# Spark Plug Heat Range



## PontiacJim (Dec 29, 2012)

This questions seems to come up, so I pulled together a few references from my library of engine builds. Selecting a spark plug is not rocket science. The factory already knows what plug to use and your *Factory Service Manual* should have this. The problem can be when you go to the auto parts store and the guy behind the counter isn't and "old" guy who knows these old cars or if the computer screen listing pulls up all kinds of different brands, designs, and prices and you and the parts guy are now more confused than ever as to what the heck to pick. But, this article is not on brand or design, but heat range and those things to consider IF you want to experiment from the factory recommended plug. You probably won't ever need to go 1 or 2 heat ranges different from the factory plug.

The term "hot" or "cold" is a reference to a spark plugs heat range. Normally, a hot plug is used in a low compression, cold engine, whereas a cold plug is used in a high horsepower, hot engine. The terms "hot" and "cold" refer to the thermal characteristics of the spark plug to transfer heat from its firing end, or the electrode, into the engine's cylinder head. All a hot or cold plug means is how hot the core temperature of the plug (or running temperature of THE PLUG) will be. It has nothing to do with combustion temperature, but the combustion temperature does have an effect on the spark plug. If you run too hot of a plug, it will allow it to retain too much of the combustion heat and it could begin to melt off the electrode. If you run too cold of a plug, it will not retain enough combustion heat and it could begin to foul the tip. Running a heat range or 2 cooler may aid in a cooler insulator (not cooler combustion chamber) where heat is your enemy and a cooler insulator could absorb just enough heat to make a difference as long as the plug still burns the air/fuel mixture and does not lead to fouling out of the plug.

The insulator tip of a spark plug is usually the hottest part of the spark plug. Its temperature is related to both preignition, and fouling. The length of the spark plug's nose and the electrode's alloy material are the primary factors that establish the heat range of a particular spark plug. Hot plugs will have a longer nose length which provides a longer path for heat transfer from the electrode to the head. Cold plugs will have a shorter nose which provide a shorter heat path for heat transfer from the electrode to the head.

A "hot" heat range plug with its longer path of heat transfer is used where combustion chamber temperatures are low. The plug transfers heat at a slower rate to avoid spark plug fouling and a more complete burning of the fuel. Spark plug fouling of the insulator tip is most likely to occur if the tip temperature drops to around 600 degrees.

A "cold" range plug with its shorter path of heat transfer is used where combustion chamber temperatures are high. The plug transfers heat rapidly to avoid overheating (melting) of the electrode or cause the electrode to become hot enough to act as a glow plug and cause preignition. Preignition is likely to occur at combustion tempertures of 1750 degrees and above.

As an example, the ideal combustion chamber temperature for a racing engine is 1350 degrees.

Heat range selection consists of selecting a plug which will keep the engine balanced thermally between fouling (too cold) and preignition (too hot) at all engine RPM's and under all driving conditions. There are five important factors that influence the heat range selection of a spark plug:

1.) *Compression*. The higher the compression ratio (cylinder pressure), the higher the combustion temperature will be.
2.) *Spark Advance*. Spark advance timing has one of the greatest effects on spark plug temerature. The greater the advance at BTDC (Before Top Dead Center), the higher the combustion chamber temperature and cylinder pressure. Too great a spark advance can lead to detonation.
3.) *Air/Fuel Mixture*. The A/F ratio can produce a "rich" or a "lean" mixture. A rich mixture has a larger fuel volume in relationship to its air volume. A lean mixture has a larger air volume in relationship to its fuel volume. An overly rich A/F mixture can rob power and foul plugs, but a slightly rich A/F mixture can offer a margin of safety as it absorbs heat from the air and cylinder surfaces that can offer detonation protection at full throttle and satisfy the leanest cylinder due in part that not all cylinders receive the same A/F mixture ratio. A lean A/F mixture can be dangerous because they burn more slowly and require a longer time to conduct heat away from the combustion chamber, plugs, and the piston crown. A lean A/F mixture invites preignition and detonation. (Note: I had to question a leaner A/F ratio as burning slower, as I thought it would burn faster. However, what I find is that it takes longer for the fuel to burn because there is less of it and because of the slower burn the engine temperatures rise because the ignition of the fuel is slower, and the slower burning fuel transfers more heat to the surrounding parts of the engine - the opposite of what rich A/F ratio does with regards to absorbing heat) 
4.) *Octane*. Octane rating is a measure of "knock resistance." It makes a difference in the burning rate of the fuel. The higher the octane number, the higher its resistance to engine detonation. The lower the octane number, the quicker the detonation and preignition will occur. The first line of defense against detonation is high-octane fuel.
5.) *Operating RPM and Temperatures*. The general operating RPM of the engine can be a contributing factor in the heat range needed. High speed versus low speed driving may require a different heat range selection. Stop and go, local driving, highway driving, or all out full throttle blasts can each have an effect on what heat range plug you may need. Engine running temperatures as well as the outside temperatures of the geographical area you live in may also require different heat ranges.

