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Racing Heat Range and Tuning Guide

12 page guide by Champion Sparking Plug Co. Ltd.'s Competition Department, 1973


With modern sophisticated engines, new ceramics and superior alloys for spark plugs, the job of selecting a plug for a specific engine has become more critical.

Dependent upon engine design, fuel and the course, the designer can select from a series of four different spark plug gap styles, his decision having a profound influence on the life of the plug and the performance of his engine.

This booklet is designed to help the designer, driver or mechanic in selecting the most suitable plug type for his engine and how other engine characteristics can affect the plug once installed.


The suffix letter to the right of the heat range number signifies gap slvle Plugs having no suffix leiter are regular gap.

Heat Range and gap style

In racing, you not only have a selection of various heat ranges in a given reach and diameter, but you have a choice of three to four gap styles in most ranges.

In many high-performance engines, insufficient piston or valve clearance exists requiring the use of 'R' gap plugs. In these engines, another condition requiring 'R' gap plugs is the extreme temperature and pressure differentials.

Many engines not subject to the above conditions may use 'Y' or 'J' gap styles. The projected insulator ('Y' type) offers many advantages for better performance. It projects the spark deeper into the combustion chamber, has wider heat range, and has more fouling protection.

Before assigning yourself to using the retracted 'R' gap because the projected types will not fit, attempt to use a regular or 'J' gap. You will have better fouling protection and superior flame front propagation.

Make sure the heat range of the 'J' or 'Y' gap is cold enough for the application.


   spark plug with 'J' gap

A conventional automotive type gap slightly modified . . . side electrode extends midway to the centre electrode . . . gap design requires less firing voltage at high RPM . . . also protects from ingested particles wedging between gap and shorting out plug. Plug design offers good fouling protection and excellent performance in many racing applications.

No suffix letter

Spark plugs without a suffix letter are usually regular gap style. The side electrode may extend fully across the bottom of the centre electrode or be cut back slightly from the midway point. The heat range rating of the plug usually determines this cut-back.


   spark plug with 'Y' gap

Commonly used in overhead-valve racing engines where sufficient physical clearance exists between piston or valves. Plug heat range characteristics are slightly different from other plugs as incoming fuel charge cools insulator tip at high speed ... long tip runs hotter at lower speeds ... an effective plug in wedge type combustion chambers ... this design offers excellent fouling protection from rich mixture conditions ... when converting to this design from retracted or regular gap plugs, ignition timing may have to be retarded slightly ... plug design is not recommended for maximum output engines, highly supercharged, or high percentage nitro-burners.


   spark plug with 'R' gap

A retracted gap plug ... electrodes are shrouded ... used in racing engines where physical clearance is limited between pistons or valves ... or cylinder design employs cartridge fire ... plug is not practical in normal road use ... it is primarily intended for competition use in highly modified ... supercharged ... or high percentage nitro-burning engines ... fouling protection is less than 'J' ... 'Y' ... or 'P' gap style plugs.

G-gap or P-gap

   'G' or 'P' gap

A fine-wire plug intended for racing applications where engine conditions require increased latitude of heat range ... plug has good resistance to fouling. The plugs are excellent performers in 2-stroke applications as well as 4-stroke ... they are not generally recommended for supercharged or high percentage nitro-burning engines ... these plugs will accept rich mixtures and are effective where cylinder-to-cylinder cooling or mixture differences exist.

Basic heat range numbers

All Champion spark plug numbers fit into four categories defining application and heat range.

Automotive &


 ... Racing


 Special &

'Hot' or 'cold' spark plugs

The words 'hot' or 'cold' when used in reference to spark plugs are often a source of confusion since normally a hot plug is used in a cold engine (low horsepower) and a cold plug in a hot engine (high horsepower). The terms actually refer to the heat rating or thermal characteristics of the plug - more specifically: to the plug's ability to transfer heat from its firing end into the engine cylinder head.


A cold running plug, by definition, transfers heat rapidly from its firing end and is used to avoid overheating where combustion chamber or cylinder head temperatures are rel atively high. A hot running plug has a much slower rate of heat transfer and is used to avoid fouling where combustion chamber or cylinder head temperatures are relatively low.

Length of core nose and electrode alloy material are the primary factors in establishing the heat rating of a particular spark plug design. Hot plugs (see illustration) have relatively long insulator noses with long heat transfer paths. Cold plugs have much shorter insulator nose lengths and thus transfer heat more rapidly.

