Resistance values can play a role in your ignition system performance—and it all starts at the plug. (Image/OnAllCylinders)

While we’ve covered resistance in spark plug wires before, the opportunity recently arose to talk about resistance in spark plugs and plug boots too.

You may have read the earlier article on my star-crossed 1970 Honda CB350’s fuel delivery woes. Truth is, fuel was only part of the story. There were some topics on spark that warranted a standalone article.

In the name of brevity, we’ll skip the electrical theory on the role resistance plays in an ignition system (though Hemming’s has a great article on it here).

Here’s a newer NGK BR8ES spark plug compared to an original B8ES. They’re virtually identical, save for the built-in resistance. Read about that distinction below. (Image/OnAllCylinders)

Resistance Built-In to Modern Spark Plugs

It all started when trying to buy new spark plugs for that aforementioned CB. Figuring it was best to start fresh with new plugs, I dove into my ancient Honda shop manual, which said the bike requires NGK B8ES spark plugs—a very common plug at the time. (Two were also already in the bike when I began its restoration.) Nowadays though, that particular spark plug has been superseded by the NGK BR8ES.

To get good resistance readings (and optimal performance in general), it’s always a good idea to keep the plugs’ electrodes clean. We used some ultra-fine grit sandpaper to clear away the buildup here. (Image/OnAlllCylinders)

But in the case of the NGK B8RES, the R means that it’s got a 5,000 Ohm (or 5 kiloohm/5 kΩ) resistor built in. The job of that integrated resistor is to reduce electromagnetic interference (electrical noise) stemming from spark discharge that could impact electronic systems like a vehicle’s radio or, more importantly, its ECU.

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

As expected, with a multimeter on the NGK BR8ES, it tested right around 5 kΩ. (Image/OnAllCylinders)

Resistance-type spark plugs are quite common these days and for a large spectrum of engine applications, they’re perfectly fine. More importantly, for vehicles that do have those high-tech electronic systems, they can be downright essential.

Problem is, that extra resistance can also affect things like spark duration and intensity—which for a small carbureted twin-cylinder engine that demands precision ignition timing, could potentially hinder the way it runs. And since there’s no radio or ECU to worry about in a 50-year-old motorcycle, I simply cleaned and re-used the old, non-resistor NGK B8ES plugs I had in there originally. (The goal here was to return the bike to as close to showroom-spec as possible.)

Yet the spark plug thing was only part of the story.

Resistance In the Spark Plug Boot

For a lot of vintage motorcycle engines, the ignition coil has an integrated spark plug wire. You simply add the correct spark plug boot for your motor’s application. In the case of the Honda, it required a right angle boot with 5K ohms of resistance.

Unlike many automobile coils, a lot of vintage motorcycles run coils with the ignition wire already attached. You just have to pick the right boot for the specific bike application (Image/OnAllCylinders)

Easy enough—while I was resurrecting the little Honda, I replaced the coils, but planned on merely swapping the old, original plug boots onto the new ignition wire ends. No need to worry about using the wrong boot there.

Here’s one of the bike’s 50-year-old OE plug boots, measuring over 8 kΩ. The other one wasn’t much better, coming in around 7.5 kΩ. (Image/OnAllCylinders)

But as anyone who works around old electronics already knows, resistors can drift off-spec with age. In my experience, it often means the resistor will gradually increase its resistance, until it ultimately fails open and you get a complete break in continuity. (In fact, we saw this with an old blower motor resistor a little while ago.)

So to get the bike to run smoothly, I checked everything first, including the spark plug boots. That’s when I found that, instead of the stock 5,000 ohms, they both were reading significantly higher, closer to 8 kΩ.

A new set of plug boots was ordered shortly thereafter.

The new spark plug boot tested right on par with the 5K ohm spec. (Image/OnAllCylinders)

Joining the Resistance

Remember, from the factory, the little Honda’s ignition system wanted to see around 5 kΩ at the spark plug electrode.

But electrically speaking, resistance values are additive when wired in series. That means that the overall resistance is equal to all the resistors in-line in the circuit. So, by using the new NGK BR8ES, it essentially added 5,000 ohms to the mix.

And when coupled with the old OE spark plug boots that were testing much higher than stock, the resistance increases even more. So, all together now:

5 kΩ plug + 8 kΩ boot = 13 kΩ total

While I’m by no means a math expert, I do know that 13K =/= 5K.

Being 8,000 ohms out-of-spec could have had an effect on spark delivery, which may result in potential performance issues.

So again, I wanted this bike to be showroom fresh. That meant running the proper 5K ohm spark plug boots with a cleaned up set of old zero-resistance NGK B8ES spark plugs. All told, with this ignition setup and the resolved fuel issue outlined in that earlier article, the Honda got a new lease on life.

NGK makes it really clear which plugs carry the “R” designation—you just have to know to look in the first place. (Image/OnAllCylinders)

As a footnote, I was a tad concerned about the dwindling supply of NOS NGK B8ES spark plugs out there. But the good news, is that you can simply move the resistance around. In other words, I’m optimistic that the new NGK BR8ES with 5 kΩ of resistance will work perfectly fine in this application, provided they’re paired with zero-resistance spark plug boots.

Regardless of how the plug- and boot-swapping game plays out, the moral of the story here is to always ensure that your ignition resistance numbers are inline with the factory specs—provided you’re running the stock setup anyway.

Aftermarket performance ignition systems are far more nuanced, but that’s a topic for a different day…

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Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or roof leaks in an old Corvette ragtop. Thanks to a penchant for vintage Honda motorcycles, he spends the rest of his time fiddling with carburetors and cleaning chain lube off his left pant leg.