Evening-out plug wear on waste-spark DIS systems

[VinceVaughnce]

Who has experience with DIS waste spark systems? Before COP took over, waste spark DIS were very common, and there are quite a few still in service. If any of you have changed plugs on a system like this, you'll notice half of the plugs show wear on the center electrode while the others remain virtually perfect. This is because there are two cylinders being fired by one coil. The two cylinders fired by one coil are referred to as "running mates" and are paired by being on opposite cycles (one fires on compression and the other 'waste sparks' on the exhaust).

Now, each coil running the mate cylinders has a reversed polarity on one output. So one plug always fires the spark from the electrode to the ground strap, while the other running mate cylinder fires from the ground strap to the electrode. Put another way, assuming an example of 35KV discharge voltage, that means that relative to Common ground (as in the cyl.head/vehicle body) plug A fires at +35KV and plub B fires at -35KV. With this, the plug firing a positive 35KV will see (proton-induced) wear on center electrode, and the other firing at negatve 35KV will see wear on the ground strap (for the same reason) but most often much less so, meaning that at plug replacement interval, one plug looks destroyed while the other remains virtually perfect.

NGK explains all of this better:
https://www.ngk.com/glossary/8/spark-plug/w

All the rambling is just so even if someone is not familiar with this system, they too can offer their opinions to the big question:
Why not even plug wear halfway through the change interval, by swapping the wire boots at the coil, between the running mates?

After all these years, I've JUST explerimented with it recently, one GM and one Ford. Although just a week or two into the experiment, no issues, both run smooth.

Can anyone think of something I might have overlooked bc I havent seen it mentioned before

 

[eljefino]

Sounds brilliant. One should also dismount and flip their tires so the inside doesn't wear faster than the outside, or vice versa, on cars so afflicted. This is the sort of thing you can do for your car if your tooling and labor is free. You could also ride a bike once in a while.

Ford had some OE platinum plugs that only had platinum "where it mattered" to their waste spark system. To them it was a penny saving engineering decision.

 

[92saturnsl2]

Waste spark has been around for the 20+ years I've been driving, and present on almost every vehicle I've owned except the most recent ones which have COP. That said, I've never had a plug fail prematurely nor have ever experienced the odd wear pattern you describe. I use copper plugs and I get an easy 50k out of them even in waste spark vehicles. When I change the plugs, I pay close attention to the wear patterns and they all have worn very similar.

Some examples of the WS-equipped vehicles I've owned: GM 3800 Series II (Bonneville, Grand Prix, etc.), '92 Saturn (1.9L I-4), Mitsu 4G63, VW VR6 Mk3.

I guess I don't see the point, spark plugs are cheap.

 

[Jethro_Bob]

The 2 hundred thousand mile spark plug change... sounds great, if you're in the business of refurbishing heads.

 

[RoadRaceJosh]

Install double-platinum plugs and forget about it. Then you can mentally massturbate about something more fun.

 

[Silk]

Originally Posted by 92saturnsl2
Waste spark has been around for the 20+ years I've been driving,


It's been around a lot longer than that in motorcycles, and even before waste spark. The magneto used on British twins fired the plugs reversed each side...so we used to swap plugs occasionaly to even the wear. BMW twins were waste spark in the '60's...I still swap plugs on my BMW.

Double plats solve the problem.

 

[VinceVaughnce]

Silk that's awesome you knew to swap em.

Personally kind of averse to platinum plugs- poorest conduction (don't like high cyl pressures ie turbo/SC) vs iridium/nickel. Iridiums are too risky/expensive to be gapping (running non-standard), double ones more so. Currently I prefer nickels; good, cheap and tossable. They perform really well, without the problems. The only downside is they erode quickly.unevenly on brute-force DIS systems, and the spark will be liable to discharge from anywhere on that big ol nickel electrode, not in a uniform spot like finewires. Despite perfect uniformity being the goal, a lack of finewire is no deal breaker

To the set-it-and-forget-guys, of course no one's going to notice variations in flame propagation speed and resulting cylinder pressure, but a good unhampered burn is good for emissions and efficiency to about the tune of a single MPG. Yes the tiny variations in speed of the flame propagation does alter the peak cylinder pressure.

The Waste-DIS system is obviously crude. Having seen DIS-ravaged plugs in the day, it's still amazing to me how an engine can run acceptably with one half at 0.06 gap and the other at 0.12

Beware the gimmick and multi-electrode plugs, they're really bad for flame propagation. If anyone's interested about that, or wants to visualise the variance in the indicated mean effective pressure (IMEP), check this out:
https://www.sciencedirect.com/science/article/pii/S2090447912000883

Also indexing and "side gapping", solid principles.

 

[DoubleWasp]

I've gapped and installed a ton of Iridium plugs. Nothing horribly risky about it. Just don't go gorilla on them, and it is fine.

A lot of DIS waste spark vehicles are the type that requires a plenum removal in order to change some of the plugs. I'd much rather go through a risky gapping session once, than to go through that nightmare on a regular basis.

 

[PandaBear]

I have done that "just for education purposes" to satisfy my curiosity.

Yes, dad's Taurus came with that "one side platinum only" OEM thing, so in theory you can make that 90k interval 120k by flipping it around and use up the steel side electrode. If you are really bored and starving. Also in theory you can run that 90k double platinum for another 60k (just a number to throw around for safety) if you flip them, making it a 150k interval plug to get every penny worth and save the environment.

The problem is, other part of the plugs wear out too. The ceramic can crack, the anti-siege coating on the thread wears out (the OEM came with uncoated plug that turns rusty and stuck at 80k), the metal can fatigue by the loosening and tightening. So yes, the electrode may last that long, but you are using up other safety margin as well. Plus the labor is not free even if you are in an African rural village, so you have to decide what is worth the trouble.

For the same effort, if you start a charity and collect used up spark plugs and send them to 3rd world, or give them away to the low income family, they will likely save more pollution (from running completely dead plug), reduce cost (of the plugs), and create job opportunities (hire the unemployed to replace spark plugs). It would also be a good experience you can put on your resume to get you into a business school, so you can make a lot of money after graduation, and buy brand new car every 3 years. (maybe not the new car but the rest is realistic).