Atmospheric pressure and altitude

Higher atmospheric pressure and/or lower altitudes tend to lean the fuel/air ratio demanding a fuel/air ratio adjustment or colder plug. Lower atmospheric pressures and/ or higher altitudes cause the fuel /air ratio to become richer, again demanding adjustment of jet size or a hotter plug.


Rich mixture-warmer plug? lean mixture-cooler plug?
Having found a suitable plug (heat range ... gap style), can spark plug heat range be precisely tailored to engine mixture settings by going another step up or down the heat range scale?

In an unblown engine not using oxygen-bearing fuel additives, there is a spectrum of efficient carburation. This spectrum, or speed ranges from best power to full rich.

Generally, a leaner mixture yields maximum horsepower, but the selling is dangerous if intake manifold design can not supply every cylinder with the proper F/A ratio, a leaner mixture is also a risk, as there is less protection from detonation, and pre-ignition. Cylinder temperatures are also increased. Consequently, most racing engines are enriched slightly to avoid cylinder starvation and dangerous temperatures.

In most instances, the mixture spread between the leaner setting and full rich can raise or lower combustion chamber temperiltures as much as 150°C. As the spark plug firing end resides within this thermal environment, it is also affected. (Its job is to transfer heat out of its nose into the cooling medium of the engine.) Consequently, in some instances, spark plug heat range must be changed one step up or down.

There is another tuning adjustment that has a profound influence on combustion chamber temperature. This is the spark advance selling.

The cautious combination of mixture and spark advance sellings can act as a 'thermostat' on combustion chamber temperature, and, in some cases, heat range substitution may be in order. Again, prudent and careful choice is recommended.

Warm-up plugs

It is good practice to warm up a racing engine with holler plugs. The warm plugs will assist the ignition and carburation to bring the engine up to proper operating temperature.

It is a fallacy to believe hot plugs assist initial starting, while cold plugs cause hard starting. When plugs are installed 'out of the box' and initially fired, both are the same temperature ... whether cold plugs or hot plugs. Warm-up plugs merely bring the engine up to temperature more smoothly and rapidly. They also save the expense or inconvenience of oil or carbon fouling race plugs.

If room exists within the combustion chamber, a projected nose plug should be used for warm-up, as it has a wider heat range and 'lights' the cylinder more easily as the spark is deeper in the combustion chamber. If projected nose plugs cannot be used, regular gap plugs are second best. If the engine, because of physical considerations, can only fit a retracted gap plug, use the warmest retracted gap heat range available in the line.

Many engines have been destroyed when operators have forgotten to remove warm-up plugs during practice, qualifying, or in racing usage.

Gear ratios

Higher gear ratios tend to load an engine. This subjects spark plugs to higher temperatures, calls for a 'colder' plug.

Ignition reserve

 mm  mm

The most important (and least understood) factor concerning good ignition performance in racing is IGNITION RESERVE. There's really no reason for any confusion about this 'reserve' ... it's simple.

Simply stated, ignition reserve is the difference in kilovolts (one KV equals 1000 volts) between how much voltage the ignition system can develop (voltage available) and how much voltage it takes for the coil or magneto to discharge to ground (across the plug gap).

Actually, when you tune an engine, you're really building ignition reserve. First by replacing components (points, condenser, etc.) and making adjustments that help increase voltage available. And second, by selecting components (plugs, distributor cap, rotor, etc.) to help decrease voltage required.

It's a bit like your bank account ... when you make a deposit you increase your financial reserve. The minute you start writing cheques, you begin reducing the reserve. Write a cheque for more than your balance and your banker will let you know about it. So will your racing car if you 'short change' the ignition reserve.

  Decrease voltage Available Increase Voltage Required
Tune-up factors
  • worn breaker points
  • defective coil
  • cracked distributor cap
  • defective ballast resistor
  • defective condenser
  • poor electrical connections
  • breaks in high tension leads
  • wide plug gaps
  • reversed coil polarity
  • retarded timing
  • corrosion in distributor towers
  • wide rotor gap
Operating factors
  • high engine speed *)
  • cold starting
  • extra electrical accessories
  • sudden acceleration
  • cold starting
  • heavy load

*)   Transistorized ignition systems, however, are rela1ively insensitive to high engine rpm.

