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.