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Most of the time, the second set is for waste spark- burns what may have not been burnt.
not really.
in aviation engines, the two plugs are fired at the same time per spec at a fixed ignition time (no spark advance on the magnetos). the difference in combustion efficiency is easily noted as every pilot does a "mag check" to see a similar amount of RPM drop when one magneto is shorted out. Of course, that on-the-ground test is done before takeoff to verify that both magnetos are firing; if one isn't firing, combustion dies when the other mag is shut off and the RPM drop can be noisily dramatic. With the advent of computerized monitoring of aviation engines, pilots can now do a similar ... even better ... "mag check" in the air with the engine under load; they can see the effects of cylinder head temp and exhaust gas temps at each cylinder with one magneto shorted out. It's a great diagnostic tool since ground running an aircraft engine with a prop on it is not very convenient.
in our Ducati single cylinder two 'plug conversions, the hemispherical combustion chamber normally has only one spark plug from the side. The additional plug is from the other side. Both plugs were fired simultaneously through a dual output ignition coil. The difference in the HP on the dyno was about 5% increased. The two plug setup addressed the flame path issue in this engine ... which otherwise would have been fine if the hemi combustion chamber shape had a centrally located spark plug over the piston. Unfortunately, Ducati's OHC design didn't allow for such access.
Similarly, many other automotive engines used dual ignition systems with some benefit. R-R used two ignition systems for reliability, a magneto and a battery & coil set for some time. Their intent was to have the battery and coil for better starting at low cranking speeds and the magneto capable of delivering stronger spark (higher voltage) as the RPM increased. But I've noticed a similar RPM drop at idle and fast idle if both systems weren't working, firing at the same time ... even though the spark plugs were placed at a comparable location in the cylinder.
More recent engine designs with improved combustion chamber shapes and better intake/exhaust flows have compensated for the combustion benefits that were achieved by two-plugging a cylinder head for flame path efficiency. Some engines had this right from way back 'when, such as the "hemi" built in the USA ... and others that had similar shapes, such as the Peugeot car engines which were pretty efficient even in pre-emissions and computer control days (not seen very much in the USA, their cars didn't sell well here for a variety of other reasons but I recall them delivering 30+ mpg in combined driving with reasonable performing cars back in the 1970's).
2.3's don't have great flow, didn't habe variable valve timing, only had about 110 hp for the dual plug models, for the last years in the ranger, the 2.5 may have had a bit more. It will however outlast whatevers in your hatchback. Lima 140s are some pretty popular race motors, especially in ministock, and midget racing. If I ever can get around to it, my goal for the one in my mustang is about 200 n/a. It's one of my favorite engines. Pretty good potential, and easy to get ahold of. I'd love to have a custom tall deck steel block made, dual plug, with volvo head, and turbo setup, bored and stroked to a 3.0.
It'll outlast it for sure, but it ain't nowhere near as fun to drive. I have 130k on the mazda, and 180k on the ranger.
I respect how reliable the ranger is. It's like a little mule. Probably add a utility rack for home depot runs and maybe swap in 4.10's to make it feel lively around town.
The hatchback has its odds stacked against it, I have a gtx3076 behind me I need to install and I'm going to keep pushing until it pops then buy a built motor.
Some BMW motorcycles from the 80's also benefit from adding a 2nd spark plug to the heads. Buddy of mine did his, the 2nd plug was smaller than the stock plug. There was a flat spot on the outside of the head, below the valve cover, that seemed to have been intended for a 2nd plug.
I have a 1989 4wd Nissan small truck with a 4 cylinder at it has 8 spark plugs. The guy I bought it from, years ago, didn't even know it had 8 plugs. He said he knew it was for sure a 4 cylinder. The truck has been very reliable overall. Lots of rust though.
A former aircraft mechanic / pilot here and yep..2 per cylinder...cost was around $28 around ten years ago per plug. Imagine the faces in the shop when someone dropped one. That was guaranteed to be cut up and used for compression testing later.
We also rarely replaced them...pulled them, cleaned out the carbon, blasted them with sand and air, checked the gap and tested them in a plug tester...clean blue spark and re-installed.
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That was guaranteed to be cut up and used for compression testing later.
