 Spark Plug Tech for Internal Combustion Engines - Part 4
This is part 4 in our series on spark plug technology. In this section we will describe radio frequency interference, ignition amplifiers, and spark plug gaps.
RFI or "Radio Frequency Interference":
Many people believe that spark plugs fire instantly. This is partly true because they fire in milliseconds, although if one looks at an oscilloscope pattern you will see much more than a single instantaneous firing event. Many things also occur that you cannot see even with the oscilloscope. Part of what you cannot see, but can in many cases hear, is the noise that is picked up in the speakers of your car stereo. This is called RFI, or Radio Frequency Interference.
Spark Plug Firing Voltages:
 When the breaker points or solid state ignition unit (switching device) interrupts current flow in the primary ignition circuit and induces current flow into the secondary windings of the coil, there is an instantaneous voltage spike. (as seen in the illustration at right in position A to B). This represents the voltage required to overcome the spark plug and distributor rotor gaps. Once the spark gaps have been bridged, the secondary voltage required to sustain the spark across the gap is much less and drops (as seen in position B to C above). The spark continues to arc across the gap at more or less constant voltage until the arc is extinguished (at position D above).
This is due to coil energy drop in that it can not sustain the spark any longer. During this arc duration (Spark Duration), the plug actually fires several times. This is caused by high frequency oscillations in the primary and secondary windings of the coil, which continues to induce voltage spikes. They continue and slowly diminish (positions D to E above) even after they are no longer strong enough to sustain spark. All of this takes place in roughly one thousandth of a second.
With our race designed ignition units, they concentrate their efforts on sustaining spark duration as well as limiting the voltage drop after the gaps have been bridged. Most aftermarket ignitions concentrate on giving us 20° of spark duration (crank degrees) as well as much higher spark energy output. A high performance coil helps this out, but the capacitive discharge and digital ignition units assist in storing and delivering this power through the coil more efficiently, faster, and give the ability to achieve higher RPMs more safely and efficiently in fuel mixture burning. The coil is only the pawn of the ignition trigger or control unit. The coil is the real workhorse and takes most of the abuse ... make sure you use a good coil. (Read below about aftermarket ignition amplifiers and controls)
The Cause of RFI:
 If we were to slow down the oscilloscope to perhaps 0.00000025 seconds and greatly expand the pattern (as pictured at right), we would see that what appears to be consistent from position C to D in the the first illustration above is actually a series of extremely high bursts of energy. These energy bursts are discharged at the same frequency band as radio and TV frequencies. It is these bursts that make your car radio snap - crackle - and pop ... as well as in just about anything electronic, including telephones, aircraft control towers and heart pace makers by causing static and interference.
Sources of RFI:
Automotive ignition systems are not the only things that spew RFI into the atmosphere. Lawn mowers, snowmobiles, ATV's, tractors, power lines, traffic control devices, and cell phones all do it. One publication refers it to "electronic air pollution". As many of us know, we live in a sea of constant electromagnetic waves.
Any time you have a flow of electric current you will have a magnetic field. Coils, relays, switches, solenoids, generators, servomotors all affect communication equipment, electronic circuits and computers. The higher the voltages, the more critical this becomes. Anytime you have the spark jump a gap or a contact, you have a miniature radio transmitter.
RFI Standards:
Back in the 1930's, engineers recognized that RFI could be a nuisance. As the years of testing and technology advancements went by, it turned into an even greater problem. Especially with the advent of high-tech communications systems, computers and electronic engine control devices. The Society of Automotive Engineers (SAE) decided to set up standards for measuring as well as the control of RFI. These are called "EMI Standards" or the more technical name for radio static of Electromagnetic Interference (EMI).
The current standard for EMI was adopted in 1961 and is known as J551. It limits RFI at frequencies between 20-1,000 MHz. All spark plug manufacturers must adhere to it. The most common method used to suppress RFI is to install a resistor in series with the spark plug's center electrode.
Other ways that control RFI include:
The metal fenders, grille and hood of your car. These provide a shielding affect which absorbs much of the RFI emitted from your ignition components. Plastic and composite body panels are basically transparent to RFI and provide little to no shielding.
The use of capacitors, silicone grease at connections, proper grounding of all circuits and routing wires to reduce electromagnetic interference are all helpful in reducing RFI.
The use of carbon impregnated secondary wiring (plug wires) and resistor spark plugs have the most impact when reducing and controlling RFI. In our race cars with our high output ignitions, it is best to use a specifically designed plug wire for our applications. These are usually the what is called "Spiral Wound" style plug wires. The construction of these wires starts with a Ferro-Magnetic impregnated inner core, helical wrapped copper alloy conductor, a high dielectric insulator then a heavy fiberglass braid. Wrapping this is a 8mm to 10mm silicone jacket. Also, secure connections of the plug wire's terminal ends are mandatory along with secure fitting boots.
Adding an Aftermarket Ignition Unit (Amplifier):
I felt I had better address this now. There are numerous ignition units available that state that they give you "X" horsepower gain, or do this or that for your engine. In most cases the ignition unit will benefit your engine, but is it needed? To decide if adding an ignition unit to your vehicle is going to be worthwhile you need to ask yourself a few simple questions:
- Am I trying to get maximum spark energy, engine efficiency, lowered emissions, and power output from my engine?
- Am I modifying the engine (camshaft, head porting, exhaust system, larger carbs/injectors, compression, or any other assortment of aftermarket performance parts) so that cylinder pressures will be increased?
