4. PLUG TYPES - FACTS AND FICTION

Resistor/ Suppressor - its function is to control electromagnetic interference.   In a racing engine with a capacitive discharge ignition it will slightly lower the energy delivered at the plug gap.  A possible benefit may be that with so much electronics being used in today's racing, (engine management systems, date acquisition systems, etc.), the reduction of the electromagnetic interference may be very important.  However with high output ignitions the resistor may burn out.

Gap Styles:

1. Standard Gap or J-gap - places spark plug approx. 1/16" in the combustion chamber.  Can be used with the ground wire in the Full coverage, Half coverage, or Corner gap configuration.  Less gap wire coverage tends to give better spark exposure, because the arc is not behind so much of the ground wire.  The arc tends to take place at the point of least resistance which typically is the shortest distance between to points.  That is normally on the back side of the center wire because of the turn down of the ground wire going towards the shell.  Also in the cold heat range plugs the ground wire becomes the hottest point in the combustion chamber and thereby the limiting heat range factor so shortening the ground wire may add a bit of insurance by reducing this hot spot.

2. Projected Core Nose - places the spark an additional 1/8" into the combustion chamber.  Originally designed to prevent fouling by exposing the insulator/ center wire to the air fuel path and heat of the cylinder.  In street applications it performs as a hotter plug at lower RPM while running cooler at higher RPM.  This is due to the cooling effect of the fuel charge on the projected tip.  In racing it does the same thing, however it has limitations because the core nose length and the long ground wire limit the ability to build the colder heat ranges in this configuration.  If this plug could be built in a colder heat range it would be ideal for super-speedway use, but as it is its use is normally limited to short tracks, some road courses, and sometimes qualifying on the big tracks.  Because it physically moves the ignition point it can more centrally locate the ignition which reduces combustion time.  It can also place the ignition point in a more efficient location based on swirl. In some cases this plug has the same effect as increasing ignition timing.

3. Retracted Gap - designed for high out-put engines where plugs could not be built cold enough with regular style ground wires. For use in F-1 and Indy style engines with very high cylinder pressures and temps. and ground wire clearance problems.  Some of these plugs have silver center wires to further help dissipate the heat.  This type of plug should only be used when absolutely necessary as it provides the least amount of combustion incentive of all the plugs.

4. Surface Gap - originally designed for outboard 2-stroke engines which had a bad fouling problem from dirty fuel causing major deposits on the insulator which in turn caused pre-ignition/denotation.  These plugs are so cold they have no measurable heat range, they also require a very high energy CD ignition system.  We have recently added a "surface air gap" plug which does have some insulator nose length giving it a measurable heat range.  These plugs are very popular in the current F-1 engines.  With the fine center wire. (.052"), and somewhat exposed core nose this plug works very well with high energy ignitions.

5. U-Groove Ground wire - the only possible advantage to this plug would be a more exposed spark due to the fact that there is no center of ground wire to fire to.  The same thing can be accomplished with a fine ground wire/corner gap design.  Nelson Crozier had a conversation with one of the engineers from Nippendenzo who was involved with the development of this plug and he relayed that the reason for the U-shaped ground wire was to eliminate a shell distortion problem they were experiencing when welding the full size ground wires on the shells. (the larger wire takes more heat in welding)

6. Split-fire/Ring-of-fire - again the only possible advantages would be (1) better spark exposure by splitting the ground wire around the center wire, however in this case the ground wire actually has more mass so it's arguable that the center wire is actually more shrouded and (2) it has more sharp edges for a longer wear life, but this additional area can also be more hot spots in a racing application resulting in pre-ignition.  As for the more spark energy claim, as long as you compare "apples to apples" there is no difference in these plugs compared to any other standard gap plug.  Where there could be a difference is if we were to compare say, resistor to non-resistor or an auxiliary gap plug to a standard plug.  Concerning the claim of multiple sparks, the natural act of lightning is the best example of "one charge/one arc" simply put if the plug receives one charge it will deliver one arc.  There are some small re-fires, due to a small amount of energy left in the coil, but they are not significant to the plugs performance.

7. Fine Center Wire/Ground Wire - originally designed to improve starting and anti-fouling characteristics in small 2-stroke engines. The small center electrode reduces the voltage required to fire the gap.  A smaller cross sectional area ground and center wire can also give a more stable ignition resulting in less cycle variations.  Another advantage is that the smaller (.052" vs. .100") center wire allows the insulator to be smaller in diameter which increases the bore clearance volume, this results in allowing more fuel charge in and out of the plug which helps to keep it clean, resisting fouling, and gives it better exposure to the fuel for improved ignitability.  These plugs have been tested and proven to be able to fire under greater pressures than the std. dia. center wire plugs with similar ignition systems.

8. Bullet Nose or Extended Surface-air Gap - with the recent problem of ground wire failure this could be a solution in the cases where no type of normal ground wire will work.

Special center wire/ground wire materials - the copper cored and silver center wires are primarily designed to pull the heat out of the firing end of the plug, all the other precious metal designs however are primarily used for their durability factor.  It should be also noted that the precious metal center wires encourage the spark to leave because of the free electron they possess.  In the street application these plugs are great as they sometimes won't need to be changed for the life of your car, but for racing they have little application unless someone comes up with an ignition or fuel that is particularly erosive/corrosive.  There is one thing that is of some interest and that is that platinum is a catalyst for alcohol, (specifically methanol and ethanol), and with the search for alternative fuels and octane boosters, with alcohol being one of the leading candidates, and the strong push for long durability plugs, and platinum being one of the popular choices there is a real potential problem.  With all this said you definitely should never run a platinum plug in your your alcohol fueled race car.

Different size plugs (10mm, 12mm, 14mm) - as plugs get moved around more there is increase in the usage of smaller diameter plugs because of the physical limitations of the cylinder head, with the increase in usage has come an increase in the number of complaints of shell failures, i.e., breakage of the shell at the base of the threads.  These failures do not occur because of poor manufacturing but because of a lack of knowledge concerning their torque specs.

Average torque rating:    14mm - 28 lb.ft.        ---       12mm - 15 lb. ft            ---          10mm - 10- lb.ft.

The cross sectional area of a 10mm shell is almost 50% less than that of a 14mm shell.  Plug manufacturers have preached to the F-1 guys for years about using a torque wrench when installing these 10mm plugs, you just cannot rely on "feel".  A 10 mm plug shell has comparable cross sectional area to a 1/4" bolt.  So the next time you install a 10mm plug think of it as a mild steel 1/4" bolt.  The reason the shells cannot be made thicker is because to do so would decrease the insulator size and the dielectric strength would not be sufficient to prevent holes from occurring in the insulator.  So it's a trade off between shell strength and insulator dielectric strength.

Tapered Seat vs. Gasketed Seat - with some of the new cylinder heads both the tapered seat and the gasketed seat are machined allowing the user to make the choice.  Originally the tapered seat design was made only because in certain applications the physical dimensions of the gasket and the 13/16" hex were not acceptable, however now with the new design 5/8" hex gasketed plug the problem is not nearly so bad and the gasketed seat plug is regaining its popularity. Concerning seating there should be no difference.  The typical arguments for  these plugs are - with aluminum heads moving around like they do, due to thermal expansion and contraction, there could be a leakage problem with the tapered seat - with the gasketed plug the gaskets could have a problem with proper crush.  Neither of these arguments hold much water as there have been virtually no problems with seating each plug.  The only possible advantage of one over the other could be that the tapered seat plug distributes its load in both the X and Y axis whereas the gasketed seat only stressed in one direction thereby exerting more of the force directly to the threads of the plugs and cylinder head.