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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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'Forget the musclecar era, this is the golden age of horsepower. Never in the history of high performance has it been easier and less expensive to make killer power. Face it, 500 hp is the new 400 hp, and it escalates from there. All this comes down to what the internal combustion engineers call BMEP: brake mean effective pressure. This cylinder pressure is what drives the pistons down. Pressure comes from heat, so that means with all this newfound power, engines are cooking up more heat in those cylinders.

When building a custom engine combination, it becomes a challenge to choose the right spark plugs, since there are often no factory reference points from which to start. And this is much more than just "if the spark plug fits, wear it." There are questions of heat range, extended nose versus standard plugs, copper versus platinum versus iridium electrode, and the incredibly diverse world of specialty or "designer" spark plugs. We'll shed some light on all these factors, but expect to do some homework to intelligently choose the right plugs for your application.

In Car Craft's own dyno testing, we've recently been victimized by overheated and pre-ignition-plastered spark plugs, and that never used to happen to street-friendly engines. This sparked an investigation into the world of fire-starters, and we learned some tricks you should know before you light a fire under your latest horsepower effort.

Where To Start
It seems every engine requires its own uniquely designed spark plugs, but despite the fact that spark-plug catalogs' pages now number in the hundreds, for the sake of domestic car-crafting needs, we can condense spark-plug construction to four separate areas: hex size, sealing seat, thread size, and thread reach. Older '60s and early '70s engines generally used 131/416-inch hex sizes, but the industry has been gradually making spark plugs smaller to fit increasingly tighter spaces, so the 51/48-inch-hex plug shell is the most common. As long as the other requirements are fulfilled, you can easily exchange 131/416-inch-hex plugs (like for the '60s big-block Chevy) with 51/48-inch-hex versions.

Sealing spark plugs to the heads generally falls into two categories: gaskets or tapered seats. Generally, iron heads use tapered seats to seal in cylinder pressure, while aluminum heads generally use gaskets. But even this isn't set in stone, since the GM Gen III small-blocks with aluminum heads use tapered-seat plugs. Some aftermarket cylinder heads are now machined for either, but gaskets are still a better idea for aluminum heads, since they prevent metal transfer between the soft aluminum and the steel plug shell.

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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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