Steed: Built to Ride

Suck, Squeeze, Bang, Blow!: Single? Dual? Choose your Spark!

Tech Tip Index

Last week at a local watering hole, I overheard two "Bikers" debating whether to install a Single-Fire ignition on a new machine that one of them was building from scratch. The first guy, we'll call him Willie, had a stock Harley and said when he ponied up for a Single-Fire ignition for his Milwaukee machine he felt like he wasted a bunch of money on his bike. He didn't notice much, or any, performance improvement. After about 3 more beers, the other guy, Sparky, decided that money was no object, and was going to put a single-fire ignition on his bike, no matter what Willie thought.

Now, I was just sitting there like a fly on the wall, listening to these two guys' liquor inspired circumlocutions. I'm still not sure either had a clue regarding what a single or dual fire ignition is, how it works, let alone what it can or can't do performance-wise for either of their machines. Mostly, their debate was about who has, or could, spend his money more foolishly.

Since I had to come up with a topic for these tech-tips, I'd like to thank Willie and Sparky for the inspiration for this issue's installment. Without becoming overly technical, for their sake, I'll do my best here to explain what your ignition system does, and what the differences are between single-fire and dual fire systems.

Suck, Squeeze, Bang, and Blow. This isn't the title of some late night Pay-per-View show on Spectra Vision. These are the four ‘strokes' of an internal combustion engine found in most street-bikes. Please bear with me here if you're an expert motorcycle technician. This article is for Willie and Sparky, not you. They've inspired me to write a simplified explanation for people who just ride bikes and want to understand how they function just a little better. Here goes:

"Suck", or the intake stroke, happens when the piston is on its downward path with your intake valve open. This action creates suction, drawing atomized fuel, in this case gasoline mixed with air, into your engine's cylinder. This is exactly the same action when liquid is drawn into a syringe.

"Squeeze", or the compression stroke, happens as the piston begins its upward stroke with all the valves in the closed position. This compresses the air-fuel mixture causing it to become more volatile, or explosive.

"Bang", or the ignition stroke, happens at a critical moment as the air-fuel mixture is at its most compressed. A supercharged voltage is sent to your spark plugs from the ‘coil', at this exact moment (timing), which ignites the fuel mixture. The valves in your engine are still closed during this period. Thus the explosion forces the piston down. This extreme energy is harnessed to turn your engines flywheels, ultimately spinning your rear tire to put a grin on your face.

"Blow" is the final of the four strokes, you might have guessed by now; this is called the exhaust stroke. The piston is now on its second upward path, and the exhaust valves are now opening, allowing the spent, or burnt fuel, to exit your engine via your exhaust pipes. At the top of this stroke your exhaust valves close and your intake valves open to start the whole Suck, Squeeze, Bang, and Blow thing ad-infinitum. This is the basic principle of the 4-Stroke internal combustion engine.

Of course, there's more than one way to skin a cat for your high school biology project. We're talking about ignition systems here, and it's about time I got around to explaining what the difference between Single and Dual fire ignitions are all about.

Early engine technology utilizes a ‘Points' and ‘Condensers' type of ignition. Simply stated, an egg-shaped lobe spinning concurrently (one to one ratio) with your engine opened and closed a small switch, or Points, to create a single that was amplified by a simple capacitor. This sends a signal to your coils, which amplifies the signal again, and then discharged this voltage across a gap in your sparkplugs, igniting the fuel/air mixture and BANG! This primitive electrical ignition system is not really any different than a guy, in the Old West, typing Morse Code on one end of a wire, and then having the signal telegraphed, amplified and transmitted by wire to a waiting interpreter on the other end, who relays the message or spark to ignite the fuel.

Since there is only one lobe on the shaft opening the points gap in this type of ignition, the coils will discharge twice during each 4 strokes of the motor, since your motor has to spin twice to get back to the 'Bang' stroke. This is all good, since you have two cylinders on most American Big-Twin bikes. Only one coil is needed without a distributor to time the spark. This is actually a brilliantly simple solution to firing a spark in time with a two-cylinder engine. Thus you ended up with a "Wasted" spark on your exhaust stroke. This is what is commonly referred to as a ‘Dual-Fire' ignition, since your ignition is firing on the ignition stroke, and the exhaust stroke. It really didn't make much sense to put the effort into designing a points type system that would be single fire. I guess you could add extra gears and more moving parts to make two separate ignition cams spin half as fast as the motor, then you'd have a single-fire points system. I've never seen one, and if you have, please inform me! In this case, less is definitely more. Harleys have run just fine with Dual-Spark ignitions since the early days.

We've become more sophisticated with our telecommunications systems in the last 25 years. It only makes sense to become more high-falootin' with motorcycle ignition systems too. The advent of electronic ignitions made for less mechanical moving parts. A sensor rotor spinning around a sensor magnet replaced the old points and condenser system on Harleys in the late 70's. Their design still had a wasted spark, or dual-fire. I guess this is the result of the engineer emulating a points-type ignition with their new designed electronic ignition. The major benefit is the reduction of mechanical parts always/usually results in increased reliability. No more points to wear out, just a magic black box, or ‘ignition module' to burn up.

In the true spirit of American innovation, and the fact that there's no end to what we'll try to improve on our beloved machines, Single-Fire ignitions were developed to eliminate the ‘wasted' spark phenomenon during the exhaust stroke. Single-Fire ignition manufacturers claim increased mileage, smoother running motor, and easier starting with the promise of increased reliability over the status quo Harley electronic ignition. Only one thing, Harley finally caught up with the American Aftermarket innovators and now offers Single-Fire ignitions on most of their machines manufactured since 2000. Only the Evo style bikes made from '84 to '99 came factory equipped with Dual-Fire electronic ignition systems.

From my perspective, this is the end of Willie and Sparky's debate. If your Evo is running fine from the factory, ride it and keep your money in your pocket! If you're building a bike, it costs the same to put a Dual-Fire or Single-Fire ignition in your creation. So, install the advanced Single-Fire unit. Here's where the debate will continue. If you've got a Stock Harley-Davidson Evo Motor equipped bike, and your ignition module fails, should you spend the money on a single fire unit?

Grab your wallet! Along with purchasing the new performance ignition system you'll need two new coils, a way to mount those coils, and new spark plug wires. Plus you'll need the skills to remove the old system and install the new wiring required, usually resulting in the loss of six greenbacks with Ben Franklin's face on them ($600+). Not a cheap operation to upgrade. You might just consider replacing your stock failed ignition module with the Harley Performance upgraded "Screamin' Eagle" ignition which will add a little pep to your old Evo for less than $150, by changing your timing advance curves and raise the RPM's on your rev-limiter (that's a whole ‘nuther tech tip).

If you're planning on building a bike from scratch, there's no debate. GO SINGLE FIRE! Several companies offer Single-Fire performance ignitions that are compact, (no external module required), and the basic components will cost about the same as a stock Dual-Fire system. That's literally "More BANG for your Buck".

Most new "Alternative American" motorcycles, Indian included, have Single-Fire ignitions as standard factory equipment. There's no end to the improvements that the aftermarket is making compared to what that big factory churns out. At the same time there's no end in sight to the mother-factory catching up with technology. With the advent of the Harley Twin-Cam, Willie and Sparky are just going to have to find some other topics to argue over next week at the pub.

Keep the rubber side down.
- John at Steeds

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