Do not Overlook the Ohm Values in Your Ignition System’s Spark Plugs & Boots

Do not Overlook the Ohm Values in Your Ignition System’s Spark Plugs & Boots


Resistance values can play a task in your ignition system efficiency—and all of it begins on the plug. (Picture/OnAllCylinders)

Whereas we’ve coated resistance in spark plug wires earlier than, the chance lately arose to speak about resistance in spark plugs and plug boots too.

You could have learn the sooner article on my star-crossed 1970 Honda CB350’s gasoline supply woes. Fact is, gasoline was solely a part of the story. There have been some matters on spark that warranted a standalone article.

Within the identify of brevity, we’ll skip {the electrical} concept on the position resistance performs in an ignition system (although Hemming’s has an excellent article on it right here).

Right here’s a more recent NGK BR8ES spark plug in comparison with an authentic B8ES. They’re just about equivalent, save for the built-in resistance. Examine that distinction beneath. (Picture/OnAllCylinders)

Resistance Constructed-In to Trendy Spark Plugs

It began when making an attempt to purchase new spark plugs for that aforementioned CB. Figuring it was finest to begin recent with new plugs, I dove into my historical Honda store guide, which mentioned the bike requires NGK B8ES spark plugs—a quite common plug on the time. (Two have been additionally already within the bike after I started its restoration.) These days although, that individual spark plug has been outdated by the NGK BR8ES.

To get good resistance readings (and optimum efficiency generally), it’s all the time a good suggestion to maintain the plugs’ electrodes clear. We used some ultra-fine grit sandpaper to clear away the buildup right here. (Picture/OnAlllCylinders)

However within the case of the NGK B8RES, the R implies that it’s received a 5,000 Ohm (or 5 kiloohm/5 kΩ) resistor in-built. The job of that built-in resistor is to cut back electromagnetic interference (electrical noise) stemming from spark discharge that might impression digital methods like a car’s radio or, extra importantly, its ECU.

Resistance is measured in Ohms, represented by the “Ω” Omega image—simply ask Charlton Heston.

As anticipated, with a multimeter on the NGK BR8ES, it examined proper round 5 kΩ. (Picture/OnAllCylinders)

Resistance-type spark plugs are fairly frequent nowadays and for a big spectrum of engine functions, they’re completely fantastic. Extra importantly, for automobiles that do have these high-tech digital methods, they are often downright important.

Drawback is, that additional resistance may also have an effect on issues like spark period and depth—which for a small carbureted twin-cylinder engine that calls for precision ignition timing, might doubtlessly hinder the way in which it runs. And since there’s no radio or ECU to fret about in a 50-year-old bike, I merely cleaned and re-used the previous, non-resistor NGK B8ES plugs I had in there initially. (The aim right here was to return the bike to as near showroom-spec as doable.)

But the spark plug factor was solely a part of the story.

Resistance Within the Spark Plug Boot

For lots of classic bike engines, the ignition coil has an built-in spark plug wire. You merely add the right spark plug boot in your motor’s software. Within the case of the Honda, it required a proper angle boot with 5K ohms of resistance.

Not like many vehicle coils, a whole lot of classic bikes run coils with the ignition wire already hooked up. You simply have to select the precise boot for the particular bike software (Picture/OnAllCylinders)

Simple sufficient—whereas I used to be resurrecting the little Honda, I changed the coils, however deliberate on merely swapping the previous, authentic plug boots onto the brand new ignition wire ends. No want to fret about utilizing the improper boot there.

Right here’s one of many bike’s 50-year-old OE plug boots, measuring over 8 kΩ. The opposite one wasn’t a lot better, coming in round 7.5 kΩ. (Picture/OnAllCylinders)

However as anybody who works round previous electronics already is aware of, resistors can drift off-spec with age. In my expertise, it typically means the resistor will step by step improve its resistance, till it in the end fails open and also you get an entire break in continuity. (In reality, we noticed this with an previous blower motor resistor a short while in the past.)

So to get the bike to run easily, I checked all the things first, together with the spark plug boots. That’s when I discovered that, as an alternative of the inventory 5,000 ohms, they each have been studying considerably greater, nearer to eight kΩ.

A brand new set of plug boots was ordered shortly thereafter.

The brand new spark plug boot examined proper on par with the 5K ohm spec. (Picture/OnAllCylinders)

Becoming a member of the Resistance

Bear in mind, from the manufacturing facility, the little Honda’s ignition system wished to see round 5 kΩ on the spark plug electrode.

However electrically talking, resistance values are additive when wired in collection. That implies that the general resistance is the same as all of the resistors in-line within the circuit. So, by utilizing the brand new NGK BR8ES, it primarily added 5,000 ohms to the combination.

And when coupled with the previous OE spark plug boots that have been testing a lot greater than inventory, the resistance will increase much more. So, all collectively now:

5 kΩ plug + 8 kΩ boot = 13 kΩ whole

Whereas I’m under no circumstances a math knowledgeable, I do know that 13K =/= 5K.

Being 8,000 ohms out-of-spec might have had an impact on spark supply, which can lead to potential efficiency points.

So once more, I wished this bike to be showroom recent. That meant working the correct 5K ohm spark plug boots with a cleaned up set of previous zero-resistance NGK B8ES spark plugs. All instructed, with this ignition setup and the resolved gasoline difficulty outlined in that earlier article, the Honda received a brand new lease on life.

NGK makes it actually clear which plugs carry the “R” designation—you simply should know to look within the first place. (Picture/OnAllCylinders)

As a footnote, I used to be a tad involved in regards to the dwindling provide of NOS NGK B8ES spark plugs on the market. However the excellent news, is that you would be able to merely transfer the resistance round. In different phrases, I’m optimistic that the brand new NGK BR8ES with 5 kΩ of resistance will work completely fantastic on this software, offered they’re paired with zero-resistance spark plug boots.

No matter how the plug- and boot-swapping recreation performs out, the ethical of the story right here is to all the time make sure that your ignition resistance numbers are inline with the manufacturing facility specs—offered you’re working the inventory setup anyway.

Aftermarket efficiency ignition methods are much more nuanced, however that’s a subject for a unique day…


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December 2022