instead of carrying only eight people, and having to ask the other team to pitch to themselves, you can carry a full baseball team in your Suburban.

Ray: In terms of engineering, it really doesn’t matter where you put the shifter. It can go almost anywhere, within reason. You just need to have some way to transmit the position of the shifter to the transmission itself.

Tom: That can be done with a cable, with rods or, these days, with an electronic signal.

Ray: The final issue is historic precedent. The column shifter was a staple for many decades. And apparently there is a group of buyers that still like it that way.

Tom: Well, I don’t want to speak for them. But everybody knows you can’t snuggle in the front seat with a gear shift in the way.

Ray: Well, you can. You just can’t snuggle pain-free.