Performance Spark Plug Buyer’s Guide
It is easy to take the humble spark plug for granted, especially when it comes to your daily driver. Modern plug designs and materials often mean a 100,000-mile spark plug change interval. So, unless you keep your cars a long time you may never even have to worry about changing your daily’s spark plugs. For those that do hit the service mileage for a plug change generally your local parts counter’s year/make/model search will return the correct direct replacement spark plugs to install, or you will find a service parts decal under the hood that lists the factory spark plug part number and plug gap (if the plugs can even be gapped!) to aid you in proper spark plug replacement. However, all is not the same when we are talking about performance spark plugs for that hot rod or race car in your shop.
For starters, we all know how frustrating it can be to grab a set of high-performance spark plugs locally. The typical year/make/model search tool does not work so well when you have a 1929 Ford hot rod with a Gen 1 small block Chevy V8 engine in it from 1963. These year/make/model designations also only provide data on the original factory spark plug type that was fitted and the parts store brand’s crossover. They do not consider such hot-rodding updates as advanced timing, higher compression, performance induction, headers, and so forth. All these upgrades can and will affect the spark plug’s job of firing the air and fuel mixture in your combustion chambers. Therefore, performance spark plugs are available in different heat ranges, conductor types, and more to provide optimum ignition and burning of your air fuel mixture. The more complete the burn, the more efficient your engine becomes and the more power it makes. It is important to not only know the basic plug specifications for your engine (see our spark plug application chart below for more details), but what you should be looking for in a performance spark plug.
Follow this link for a larger version of our spark plug application chart
Follow this link for a spark plug brand conversion chart for Autolite, Champion, and NGK spark plug conversion chart
The life of a spark plug is not an easy one. Their job is to take the high current electrical voltage from the secondary ignition system (your distributor’s cap and rotor and spark plug wires) and provide a ground path for the voltage to jump across, creating a spark that will ignite the air and fuel mixture in your combustion chamber. The spark itself along with the heat of the combustion chamber puts a lot of wear on the spark plug and over the years many different electrode materials, which we will get into shortly, have been used to prolong the life of the spark plug in such a harsh environment.
The spark plug body is made of up a steel shell that threads into the cylinder head and contains the side electrode (sometimes referred to as the ground electrode or ground strap) with a porcelain insulator that resides inside the shell. The body’s steel shell is machine crimped to retain the porcelain insulator. Inside the insulator is the center electrode. This center electrode is what connects directly to the spark plug wire and is the path the voltage takes to reach the side electrode inside the combustion chamber. Automotive spark plug lengths are fairly standard; however, a few companies offer “shorty” plugs that often aid in header clearance situations. Now that you understand what makes up a standard spark plug, we will look at each component individually and how they are utilized in a performance application.
The spark plug seat is a machined aspect of the spark plug’s steel shell. There are two types of spark plug seats: tapered seat and gasket seat. The style of seat is determined by the opening machined into the cylinder head’s combustion chamber and they are not interchangeable. The seat of the spark plug shell is what seals the combustion chamber and transfers the heat from the plug to the head to keep the plug cool (more on heat ranges later in the guide). On a tapered seat spark plug the taper itself seals, much like a brake line flare fitting, while on a gasket seat spark plug there is a crush washer on the spark plug that helps seal the plug to the combustion chamber.
The big three have all used both style of plug seats over the years and most aftermarket performance cylinder heads uses the gasket seat style of plug. If you are not sure what your engine uses, you can simply remove a spark plug and inspect it. If the plugs are missing from the engine, you can look at the spark plug hole itself to determine the style of seat or use our spark plug fitment chart we mentioned above.
The only thing you must consider regarding spark plug thread size is that it matches the threads in your cylinder head. While there are several spark plug thread sizes, the two most pertinent ones when it comes to American V8 engines are 14mm and 18mm threads. Pre-1974 Ford V8 engines make up the majority of the 18mm thread applications, while General Motors and Ma Mopar generally stuck to the 14mm threads throughout the years.
While we are discussing spark plug threads another measurement to consider is the spark plug’s reach. This is the threaded length of the steel shell. It can be listed in decimal or fractional measurements (3/4-inch or .750-inch for example) and is dictated by the cylinder head’s specifications. The measurement is from the spark plug’s seat to the end of the threads on the shell but does not include the center electrode tip or ground electrode or any extended non-threaded portion of the shell. For gasket seat plugs it is measured from the face of the seat, while tapered seat plugs the measurement is from the top of the taper. It is critical you match the cylinder head’s reach specification, as too short a spark plug reach will shroud the spark plug, resulting in poor combustion since the spark is so far away from the air/fuel mixture in the combustion chamber. A spark plug with too much reach can possibly hit the piston surface, causing damage to the plug and the piston.
This thread sizing is an SAE standard and has no real correlation to the socket hex size machined into the spark plug’s shell. Most V8 engines through the 1970s were made with the larger 13/16-inch socket hex. This made for a larger plug body that many find harder to install with the larger socket. The smaller 5/8-inch socket hex design, often called a “peanut plug” at the time, was implemented to aid in easier installation by using a smaller socket and having to machine less clearance in the cylinder head’s spark plug areas. You will find most gasket seat plugs utilize the larger 13/16-inch hex shell, while tapered seat plugs are most often associated with the smaller 5/8-inch hex shell.
