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Choosing The Right Spark Plugs For Your Custom Engine

Don't Get it Wrong! Know Something About Reach and Heat Range and Everything Else You Need to Pick the Right Plug.

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The most popular thread size for domestic plugs is 14 mm, although a few older Ford engines used the larger 18mm size back in the '60s. Thread reach does vary, but the two most common flavors are the short 0.460-inch and the longer 0.750-inch versions. The longer thread reach is generally used most often with aluminum heads, while the shorter versions are used with iron heads.

The Cold War Heats Up
Big horsepower means more heat, so when it comes to choosing a spark-plug heat range, the best advice is also the simplest-put spark plugs in the engine that will live. This goes for both race and street engines. Spark plugs are especially sensitive to heat, and as a result, designers offer spark plugs in several heat ranges to be able to handle the wide range of cylinder temperatures. A mild street engine idling around town prefers hot plugs to keep the center electrode at the optimum temperature to burn off deposits that otherwise lead to misfires and lost efficiency. An 850hp drag-race motor that spins to 9,000 rpm will require much colder plugs capable of quickly transferring all that additional heat away from the center electrode.

As you might have guessed, a strong street engine might actually need two different sets of plugs, depending on how the engine is being used. Those hot plugs for around-town cruising might well need to be one or perhaps two steps colder for those high-rpm, Saturday-night blasts at the strip if you're using nitrous or cranking up the boost on a supercharger.

How To Read Spark Plugs
Here is where the science of spark plugs diverges into more of an art form. We will begin to outline all the details of spark-plug reading, and even though there is much left to interpretation, there are a few simple techniques that can help you get closer to understanding what your plugs are telling you.

With unleaded gasoline, and especially the new ethanol-blended fuels like E85, the signs present on spark plugs will take some close scrutiny. Begin by finding a good spark-plug viewer. ZEX and others sell these inexpensive tools. The most expensive ones we've seen are medical-grade tools that let you see right down into the bottom of the plug porcelain, but they cost $150. It's also best to tune with new plugs that have perhaps two or three passes on them to help you read the results.

Think of spark plugs as windows into the combustion process. The end of the porcelain closest to the center electrode is used to read the idle mixture. The center portion of the porcelain will indicate part-throttle air/fuel (A/F) mixtures, while the bottom of the porcelain will indicate the wide-open-throttle (WOT) air/fuel ratio. Obviously, a black, sooty plug is way rich and a bone-white porcelain is probably way lean. The best way to judge WOT mixtures is to run the engine at WOT and then cut the ignition clean at the end of the track and safely shift into Neutral. Coast to a safe stop on the return road, pull a spark plug out, and use the viewer to closely inspect the plug.

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13 comments
David Rico
David Rico

Cold as they get,blown methanol don't foul.

Dean L. Thomas
Dean L. Thomas

dont even go there. do your own reaserch and order parts by #'s

Brian Bavington
Brian Bavington

lots of good info but got a? i came to this site trying to find a good plug to run in a pare of 305 marine engines caribrated engines here my problem engines run cold for long periods of time at low rpm it seems to me that all new plugs with the gas of to day seem to foul and unable to clean them self's  i have had great luck with ND plugs IN THE PAST how dose one find a replacement for a older engine by that i mean colder running except under load then hotter than hell not water temp head temp crack porcelain then the plug fouls  w16epru  i think was the plug i was running and i will be darn if i can keep track of the heat range any more 

野口強
野口強

モロソ オイルパン!

Jeff Griffith
Jeff Griffith

Well that was pretty vague! Lol That's just common sense to most of us, maybe explain the heat range next time?

Briisdow Vergara
Briisdow Vergara

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Drue Taylor
Drue Taylor

One steep colder worked great for me 10.3:1 dodge 406 big block. Motorcraft plug of course

Karl Loper
Karl Loper

Try explaining heat range period to the counter monkeys.

Johnny Baker
Johnny Baker

Try explaining you need a colder heat range plug for your aftermarket aluminum chevy heads to the dufus at the parts store and they stare blankly and ask "what year?"

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