Best spark plug gap for MPG

[tradosaurus]

I found this article a few years ago and would like to discuss the best spark plug gap for fuel efficiency.

"Get NGK V-power plugs and set the spark plug gap to 0.030 for best results. Or close your wide factory gap by .020-.030. Use dielectric grease on your plugs and in the plug wire boots and on the metal contacts. Silicone spray is okay but the grease is better because the next time you remove the boots, it is possible to rip the cables and damage the wires to introduce a missing condition in the ignition. It is also a good idea to put anti-seize on the plug threads. Champion and AC plugs have tremendous resistance levels that simply waste spark energy. Measure the resistance for yourself. Please use compressed air to blow out dirt from around the plugs BEFORE you remove them. Otherwise dirt falls into the cylinders and causes rapid ring wear. I use fish hook extractor pliers to grab my plugs in and out of a deep hole. Be sure your plug cables are like new by viewing the ignition system on a scope if possible. Replace cables if older than five years with NGK or Federal wires. With a tighter gap, voltage is less but amperage is much greater. Amps mean HEAT and that is what fires the air-fuel mixture, not voltage. The more heat, the better the ignition and mileage. Ordinary wires should be replaced every 50,000 miles. It is best to use NGK V-power or get Bosch Super spark plugs as they have the proper (lower) resistance levels. Some AutoLite plugs have a lower resistance and are okay. But ALWAYS check the plug resistance to be sure it does not exceed 7000 ohms. Please ignore most mechanics opinions about plugs. For instance we avoid platinum and iridium plugs because people tell us their cars do not run as well. We pay close attention to what people tell us about their actual results. Remember mileage and power go together."

From wikipedia:

"The gap adjustment can be fairly critical, and if it is maladjusted the engine may run badly, or not at all. A narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture, while a gap that is too wide might prevent a spark from firing at all. Either way, a spark which only intermittently fails to ignite the fuel-air mixture may not be noticeable directly, but will show up as a reduction in the engine's power and fuel efficiency.

With a narrow gap, the spark might be too weak/small to ignite fuel, but the plug will almost always fire on each cycle; a plug with a wide gap might not fire, or missfire at high speeds, but will usually have a spark is strong for a clean burn. A properly gapped plug will be wide enough to burn hot, but not so wide that it skips or misses at high speeds, causing that cylinder to drag, or the engine to begin to rattle."

[theholycow]

I am not qualified to provide an educated opinion. However, my guess would be that a narrower gap is detrimental. I doubt it will make the spark any hotter, but either way that spark will 100% ignite any fuel it touches. Ignited is ignited. A wider gap would make the spark travel through more fuel, directly igniting more fuel; if anything, wider might be better.

Don't forget that you can't adjust iridium and platinum gaps. If such plugs aren't gapped right from the factory then they are defective. Check the gap and either install or replace.

On resistance: It's there for a purpose.
http://www.ngksparkplugs.com/techinf...aqresistor.asp

Q: When should I use a resistor spark plug?

A: NGK "R" or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking.

NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.

They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer.

In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer undesirable side effects such as an erratic idle, high-rpm misfire, engine run-on, power drop off at certain rpm levels and abnormal combustion.

[tradosaurus]

I would say that a wider gap being measured in 0.010" wouldn't travel across much more fuel. The spark is igniting the fuel/air mixture in the cylinder which will be constant no matter how small or wide the spark plug gap.

However if the spark is hotter it stands to reason that the ignition in each cylinder should be more efficient.

I guess I want to know at what gap is the most optimum and does it depend upon how many cylinders are in the car?

[GasSavers_Erik]

I think the number of cylinders was relevant to how hot the spark was at high rpm back in the days of breaker point ignition systems (stated more accurately is cylinders per set of points) some 8 cylinder engines used two sets of points for a hotter spark since an extra set of points allowed extra dwell (coil charge time). Although dwell is actually measured as an angle.

I don't think the # of cylinders matters today. I say to go with the engine manufacturer's plug gap recommendation. Some ignition systems produce a hotter spark (ex. GM HEI) so the gap should be wider.

The only time I ever narrow a plug gap is when I am working on an old lawnmower engine with a weak spark and breaker points. I only narrow the plug gap because I am too lazy to pull the flywheel off and clean or change the points.

[VetteOwner]

X2 on the mower^^^

i just go with whatever the specs are. I know the 2.2L gm engine loves the factory plugs or NGK's and doesnt run as good on any other plugs (might have something to do with why the delco plugs are $6 a piece double platinums...)

and yes you can and are supposed to adjust most iridium and platinum plugs THC...you may be thinking of those crappy snake oil bosch +2 +4 crap in which yea your not supposed to on those.

[theholycow]

Interesting. I have always known that you are not supposed to gap the brittle platinum and iridium plugs, but further research shows that it's not that simple. The manufacturers recommend against it or advise caution but don't flat-out say you can't.

What you can't do is gap a used platinum plug. From good ol' Chilton:

NOTE: NEVER adjust the gap on a used platinum type spark plug.

[VetteOwner]

yea they are fragile but if you use the right gaping tool you should never come in contact with the tip just the ground prong. then lightly drop the plug on the prong if you gap it too far. i always have because the 6 i recently put in my blazer 2 were way over 2 were under and 2 were just about right on size i needed.