Plug gap theory

[NGKTECH]

Here is an email I got from a kid that pertains to gap theory, thought this may be a good place to post it and it is interesting info. My email reply back to him is first and his is at the bottom so start there.

Hello Tyler,

I must apologize as I double checked this information which unfortunately is not entirely accurate, It should be changed to read:

"The incorrect plug gap for your engine can contribute to a high rate of misfires, loss of power, plug fouling, poor fuel economy, and accelerated plug wear."

You see the plugs gap regardless of too large or small should not result in a big enough change in electrode temperature of either electrode that it should result in abnormal combustion (pre-ignition or detonation).

Actually a smaller gap has the opposite effect of what you described. A smaller gap will require less initial voltage to fire which means the secondary ignition voltage would actually begin to ionize the air gap sooner than a larger plug gap would, which in effect is like advancing the ignition timing and vice versa with a larger gap. This happens to a very small degree, so chances are you wouldn’t actually notice this small difference in firing time with small gap changes.

It is easy to imagine your ignitions available voltage output as a piece of rope and lets for the sake of argument say this rope is 10 inches long and represents 40k volts. If the initial voltage required to ionize the air gap is 24k volts or 6 inches of rope then we only have 4 inches of rope left to keep the plug burning (14k volts). So theoretically if we raised the gap and now the voltage required on this same engine increased to 28k volts (7 inches), we would then have only 3 inches of rope left to keep the plug firing.

Leaner air fuel ratios which are less dense and much harder to ignite will require a larger gap which will require more initial voltage (higher initial heat energy) to start the burn, but results in a shorter burn time. Lean air fuel ratios also tend to burn more quickly than rich AFRs so the firing time is not as critical.

A smaller gap will usually be needed for high performance racing applications because under high cylinder pressures and high RPMs the voltage requirement is much higher than it is in your moms Taurus. The available time to fire the plug is very short at high Rpms and the A/F ratios are rich and subsequently dense, so we do not need as high initial voltages (heat) to start the burn like we do with lean A/F ratios.

Remember a large gap would result in a shorter burn time which would not effectively ignite our entire dense fuel charge resulting in wasted energy (less power output). By lowering the gap in these high power applications we can lower the voltage requirement and get the plug to spark sooner and for a longer period which will help give us a better chance to more completely ignite our slow burning rich air fuel charge and create higher peak cylinder pressure and ultimately more power.

This is pretty technical stuff so I hope that it makes this more clear for you. Also keep in mind that while a lower gap may help in a high Rpm race engine it will not be well suited to getting the best possible gas, part throttle, and idling efficiency (which require a larger gap for greater ignitability) Hope this helps, thank you.

Best regards,



Brandon Peeler

Technical Support Representative

Aftermarket Division

NGK Spark Plugs (U.S.A.), INC.

46929 Magellan Dr.

Wixom, MI 48393

Bpeeler@ngksparkplugs.com




-----Original Message-----
From: tylerb@charter.net [mailto:tylerb@charter.net]
Sent: Thursday, April 10, 2008 7:50 PM
To: Info
Subject: [Tagged as Possible Spam] NGK Form Mail
Importance: Low

**** Contact Form & Tech Info Feedback ****

Email From: tylerb@charter.net
VehicleType: Car
Make: Hyundai
Model: Tiburon
Year: 2003
Engine Type: S/C V6
Comments: I was wondering is you could elaborate as to WHY this statement (directly from the NGK website) is true?

"Insufficient spark plug gap can cause pre-ignition, detonation and even engine damage."

We're having a discussion about the theory behind this on one of my enthusiast forums, and everything we've read elsewhere says that reducing plug gap has an effect similar to retarding timing (due to a smaller initial flame kernel). In turn, this should reduce tip temperature, and (to the contrary) should reduce the chances of pre-ignition.

As such I'm just wondering what additional factors would come into play to actually contribute to the chance of pre-ignition with a smaller plug gap? ANY additional info your techs could provide on the facts and theory behind this situation would be GREATLY appreciated!!

Knowledge Hungry,

Tyler