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Ignition Components Ltd

Understanding the Different Types of Plug Caps: Suppressed vs. Non-Suppressed, Straight vs. Angled, and SAE vs. 4mm

If you’ve worked on ignition systems—whether it's in your car or industrial machinery—you’ve probably encountered plug caps. These small but essential components connect the ignition wire to the spark plug.  However, not all plug caps are the same. Different designs, fittings, and functionalities cater to various needs. In this blog, we'll break down the types of plug caps, focusing on the differences between suppressed and non-suppressed versions, straight and angled designs, and the two types of fittings: SAE and 4mm.


1. Suppressed vs. Non-Suppressed Plug Caps

One of the most significant distinctions in plug caps is whether they are suppressed or non-suppressed.


  • Suppressed Plug Caps: These have a built-in resistor, which serves a crucial role in reducing electrical noise and interference. This is especially important for modern systems equipped with electronic systems, or sensitive components that could be affected by electromagnetic interference (EMI). The resistor in a suppressed cap helps minimize the noise that the spark plug generates during ignition, preventing disruptions to these systems.

  • Non-Suppressed Plug Caps: As the name implies, these caps don’t have a resistor and thus don’t provide suppression of electrical noise. While they may offer slightly better performance by allowing the maximum electrical energy to reach the spark plug, they are more likely to cause interference with nearby electronics.


    Key takeaway: If your ignition system has electronics or a radio system, a suppressed plug cap is likely the best choice to avoid interference. Non-suppressed caps may be better suited for ignitions without sensitive electronics or where a strong spark or signal is required.

 

2. Straight vs. Angled Plug Caps

Another design consideration is whether to choose a straight or angled plug cap. This primarily affects how the cap fits within the ignition system.


  • Straight Plug Caps: These have a linear design, where the wire connects in a straight line to the spark plug. Straight caps are ideal when there’s plenty of space around the plug and wire, allowing for easy access and a neat connection.

  • Angled Plug Caps: These caps are bent, typically at a 90-degree angle, allowing for better routing of the wire in confined or awkward spaces. Burner units and engines with compact layouts, where space is a premium, often benefit from angled caps because they reduce the strain on the ignition wire and fit easier within tight areas.


    Key takeaway: Choose a straight cap when space allows for a clean, direct connection. Opt for an angled cap when working in tighter spaces or when the wiring needs more flexibility.


3. SAE vs. 4mm Fittings

The type of fitting refers to how the plug cap attaches to the electrode or spark plug itself. There are two common types of fittings: SAE and 4mm.


  • SAE Fitting: The SAE standard fitting is designed to attach to a standard 6.35mm bullet-type terminal. This is a common size and is widely used in many industrial and automotive ignitions.  The bullet-type connection offers a secure fit with minimal effort, making it a go-to option for most standard engines.

  • 4mm Fitting: This fitting is designed to fit onto M4 (4mm diameter) threaded posts. For where the electrode or spark plug has a threaded terminal.  Terminal nuts are also available to screw onto threaded posts allowing the electrode to be used with a standard SAE plug cap.


    Key takeaway: The SAE fitting is more common, but in applications where you have two similar electrodes next to each other doing a different job, using a SAE fitting on one and 4mm fitting on the other can ensure ignition cables are plugged on to the correct electrode.  And example of this would be where you have a flame probe and ignition electrode installed next to each other in a burner unit, when installed both items could look similar but mixing up the cables will mean the system cannot function correctly.

 

Choosing the Right Plug Cap for Your Needs

Selecting the right plug cap depends on your ignition system, the surrounding electronic components, and the specific performance requirements. Here’s a quick recap to help guide your choice:


  • Suppressed vs. Non-Suppressed: If you’re running a modern system with electronics or want to reduce interference, go for a suppressed cap. If you’re in a high-performance or older system, a non-suppressed cap might be the right fit.

  • Straight vs. Angled: Choose based on the space available in your engine bay. Straight caps are best for open, spacious layouts, while angled caps work in tighter areas.

  • SAE vs. 4mm: Most standard applications will benefit from an SAE fitting. However, if you have more than one electrode and need to ensure the wires do not become mixed up consider using a mix of both items.



Understanding these differences helps ensure you pick the right plug cap for your application.  If you are still struggling to understand what is best for your application please get in touch with us and we will be happy to advise.









90° Angled Plug Cap Straight Plug Cap 4mm Terminal SAE (6mm) Terminal


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