[beatr911]

Dual plugs are for bad (slow) combustion chambers and for using lots of EGR where the flame spread is poor. It also allows about a point higher in compression on the same fuel grade to get a little more power. Among others, Porsche used it in the 964 (I think) version of the 911, Ford used it as a last gasp development of the Lima 2.3 from '88 to 2001. BMW and Suzuki use dual plugs on 650cc singles. Dual ignition also takes twice the amount of parasitic power to fire as single plug.



MFR recommended plug gap is not the ultimate for MPG or power. The gap is narrower to account for wear. For an old vehicle, I had purchased a Jacobs Omni something ignition. Basically it was a seperately powered coil with a 30A fuse to the battery. Very strong spark.

Anyway, with it came a spark plug tuning guide. It basically said to open the gap at .005" increments until a slight miss occurred at high load and high speed. Then back off .005" and that is your optimum gap. Re-inspect gap every 10,000 miles.

So when the Jacobs ignition eventually failed, I tried this same procedure with the stock ignition. Ya know what? Nearly the same benefit and it didn't cost $400 for a fancy coil. Sure, it wasn't as good as the Jacobs spark, but had I known this procedure in the first place I probably wouldn't have purchased the Jacobs coil.

Now I use the procedure on every ICE I own where I can get to the plugs relatively easily. This due to the more frequent inspection/re-gap interval. Results have been easier starting and smoother idle but I must say I haven't done ABA mpg testing. I can only infer that the part throttle burn is better though.

Edit: I'm stating this because I think a sparkplug discussion is not complete without knowing how to actually use the sparkplug to it's full benefit. Some racers make a black art of modifying plugs if ya really want to go nuts.