Ignition & Electrical / Tech Articles

Ask Away! with Jeff Smith: Determining Correct Spark Plug Heat Range

spark plug heat range chart

This is a comparison of a cold heat range plug (left) versus a much hotter heat range plug (right). Notice how the distance between the ceramic and the spark plug shell on the hotter plug requires heat to travel a farther distance, which maintains more heat in the spark plug. A hotter heat range plug is preferable for engines that only see light duty applications as in mild street driving. (Image/Autolite)

How do I tell the correct heat range spark plug for an engine? – Mark S.

Jeff Smith: Simple questions sometimes require a bit longer answers. This is one of those cases.

Let’s start with a typical street engine. For a stock engine, the answer is simply to use the recommended plug for the application.

Clearly, that’s not your question.

We’ll assume that the engine has been modified. Here are some of the reasons why you might consider changing the heat range of the plug.

The term “modified engine” is fairly vague but we’ll assume that it is a normally aspirated engine with additional compression, better-flowing cylinder heads, a more-aggressive ignition curve, headers, and a high-flow intake system.

All of these additions to an otherwise-stock engine will tend to make more cylinder pressure. As cylinder pressure increases, combustion creates more heat in the oxidation process. More heat in the chamber makes life harder for the spark plug, so in some cases this may call for a one-step-colder heat range plug.

To cut to the chase before getting into the details, the best advice for a performance street engine is to put the hottest plug in the engine that will survive.

That sounds overly simplistic but it’s accurate. A plug needs sufficient heat to keep the center electrode mostly free of carbon deposits that can cause a misfire.

Hot vs. Cold Spark Plugs

Let’s first define what is a hot versus a cold plug.

Looking at a spark plug center ceramic insulator, if the area between the ceramic and the spark plug outer steel shell is deep, that is a hotter heat range plug. Conversely, if the ceramic contacts the shell very close to the open end of the shell, this would be a much colder spark plug. The shallow style shortens the heat path to the shell, making the spark plug operate as a colder plug compared to a deeper style where heat must travel further from the end of the plug to the shell.

The Importance of Spark Plug Design

Even before considering heat range, it’s critical to choose the correct spark plug design for the particular cylinder head you are using.

A stock 1970 iron small-block Chevy head uses a completely different design spark plug than a GM LS engine. Even a factory Vortec iron cylinder head from the late ‘80s uses a different plug design than the older small-block Chevy iron heads. If you don’t pay attention, it’s very easy to install the wrong spark plug.

Let’s use a small-block Chevy as an example.

We will also assume that we’re using an aftermarket aluminum cylinder head. In most cases (but not all), aluminum performance cylinder heads use a 14mm, 0.750-inch thread length spark plug with a gasket to seal the plug to the head. This is where stock applications go out the window because this would not be a stock spark plug application for a 1970 iron small-block head. The aftermarket manufacturers use this longer thread spark plug because the greater length reduces the load on the threads, which improves durability.

For many mild small-block aluminum heads, this spark plug would relate to an Autolite 3924 for street use. Within the Autolite family, this particular plug is toward the hotter side of the range that runs (cold-to-hot) 3922 – 3923 – 3924 – 3926.

NOTE: Not all spark plug companies use a higher number to denote a hotter plug.

Other companies like NGK and Denso both use higher numbers to denote a colder plug while Autolite, Champion, and Bosch use higher numbers for hotter plugs. The key to a heat range selection is related to selecting a plug that will efficiently burn-off carbon deposits from the center ceramic during normal use of the engine. So for a normal, street-driven engine, that 3924 recommendation is a great place to start.

After a few hundred miles of operation, you could pull one or more spark plugs and evaluate their condition. As long as the center electrode is burning off the carbon deposits and appears relatively clean, then the heat range is probably correct.

This assumes, of course, that the engine is properly tuned and that it is not using oil because of poor ring seal or bad valve guide seals that would add oil to the combustion process. These issues would need to be solved before you can make a true decision as to spark plug heat range. While some people suggest running a hotter plug to burn off the oil, that is really a crutch. The proper solution would be to solve the oil consumption issue first.

But now let’s say that you want to run the car at the drag strip and plan to make multiple passes. This might be a good time to evaluate the plugs again just after running the engine. This would be a case where installing one-step-colder spark plugs might contribute to slightly better power. A colder plug allows more heat to be transferred away from the center electrode. This allows the plug to survive in the more-aggressive environment of multiple drag strip passes at wide-open-throttle (WOT).

Or, perhaps you are considering racing your car in autocross or a track day at a road course. In these cases, a colder heat range plug is preferable to minimize the chance the spark plug could overheat and potentially cause pre-ignition. Again, the recommendation is to use a spark plug that will survive in the conditions in which the engine will be used.

Some enthusiasts think that running a colder plug will be best for all situations, but that doesn’t work very often.

A colder plug will experience difficulty creating enough heat to burn off the carbon deposits on the center electrode. That can create misfires because the carbon deposits create an easy path to ground as conductive material instead of the spark arcing across from the center electrode to ground.

