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 three 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.
I was 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 my monthly Bikernet tech-tip, I'd like to thank Willie and Sparky for the inspiration.
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. 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), it 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 engine's 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 (or until you run out of gas).
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 the differences between Single and Dual fire ignitions.
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 signal, that was amplified by a simple capacitor. This signal jams to your coil, which amplifies the signal again, and then (through sparkplug wires) discharged this voltage across a gap in your sparkplugs, igniting the fuel/air mixture and BANG!
Since there were two lobes on the shaft opening the points gap, one fired the front cylinder and one the rear, through one coil with two plug wires. When the coil fires on the Bang stroke for one cylinder, it also fires on the exhaust stroke (Blow) for the other. This is all good, since you have two cylinders on most American Big-Twin bikes. This is actually a brilliantly simple solution, for firing a spark in time with a two-cylinder engine. But 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 for one cylinder and the exhaust stroke for the other cylinder simultaneously. It really didn't make much sense to put the effort into designing a points type system that would be single fire and demand more points and another coil.
It was the end of the 6-Volt era, and they went to the extra expense of adding two sets of points and two coils, more parts, mo' cost, to enhance the spark from the piss-poor 6-volt system. Riders were also forced to time each cylinder separately and adjust two sets of point with feeler gauges. As soon as 12-volt batteries and generators were installed they saved on parts expenditures and more complicated timing procedures and jumped back to dual fire systems.
We've become vastly more sophisticated 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 '70s. Their design still had a wasted spark, or dual-fire and one coil. I guess this is the result of the engineer emulating a points-type ignition with their new designed electronic ignition.
In the true spirit of American innovation drive for more performance, 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. Think about it. With a single fire system the coil is focused on one plug each time it fires for more efficient spart, which means more powerful and efficient spark plug activity in the cylinder. It also eliminates that wasted exhaust fire which smooths the idle of most bikes. The bottom line is an ignition system for each cylinder, not shared.
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 continues. 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 RPMs 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 come equipped with Single-Fire ignitions. 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 will stumble on more hot topics to argue over next week at the pub.
Keep the rubber side down,
John at Steeds
http://www.musclebikes.com