Detonation will hammer a hole in the top of your piston. Preignition will melt a hole in th top of a piston. Detonation is that "pinging" sound you typically hear, but can be easily masked by a loud exhaust system or open headers. Detonation is the uncontrolled burning of the fuel after the spark plug has fired. It is the violent collision of flame fronts within the combustion chamber. The combustion chamber should provide for a smooth flame front that burns evenly.

Preignition is when part of the combustion chamber reaches the point of incandescence (hot enough to glow) causing the A/F mixture to ignite without the aide of a spark plug. Sharp edges of a combustion chamber that hang over into the cylinder after it has been milled can be such a cause. This is why it is always a good idea to use a fine grit sandpaper to smooth out any sharp edges found on the combustion chamber or even the valve reliefs found on piston tops.

I have personally never played around with spark plug heat ranges. I have always used whatever the factory plug was and then adjusted my timing to get out any detonation if I had any. I have however tried different brands, designs, and electrode materials as new ones seem to pop up all claiming to be "the best." It seems I always fall back to whatever the factory recommendation is. :thumbsup:


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## GTO44 (Apr 11, 2016)

Great write up Jim! 

Working a lot on nitrous cars, its always a balance of getting a cold enough spark plug for the tune but then not letting it foul from idling or driving back from the pits. 

One indicator of heat-range is the threads. If there is color change past the first couple threads, it most likely needs a colder plug.

Also, the slow burning lean mixture is why street cars should use a vacuum advance distributor. The additional advance at part throttle aids in the combustion of the slow burning lean mixture. Same at idle... but manifold vacuum vs ported vacuum is a whole different topic ?


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## PontiacJim (Dec 29, 2012)

GTO44 said:


> Great write up Jim!
> 
> Working a lot on nitrous cars, its always a balance of getting a cold enough spark plug for the tune but then not letting it foul from idling or driving back from the pits.
> 
> ...



Thanks. Yes, I read about reading plugs and it mentioned the color change past the first couple treads. Plug are not so easy to read with the newer fuels as they were with the old leaded gas. So plug reading can be a tool, but not always.

The use of nitrous. blower, or turbo takes in a few different considerations when selecting plugs. More cylinder pressure means more heat once again. Here is a little coverage on nitrous seeing you mentioned it and I plan on using a small dose on my engine build as well.

From what I read, with the power adders, you do not want to use the extended nose plugs. I enclosed a photo that shows the difference in the extended tip plug and the non-extended tip plug. "An extended tip electrode plug should not be used in a nitrous or forced induction engine because that thin, protruding electrode sticking out further into the combustion area can become a glow plug real easy under WOT which will cause pre-ignition and detonation. The plug on the left is a typical example of an extended tip electrode plug. On nitrous or forced induction engines use the non-extended tip electrode plugs such as the plug on the right." 

It is also recommended to back the timing off 2 degrees for a 75 - 100 HP shot (bigger amounts need more) and switch to 1 step colder spark plugs with NON-extended tip electrodes. 