- Am I adding a serious power enhancer (Nitrous, Supercharger, Turbocharger, etc)?
- Am I planning on racing the vehicle?
- Am I increasing the RPM range of the engine?
If you answered YES to even one of the questions above you could consider the addition of one of the available ignition units. If you answered YES to two or more of the above questions, you'd better install a quality ignition amplifier (Mallory Hyfire, Accel, Jacobs, Crane Hi-6, etc). Of course there is going to be B.S, hype from many ignition manufacturers, and the facts surrounding what an ignition unit can really do.
Ignition Amplifier Facts:
- A small power increase and strides in efficiency will be seen in most cases. Note that if you are racing, not having a good ignition amplifier can cause serious power loss and engine damage if the air/fuel mixture is not ignited properly
- Without the correct or adequate coil the ignition unit cannot do its job properly. The amplifier is telling the coil what to do.
- Analog ignition units are slow and not as good as digital units
- Opening up your plug gaps just because you have added an amplifier CAN SLOW YOU DOWN and cost your horsepower. (read below)
- Not all ignition units are the same (Inductive amplifier and Capacitive Discharge amplifier, not to mention brands)
- If it takes 10k volts to fire your spark plug, that is what you will get. Just because you have a bazillion volt system does not mean the box will give that to your plugs and combustion process. Don't believe that just because the ignition has the highest "millijoule" rating that it is the best.
- Using the wrong coil can have serious implications (we have seen coils overheat, boil over, and catch the vehicle on fire because the coil could not handle what the box was telling it to do).
- Not all ignition triggers are equal (breaker point, magnetic, magnetic breakerless, hall effect, photo optic, etc). Each can affect the efficiency of your ignition unit.
- One of the main benefits of an ignition amplifier is the spark duration of 20° crank degrees per spark. (this is the B to C distance on the chart above under RFI)
Ignition Amplifier Hype:
You should be cautious if when shopping for an ignition amplifier you see comments in their advertising, or hear from one of their sales or support reps statements or claims such as these:
- "Guaranteed horsepower"
- "Our amplifiers will help ALL vehicles"
- "It will work with any coil" (this usually means that their unit does "squat"). Remember, the coil is the workhorse and the box is managing the coil.
- "You can now increase the spark gap for better performance". This usually means again that their box does "squat". If you "have to" open the gaps up to get spark energy, the box is not supplying what they claim. Increasing plug gaps should be decided on an individual engine basis, not by the box. Note that opening gaps is not a bad idea, but when used as a teaser, don't trust it.
- The lowest plug wire resistance is not always the best either. A coil has to build energy (resistance) and then release it. This resistance is a combined part of the coil, plug wires, plugs, etc. The more efficient units can operate with slightly higher plug wire resistances to eliminate outside interferences. Weaker units require extremely low resistance plug wires and ignition coils, virtually just blowing the spark through the coil and wires to get to the plugs.Using to low a resistance increases the possibility of secondary voltage leaks.
- "All these controls will help you achieve more performance". Many of the available "gadget boxes" are just that. If you have a real need for timing controls, high speed retards, etc, then get a box that uses these features. Just because the box has these controls does not mean you will get more performance. Computer programmable ignitions (where you hook your laptop or PC to the ignition) are for dyno rooms, and those of us that have the time to tweak and test small incremental adjustments. Once you have a setting that works for your specific engine, it probably never needs to be tweaked again. You can play with those settings for months and never get a single HP gain from them.
- Sure, we have our preference of what brand(s) of ignition units we like and those we don't. There are reasons for these decisions (reliability, factual claims, performance output, consistency). We have tested numerous ignition brands/styles and know what works for an individual's application. The term "Ignition System" fits. If you want your ignition to work correctly it needs to be a compatible system for your specific application.
 (example of an excellent ignition unit ... and one I have installed on one of my personal vehicles)
If you cannot decide how to choose your ignition system, use the Live Support link at the top of this page or E-Mail Us and a representative will assist you.
Proper Plug Gapping:
 Proper gapping of the spark plug is necessary to get maximum spark energy, lowest RFI release as well as what is best for the longevity of the secondary ignition components (coil, cap, rotor, wires, plugs).- When checking plug gaps, the correct way is to use only wire gauges, though many of us are using the slider style gapping tools. These flat or feeler gauge style gauges do not accurately measure true width of spark plug gap. The best tool is a a set of spark plug gapping pliers (as shown at right).
- When increasing the gap size for our high performance applications utilizing advanced ignition systems such as Mallory, Accel, Jacobs, Crane and Holley ... it is important never to go more than plus or minus .008". This is to maintain parallel surfaces between ground and the center electrodes.
- Something many do not know, is that with Higher Compression ratios and Superchargers as well as Nitrous, in many cases smaller spark plug gaps must be used as well as the use of a much hotter ignition system (see above). These higher cylinder pressures require more energy to jump the spark plug gap.
- The rule of thumb on plug gaps is to open them up in .002" increments at a time. When the car (race vehicle) begins to lose power or slow down then go back .001-.002" and this will be (in most cases) the optimum gap.
Remember that the Ignition Unit, plug brand as well as heat range, cap and/or rotor styles and in many cases fuel type or brand can change the optimum spark plug gap settings.
Lastly, NEVER use the porcelain insulator as a fulcrum point when setting these gaps, this can cause damage to the spark plug.
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