Beyond just heat range applications are issues with the way the spark plug is configured. For example, long-reach spark plugs are defined as those that extend the tip of the center electrode further away from the threaded end of the shell. This design is very popular with near-stock engines as this improves combustion efficiency by moving the hot end of the spark plug nearer to the center of the cylinder.

But this design also demands a longer ground strap.

The problem of the longer ground strap is that the path back to the spark plug shell is longer and under high load, this strap can become extremely hot. If the fresh, incoming air-fuel charge doesn’t completely cool the ground strap, this glowing ground strap will become its own ignition source in short order.

This is one of the primary ways of causing pre-ignition where combustion is initiated before the ignition fires the plug. If this occurs too soon with the piston travelling upward, catastrophic cylinder pressure is created that will near-instantly destroy a piston or otherwise seriously damage the piston and/or connecting rod.

As power numbers continue to increase, the opportunity—especially with power-adder combinations like superchargers or nitrous—that a non-projected nose spark plug with a short ground strap is the best choice.

Many race plugs like the Autolite AR series plugs come with side-cut ground straps that are much shorter and also eliminate shrouding of the spark into the combustion chamber.

This is equal in importance to the heat range question.

There are increasingly greater inputs into selecting the proper spark plug design, application, and heat range in order to employ the best plug for the engine. Asking lots of questions and seeking professional guidance from engine builders can help with the process. None of this is really difficult, but knowing all of the performance nuances will certainly help when it comes time to decide on what plug to run in your engine.

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  1. Curtis Hammond says:

    What is the best recommended spark plugs for 2006 300c

    • Jeff Smith says:

      I looked up the 300C and both the 5.7L and 6.1L SRT engines use the same plug. It is a 1-inch reach tapered seat plug. This would be an Iridium plug from Autolite as an XP5263. A double Platinum would be an APP 5263. I would not recommend installing a copper style plug. They are least expensive but not as durable. This is the stock heat range plug and there is likely no reason to run anything but a stock heat range.

      • James Stafford says:

        Be careful on Chrysler engine controllers. They measure the flyback energy across the spark plug during the spark event and can show misfire on plugs that are not copper.

  2. Zeppo Jaworski says:

    Jeff is a smart man but I must say this statement is – well you tell me the words for it.
    .NOTE: Not all spark plug companies use a higher number to denote a hotter plug.

    Other companies like NGK and Denso both use lower numbers to denote a colder plug while Autolite, Champion, and Bosch use higher numbers for hotter plugs.

  3. Jeremey Denton says:

    What is the best recommended spark plug for a 2008 chev aveo 1.5 LT

  4. Fred schram says:

    I have a ford 460 stroked to a 552 with aluminum heads 3/4 reach 10.1 compression holley 850 eldel brock intake msd ignition with a 6a box which plug do i start with

  5. I have a T bucket with a 1969 small block Chevy 327 it is a stock engine with headers and side exhaust wondering what plug to run. Has an Edlebrock four barrel 1405 carb. It had AcDelco 7 plugs in it and I just bought it really haven’t had it on the road that much

  6. Michael I Lester says:

    I have an 85 350 with vortec heads,comp cam 268,280 duration 110 degree,478-480 lift,Edelbrock rpm airgap,hughes 2000-2200 stahl, 650 edelbrock.What spark plug would you recomend i start with?In aan 1988 C2500 With 3:42

  7. I have a 2017 Mustang Ecoboost 2.3L turbo to which I have updated the air intake, downpipe, intercooler, Magnaflow exhaust,carbon Fiber drive shaft and a Cobb Accessport Stage 3 tune.
    I am thinking of replacing my factory turbo with a Turbonetics 11910 Direct replacement turbo made for the 2.3L ecoboost and was wondering if I need to go to a one range cooler plug?
    The Air/fuel ratio is right at factory spec…14.7.

  8. Luis Santiago-delavega says:

    Cold spark plugs for 2017 Ford Taurus SHO

  9. George Houk says:

    I’m building a Ford 427 FE Hi-Riser engine with 14 to 1 compression ratio. It will be used mostly in car shows and some drag racing. What spark plug and heat range would you recommend? The engine will be put in a real 64 Ford Thunderbolt?

  10. I’m looking for 1 heat range cooler plugs for 2 applications. Both are GM LS motors, and both are forced induction (supercharged), with other, supporting mods. (intake / exhaust, cam, etc) One motor is the iron block 5.3 (L59), and the other is the 6.0 iron block (LQ4) Prefer AC Delco plugs, although I I hear NGK makes the AC Delco OEM plugs these days. Any assistance would be much appreciated!

