In light of some of the advice offered on this forum, thought the following may be of interest
For the average stock bike in good running condition, spark plug selection is a fairly easy ... choose whatever brand’s on sale, look up the plug part number in the catalog, and stick it in ... At the opposite extreme is the professional racer.
Based on years of experience and reams of dyno and on-track data, special plug configurations have been developed by plug makers, (generally for racing purposes
). But most bikers fall between these extremes; their rides are not by any means stock, but neither are they used purely for racing.
So how does the average person figure out the proper plug for a nonstock combination with an altered compression ratio, a bigger cam, aftermarket cylinder heads, and a power-adder like a turbo, supercharger, or even nitrous oxide?
You either refer / consult, the leading spark plug manufacturers, (Bosch, Champion, Denso, SplitFire and others
), or ... read on below to get the lowdown on modern high-performance spark plug design and application;
Basically, high-performance spark plug selection can be boiled down to four basic steps:
• Pick a shell design,
• Choose an electrode gap style
• Determine preliminary heat range choice, and;
• Evaluate the preliminary selection in/on the bike or on a dyno.
Most custom-built engines no longer have original cylinder heads, so the first thing you need to do is figure out the necessary spark plug thread diameter, thread length or “reach
” and the type of seat design required by the currently installed heads. Failure to select the right plug can result in inconsistent heat range and potential engine damage.
Some heads may accept several different shell designs. This can be important if you have physical clearance problems, or if the proper heat range is not offered in every shell. It pays to become familiar with your favorite plug maker’s catalog offerings and available heat ranges.
Also ... the porcelain end of today’s standard plugs is longer than that of equivalent plugs from 10 years ago. This may result in plug or plug wire interference on older headers. Possible solutions include special “shorty
” plugs available from ACCEL and other manufacturers.
Electrode and Gap Design
When selecting the spark plug “nose
” configuration, the simple rule to remember is: The more the spark plug is exposed to the air/fuel mixture, the easier it is to initiate combustion. Many specialized plugs have been developed for high-end race cars, but for most dual-purpose vehicles the choice typically boils down to either regular-gap (conventional) or projected-nose styles.
The regular-gap plug is the traditional configuration factory-installed on most standard bikes. For modern high-performance work, it should only be used if there isn’t enough clearance for a projected-nose plug. The latter style “projects
” the spark further into the chamber than a standard plug, and will nearly always offer improved performance if there is sufficient valve and piston clearance
, although many users prefer to stay away from them because of excessive heat buildup in the tip that can cause detonation
Projected plugs initiate the flame-front closer to the center of the combustion chamber, which has an effect similar to advancing the timing. This lets the total ignition advance be reduced, decreasing the chances of detonation while providing superior throttle response. A projected plug’s longer core nose provides a hotter plug at low speed to help prevent fouling. As engine speed increases, the incoming air/fuel mixture flows across the core nose tip, providing charge cooling that effectively reduces the heat range for increased top-end detonation resistance. Today many race cars also used projected-nose plugs, albeit in highly modified form from the “civilian
” versions, the ground electrodes are often cut back to help improve the flame kernel and reduce the voltage amount needed to fire the plug.
Controlling the operating temperature of the plug’s firing tip is the single most important factor in spark plug design. “Heat range
” is the relative temperature of the spark plug’s core nose, and it is determined by the length and diameter of the insulator tip, as well as the ability of the plug to transfer heat into the cooling system. A “cold plug"
transfers heat rapidly from its firing end into the cooling system and is used to avoid core nose heat saturation where combustion-chamber or cylinder-head temperatures are relatively high. A “hot plug"
has a slower heat transfer rate and is used to avoid fouling under relatively low chamber or head temperatures.
What’s confusing is that a “hotter
” (higher performance level
) engine requires a colder plug
because more power equals higher cylinder temperatures
Critical factors affecting heat range include:
• Air/fuel mixture:
Lean air/fuel ratios raise cylinder-head temperatures, requiring a colder plug. Rich air/fuel ratios require a hotter plug to prevent fouling. Mixtures that cause the plugs to read lean may contribute to pre-ignition or detonation. If not running an electronic engine management system, it pays to tune slightly on the rich side to avoid detonation.
• Spark advance:
Ignition timing has one of the greatest effects on plug temperatures. It becomes more critical as compression ratios increase. More timing raises combustion temperatures, calling for colder plugs.
• Compression ratio:
Increasing the mechanical compression ratio raises cylinder pressure, resulting in higher cylinder temperature. The higher the compression ratio, the colder the spark plug needs to be. According to Champion Spark Plugs, for normally aspirated, gasoline-fueled engines, a good rule of thumb is to go about one heat range colder for each full point in compression ratio increase from 9:1 through about 12.5:1, and two heat ranges colder for each point increase between 12.5:1 and 14.5:1. Beyond 14.5:1, 3-4 heat range reductions per point may be needed.
• Fuel quality:
With leaded gas, the lead is attracted to the hotter (core-nose) part of the plug, causing glazing. The spark runs down the core nose instead of jumping the gap. Going to a slightly colder plug helps prevent lead-glazing. However, with today’s cleaner-burning oxygenated unleaded gas, an equivalent engine needs to run plugs about 1-2 heat ranges hotter than originally specified, (many plug manufacturers have revised their catalogs accordingly
). The same rule hold true with two strokes!
Methanol has a higher octane level compared to gasoline, (allowing an increased compression ratio
), contains 50 percent oxygen by mass, (requiring a much richer air/fuel ratio
), and has a reduced latent heat of evaporation, (which cools the incoming air/fuel charge and allows a denser mixture
). The net effect is to require a plug that’s at least one step colder than normal for an equivalent gasoline-fueled application.
• Nitrous oxide:
N2O raises cylinder temperatures and may require a plug 1-2 heat ranges colder. Lower output street systems may get by with standard heat ranges if nitrous use is held under 10 seconds.
(no-one has done this to their bikes hmmm
?) With increased pressure and temperature in the chamber, two or more heat ranges colder may be needed. Extreme high-boost race-only applications may need a surface-gap plug.
• Sustained acceleration:
Prolonged acceleration and/or high-speed riding raises temperatures and calls for colder plugs.
Leaning the mixture and advancing the timing can partially compensate for lost power and efficiency caused by increasing elevation, (height above sea level
). Spark plug heat ranges should stay the same as at sea level unless racing above 3,000 feet, where one step hotter usually suffices.
To part 2 ...