![]() ![]() 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. ![]() 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. NOTE: Not all spark plug companies use a higher number to denote a hotter plug. Within the Autolite family, this particular plug is toward the hotter side of the range that runs (cold-to-hot) 3922 – 3923 – 3924 – 3926. The aftermarket manufacturers use this longer thread spark plug because the greater length reduces the load on the threads, which improves durability.įor many mild small-block aluminum heads, this spark plug would relate to an Autolite 3924 for street use. 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. 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. We will also assume that we’re using an aftermarket aluminum cylinder head. Let’s use a small-block Chevy as an example. If you don’t pay attention, it’s very easy to install the wrong spark plug. Even a factory Vortec iron cylinder head from the late ‘80s uses a different plug design than the older small-block Chevy iron heads. The Importance of Spark Plug DesignĮven before considering heat range, it’s critical to choose the correct spark plug design for the particular cylinder head you are using.Ī stock 1970 iron small-block Chevy head uses a completely different design spark plug than a GM LS engine. 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. Conversely, if the ceramic contacts the shell very close to the open end of the shell, this would be a much colder spark 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. Let’s first define what is a hot versus a cold plug. A plug needs sufficient heat to keep the center electrode mostly free of carbon deposits that can cause a misfire. That sounds overly simplistic but it’s accurate. 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. 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. As cylinder pressure increases, combustion creates more heat in the oxidation process. 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.Īll of these additions to an otherwise-stock engine will tend to make more cylinder pressure. Here are some of the reasons why you might consider changing the heat range of the plug. We’ll assume that the engine has been modified. For a stock engine, the answer is simply to use the recommended plug for the application. Let’s start with a typical street engine. Jeff Smith: Simple questions sometimes require a bit longer answers. How do I tell the correct heat range spark plug for an engine? – Mark S. A hotter heat range plug is preferable for engines that only see light duty applications as in mild street driving. 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. This is a comparison of a cold heat range plug (left) versus a much hotter heat range plug (right). ![]()
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