BUT, a hot plug may also be a wise choice that could save your engine. "Sometimes using a heat range that is a bit too much on the hot side can be to your advantage when using serious amounts of nitrous or when running high blower boost levels. A trick with very experienced guys is to run a plug that is on the hot side, so you can literally use the plug as a "safety fuse". In other words, if your plugs run just under the melting point, then when you have the misfortune of leaning the engine out too much, (which is THE worst thing you can do to a nitrous or supercharged engine), it will instantly increase the cylinder heat which in turn instantly increases the plug's heat and will actually melt off the electrode causing that cylinder to not fire anymore. No more fire = no more heat. No more heat means you may save your engine from a lean-out melt down. You'll notice the sudden loss of power and misfiring and you'll quickly get out of the throttle. Obviously it takes time and testing to figure out exactly what heat range and type of plug to run for any given application."

Now if you want to try to gain a little free HP from what is called a "regular plug" (not one of the many other configurations like split fire or multiple ground straps, etc.), there is a trick known as "side gapping" the plugs. Take a small file or a fine grinder to the ground strap and cut it back so it only covers about half of the center electrode. This exposes the spark to the open cylinder and combustion area and prevents shrouding of the electrode that the full length ground strap tends to do on regular plugs. Enclosed a picture of this. The plug on the left is the standard plug we pull out of the box. The one on the right has been side gapped.


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## GTO44 (Apr 11, 2016)

Yes sir I always use non projected plugs for nitrous and boost applications. And a tighter gap.

For nitrous I always start on the safe side... 2 degree’s of timing for every 50hp added. That’s 6 degree’s for a standard 150shot. That’s usually overkill but i always start on the safe side for customer cars and work back. Read the plug after the run and start bringing the timing back in 1 degree at a time. The ground strap will tell you where the tune is in terms of timing. In my experience the ground strap shows a more defined color change with nitrous. You want the color change half way to 3/4way toward the base of the strap. Too close to the top, add more timing. Too close to the base, pull timing. For N/A the change can be almost at the base. (Pics attached to show the transition on a nitrous plug from a 408stroker i built for a customer. This was the “safe” tune on a 150shot before we started ringing its neck)

In all actuality, anything under 150 shot on a 400hp+ motor could run with no timing be pulled and on the same heat range plug. But the guidelines are there to start with and work your way back. 

For a street car running some nitrous, its best to use the N/A plug heat range on the street and change to a colder plug when you spray. With that being said you can usually get away with one step colder on the street without fouling the plug. If it’s under 125-150 shot on a decent V8 no need for colder plug. 

Also i always use a retard box to pull timing for nitrous. That way the tune is right and the motor is snappy up until the point the nitrous engages. I usually install some type of controller or progressive controller to bring the nitrous in. It may not hit until the 60foot and thats where the timing will be pulled, not a moment sooner.

If not running a controller it’s best to run at least a window switch. Come in no sooner than 3,000rpm. (Maybe 2,500 if its a torquey low rpm motor). Too much load on the motor at too low of an rpm can do some serious damage to an engine. Then cut it off about 500 rpm short of redline. Dont want to be spraying into the rev limiter if you have one. For EFI cars the computer kills fuel and not spark for rev limit... so if you accidentally spray into the rev limiter, you have nitrous and no fuel... and boom. You obviously shouldn't have to worry about that being carb. 

Also for anyone deciding how much nitrous to use safely on their street car... here’s a general rule of thumb. A typical run of the mill stock V8 can take up to 35% of it’s own HP rating all day everyday and never hurt it (assuming the tune is good). 400hp * 35% ~ 140shot, 350hp * 35% ~ 120shot, 300hp * 35% ~ 100shot. That’s a guideline i learned from an old nitrous guru at Nitrous Express. Obviously the numbers go up based off how well your engine is built and/or if setup for nitrous (forged internals and large ring gap for the latter)

Oh and never run off a push button. Always run a wide-open-throttle micro switch on the carb.


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## PontiacJim (Dec 29, 2012)

Good info. I have always been interested and followed the various nitrous offerings and applications since the mid 1970's when the big name in systems was Marvin Miller. Nitrous Oxide Systems (NOS) and 10,000 RPM Speed Equipment were a couple of others. :thumbsup:


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