  11. Michael Parker says:

    This is a first for me and a site like so here goes. I have a 350 crate engine .. 330 hp 650 holly carb . Everything is basically out of the box .. plugs in right now are ac delco R44LTS… any recommendation on better or keep the same … thanks Mike

  12. Michael Parker says:

    This is a first for me and a site like so here goes. I have a 350 crate engine .. 330 hp 650 holly carb . Everything is basically out of the box .. plugs in right now are ac delco R44LTS… any recommendation on better or keep the same … thanks Mike

    • Wayne Centore says:

      Have same motor 600 cfm edelbrock, jetted up 8%,headers,auto,3.42,sheet called for r43lts,chev,said to use r42ltsm car run 13.7 all day long plugs overall look good have no issues shift at 5400 have used these plugs 10k miles in 68 camaro

  13. Charles Espenshade says:

    Ok, I’m redoing a 1985 Stang H.O. motor.
    It’s bored .30 over and decked .15. the heads are shaved .08 but stock E5’s.
    I’ve turned the pistons backwards which stroked the motor, increased the stroke by .03 thousandth higher increasing compression.
    I’m running a Crane 484 lift 284 duration cam
    It’s also running straight timing gears.
    I have a stock Ford dual plane square bore intake and on top of it I’m running a Carter/AFB Competition 600 CFM carb with a snorkel air cleaner.
    I have stock cast iron shorty exhaust manifolds.
    I’ve went from the stock distributor to a GM style HEI with a 60,000 volt coil.
    Stock Autolite plugs should be the #26. Am I thinking correctly in I should step down to the #24?

  14. I have a chevy 355/360it’s been bored to .60 over it has vortec heads on it! I bought the engine from a friend. I pulled the heads off and found the plug used R45TS. These plugs are really shallow in the head! Not sure if this is enough information or not! But what plug should I use! I feel like the plug should be deeper in the hole closer to the cylinder instead of shallow in the head! I’d say the plugs go about halfway down and there’s another 8 threads before it even starts to stick out! Hope this helps!

  15. I’ve been looking everywhere for an answer to my question to no avail. Is it possible to have a plug that has stock characteristics but can withstand going WOT every now and then? A daily driver/part time drag racer…can I have my cake and eat it, too? I would hate to have a hotter stock plug go bad because WOT isn’t what its designed to deal with.

  16. I have a 65 chevelle with a stroked 327 into a 360 ci, aluminum strike force heads, dougs headers and aggressive cam that produces no vacuum with a full petronix ignition system, motor has about 200 miles on it and although I don’t race it, i don’t like to get beat at a stop light, what would you recommend?

  17. stephen A edwards says:

    my new big block,chevy 427 only has about 8oo miles on it . It has aluminum heads mild cam but keeps gas fouling the spark plugs all the time. what can i do?

  18. Ian racer says:

    Hi, for brodix track 1 sbf heads the book says autolite 52. Doesn’t give me much to go on. I need a starting point. 393w, 12.5 to 1 compression, flat top pistons. Street/ strip gasoline engine. What would you suggest?

  19. Jeff. I have problem with filed plugs. Engine is sbc bored 040. Compression is 9.5 heads are EQ HR350e. Cam comp 268 flat tap. Carb750 Holley or 650 quick fuel. Dist is protronix multi sparks voltage to dist is 11volts. 8 mm wires. All parts on engine are new or low milage. Plugs used E3. Both 3.40 and hotter 3.42. Engine with fresh plugs runs fine for a few days then starts misfiring, backfiring. Running like junk. Plugs are covered in black soot. I’ve had 3 distributors in it, different plug wires, checked input voltage, 2 different quality carbs that worked great on another old sbc engine. Any ideas? I’m stumped. Will NGK-R5673-6 work? Headers make spark plug changes challenging at best. Thank you

  20. steve turner says:

    i have a 359 chevy with 12,5 compression afr heads 67 combustion chambers ported victor jr intake 1 3/4 headers 750 deamon carb block has been decked ,010 645 lift solid roller cam what heat range plug do you recommend

  21. Jeremy Chatelain says:

    Needing spark plugs for 76 k10 350 engine. Ran a new set for a week after tune up and timing until they fouled. Any help and advice would be appreciated

    • Timthefoolmantailor says:

      Is it stock? If not you will need to post the mods. I would use a non resistor copper core spark plug that fits the stock plug heat range if no mods to the engine were done.

  22. Blaine McKivison says:

    I have a 1965 327ci bored to 331ci (.030 over). I have a mild cam in it and 400ci heads. I am using a MSD HEI distributor. I am using a short spark plug as the longer ones placed my spark plug wires too close to my headers.
    I want to try some e3 spark plugs. What e3 should I get?

    • Timthefoolmantailor says:

      Stay away from those spark plugs and go for copper core with no resistor in them. I can’t recommend a brand as I need to know more about your 400ci heads and compression ratio and type of gas you are using. I think with the colder plug pre-detonation might be an issue.

  23. Sergio Esquivel says:

    I’m installing aluminum heads on my sbc 350 and the sparkplug threads are longer than my stock heads do I need to use longer sparkplug?

  24. I have a turbo drag strip-street 830 hp engine using VQ C16 {118 octain} at the track at 30psi boost. Which is a better autolite plug for the track, Ar3932vs AR3933 vs AR3934? What would be good for the street using 10 psi boost on 92 octain from the gas station pump? Your article suggests hotter plug is generally used for the street vs cooler Racing plug?

  25. Timthefoolmantailor says:

    I want to run a spark plug with a copper core and I do file the ground strap to be over the electrode too. Also, what are the advantages and disadvantages to running a spark plug with a resistor and no resistor and does the resistor have the potential to degrade the hot spark?

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