Nitro methanol and alcohol

Spark plug requirements for straight methanol fuel are generally one heat range colder than those used for petrol. When oxygen-bearing additives are used, spark plugs one to two heat ranges still colder are required. Richer jets must be installed and the spark advance adjusted to match the burning rate of the fuel.


Supercharging is not compatible with high compression ratio ... generally C.R. is lowered from the original specifications. Total spark advance cannot be as great because of high combustion pressures ... extreme detonation and pre-ignition can result, fracturing the plug firing ends. When nitro methanol fuels are used in super-charged engines, extremely cold plugs are required.

Spark advance

Pressure and heat resulting from an excessively overadvanced spark can cause detonation or pre-ignition, which can result in engine damage or plug failure. Total spark advance should be known. In a 4-cycle engine, total spark advance is the distributor degrees doubled, plus the flywheel or initial timing.

'Hot' ignition systems - 'cold' plugs

'Hot' coils, special magneto, and transistorized ignition systems do not usually require changes in spark plug heat range. It is not the temperature of the spark but the temperature of the combustion chamber that influences heat range. The heat of the spark is insignificant. Spark erosion cannot be materially reduced by colder heat range substitution. If the engine timing is changed, heat range substitution may be in order.

Capacitor discharge ignition systems have the ability to fire fouled plugs. Extremely 'cold' race plugs do give better pre-ignition protection when fired by capacitor systems. However, plugs thermally matched to combustion chamber temperatures are usually superior to colder plugs mated to 'exotic' ignition systems.

Setting extra-wide plug gaps because of exotic ignition systems presents some risk ... if ignition becomes marginal during a race, a closer gap might have prevented a breakdown. Some magneto systems cannot tolerate wide gaps, as starting ability suffers. Transistorized or capacitor discharge systems have very fast 'rise times' and highly stress ignition system components. Consequently, perfect ignition system maintenance is a must.

Comparison chart

Important: Comparative types listed below are seldom duplicated exactly in heat range.
Always consult Champion Competitions Department for racing applications. See footnote on back page.

spark plug comparison chart; table Champion, AC, Autolite, Bosch, Lodge, NGK, KLG
spark plug comparison chart; size of reach Champion, NGK, Bosch

Plugs should all 'read' alike

Every set of plugs removed from a racing engine should look alike ... in colour and condition. Any difference in colour or condition among a set of plugs is an indication the combustion chamber temperature or fuel/ air ratios are not the same in every cylinder, or that related engine components need attention.

If differences exist in firing end condition, they can generally be traced to the following:

Close inspection of the respective systems may reveal the following:

steam pockets, leaking head gaskets, cracked header tank, water pump cavitation, insufficient radiator area, restricted water passages.
unbalanced multi-carbs, throttle valves out of sync, air leaks in the induction system, disturbances in air flow, improper ram tube length, exhaust obstructions, dirty fuel, unequal float levels, dirty nozzles (fuel injection), or air cleaner restrictions, mismatched intake manifold.
weak battery, points out of sync, distributor cap arcing, defective rotor, 'cross-fire', defective spark plug leads, grounded resistors.
Poor oil control
new rings not seated, broken rings, worn valve guides or seals, excessive oil pressure, improper crankcase ventilation. (Detonation can also cause poor oil control.)
Valve timing
improper valve lash, worn cups, defective springs.
Unequal cylinder pressures
piston deck heights unequal, rod-crank-piston tolerances not the same, leaking cylinders, defective or unseated rings, unequal C.C.'s, improper valve lash.

Note that some cylinder temperatures or F/A ratios may be so extreme among cylinders that it would require two spark plug heat ranges to successfully 'satisfy' the engines in various cylinders. If such differences exist, it is impossible to expect top engine performance ... conditions must be corrected to insure equal compression, across-the-board temperatures, equal F/A ratio, ignition equal in all cylinders.

This is most essential in race tuning, as wide difference in plug 'readings' cannot be tolerated. A racing engine that reflects this condition will never perform at peak efficiency and will be in frequent spark plug 'trouble'.

All enquiries concerning recommendations for racing engines should contain the following information: Make of engine, cubic capacity. compression ratio, camshaft type, piston type, plug type currently used. An indication of the type of event normally entered.

Please address enquiries to:
Competitions Department
Champion Sparking Plug Co. Ltd.
P.O. Box 7
Great South West Road
Feltham, Middlesex, TW14 OPN

73 / 11 / L40

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