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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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Unleaded fuels do not color plugs as clearly as leaded fuels, so you have to look closely for the A/F ratio ring at the bottom of the center porcelain. The big things to look for are tiny flecks of aluminum that have transferred to the ceramic from the piston or perhaps the combustion chamber during an oh-too-lean condition that has caused aluminum to melt.

Ignition timing plays a huge part in terms of spark-plug temperature. The best place to read this is on the spark plug's ground strap. The tip of the ground strap will tend to color first, and then, as temperature increases, this coloring (called annealing) will travel down the length of the strap toward the shell. An ideal ignition-timing temperature range for a given spark plug will create a light blue ring around the strap about halfway down its length. If the ground strap shows no coloration, the engine could respond to more timing. If the ground electrode begins to color in blues and greens or the tip begins to melt, the timing is over-advanced. Of course, if you are tuning at the dragstrip, these conditions will have raised red flags in terms of lost e.t. or trap speed as well.

The Demands of Superchargers, Turbos, and Nitrous Power-adders do a fantastic job of increasing cylinder pressure and heat. This also places much greater demands on the spark plugs. Nitrous is perhaps the most abusive because of its reputation for extremely fast cylinder-pressure rise time. Some rules of thumb include running a colder heat range and using nonprojected-nose plugs. The general consensus among power-adder tuners is that it's best not to use platinum plugs, since this material acts as a kind of heat sink that will cause pre-ignition problems. Other tips include running a tighter gap, since high cylinder pressures place much greater demands on the ignition system. The best plug gaps are the largest ones that will survive; start with 0.030 inch.

For nitrous, a safe recommendation is one step colder for every 75 to 100hp increase in power. So, for a 300hp engine adding 200hp of nitrous, this would mean using plugs that are two steps colder. This is also a good starting point for similar power increases either normally aspirated or with a supercharger or turbo.

Specialty Plugs
This is the age of designer spark plugs. SplitFire rode the first wave of specialty plugs with its V'd ground strap, and now it seems that every spark-plug manufacturer has its own version of what's hot. While the debate will continue to rage over whether diamond-shaped or split-V ground straps are worth more power/mileage/emissions, we do know that plugs using more exotic metals enhance durability. Platinum arrived a few years ago, with Bosch leading the charge with three different versions of its platinum-tipped spark plugs. Then NGK upped the ante with its Iridium IX series of plugs. The plan with virtually all these designs revolves around reducing tip or ground-strap erosion that increases the gap between the center electrode and the ground strap. Some plugs, like the Bosch Platinum +4, use four ground straps to also minimize the overall gap to ground.

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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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