The center electrode of the spark plug carries the conductive material that helps create the spark at the tip of the plug. Almost all spark plugs use copper for the center electrode material, as it is a very efficient electrical conductor (much like your car or home’s electrical wiring use copper). However, unlike standard copper wiring, the copper conductor in your spark plug must survive in the harsh environment of a combustion chamber’s extreme heat and pressure. So, while copper is quite efficient at its job, the standard copper plug has a relatively short lifespan. Spark plug manufacturers have created longer wearing plugs by adding special coatings or welding special tips/discs to the copper conductor’s end. These coatings, mainly Platinum and Iridium but even Nickel as well, allow a plug to last a very long time at the expense of a hotter plug and higher initial cost. We have listed the main benefits and detractors of each conductor type below so that you can make your own informed decision based on your needs and not just “Iridium vs Platinum spark plugs” Internet arguments.
Copper Spark Plugs
- Cheapest of all conductor materials
- Low cost makes them a great choice for applications which require frequent replacement of spark plugs
- Shortest lifespan at about 20k to 30k mile change intervals
- Commonly used in motorsports applications where racing spark plugs are replaced frequently
- Good choice for street rods that will see limited mileage or with radical engines that may foul plugs easily
Platinum and Double Platinum Spark Plugs
- Medium cost level
- Single platinum plugs have a platinum coating on the center electrode only while double platinum is coated on both the center and side/ground electrode
- Lifespan of 75k to 100k miles
- Common plug material for OEM manufacturers
- Good choice for hot rods and muscle cars with mild engines that will travel a good number of miles annually
Iridium Spark Plugs
- Highest cost level
- Lifespan of 100k to 125k miles
- Common plug material for OEM manufacturers
- Good choice for hot rods and muscle cars with mild engines that are daily drivers and see extensive miles annually
- Not typically used in racing applications due to the high cost and frequent replacement
Since it relates to the center electrode and conductor, we wanted to discuss the need for a resistor or non-resistor spark plug. Originally all spark plug types came without a resistor, but in the 1960s, as car radios became standard, and some vehicles were beginning to be fitted with electronic ignition or other electronic components, the need to suppress the inherent radio frequency interference (RFI) from the spark plug arose. Adding a resistor to the center conductor lowers the RFI to an acceptable level, preventing interference with sensitive electronics and that annoying ignition whine heard through the radio. As its name suggests, the resistor means some of the spark energy is resisted, lowering the spark plug’s spark output, but not usually enough to be an issue with your hot rod or classic muscle car. Resistor plugs are used by all OEMs today and we highly recommend them for all street/strip applications and for cars with EFI and other advanced electronics.
Non-resistor plugs have their place still in the automotive performance world though. A non-resistor plug will allow the maximum spark output possible to reach the plug tip for a hotter, stronger spark to light off the air/fuel mixture in the combustion chamber. For that very reason non-resistor racing spark plugs are still extremely popular for motorsports applications. However, even with some motorsports applications RFI shielding, such as a metal firewall or shielding wrap will need to be implemented or the ignition control module, EFI ECM, etc., will need to be mounted as far away as possible from the secondary ignition system.
While a lot of marketing has gone into various spark plugs with fancy ground electrode designs, the truth of the matter is, when the spark energy jumps from the positive center electrode to the negative ground electrode the sharper the edge on both electrodes the more concentrated the spark energy. Every time a spark plug fires over the course of a spark plug’s life a little microscopic bit of the electrode’s edge erodes away. As these edges wear the softer, rounded edge not only increases the spark plug’s gap and the plug’s ability to even fire since the ignition coil cannot provide enough voltage to cross such a large gap, but the rounded edges create spark scatter/jumping. The idea behind multiple ground electrodes, be it individual electrodes or a stamped metal plate, is to provide additional sharp edges so that the plug can produce a nice, solid concentrated spark to light the air/fuel mixture. Some suggest they are best used with ignition systems that provide multiple spark output capabilities as well, such as those from MSD Ignition and others.
Quite simply, the difference in the two tips is right in their naming. A projected tip, sometimes called a power tip, extends the center electrode past the steel body of the spark plug, placing it further into the combustion chamber. Whereas a standard tip is generally found to be flush with the steel body of the spark plug. Why would you want to have the tip projected? Moving the tip deeper into the combustion chamber often provides better idle and cruise rpm operation, and because projected tip plugs run hotter, they are known to be less prone to fouling than a standard tip plug, which runs cooler since the tip is slightly shrouded. Projected tip spark plugs are not a fix-all however, as a standard tip plug will be a better option when using dome pistons to ensure adequate clearance between the piston and the spark plug. Additionally, due to their colder running, standard tip plugs are still the preferred plug of choice for high horsepower and racing applications.
A spark plug’s heat range is a function of the ceramic insulator length around the center electrode. The shorter the insulator the less material to transfer thermal energy to the steel spark plug shell and on to the cylinder head. This is how a “hot” plug works. The opposite is true with a “cold” plug by increasing the length of the insulator, allowing the plug to cool more quickly between firings of the secondary ignition system. A hot plug will dissipate heat slowly, keeping the plug tip hot. This is good for emissions and prevents plug fouling. Hotter plugs are generally used in stock or mildly modified engines. The need to run a cooler plug arises with more advanced ignition timing, higher compression, and when a power adder like a turbo or supercharger is used. The cooler plug resists detonation, which is harmful to not only the piston, but can completely melt the tip of the plug if an excessive heat range is used in the wrong application.
Unfortunately, there is no magic bullet and the only way to know the best spark plug heat range your application requires is with proper tuning, reading the spark plugs, and some engine stress testing; be it on track or on a dyno. Ultimately you want a plug hot enough to prevent fouling, but cool enough to resist detonation with your final carb or EFI tuning, timing, and other parameters set. To further complicate matters, the major plug manufacturers all use different heat range part number values, with some going up as the plug gets hotter and some going down as the plug gets hotter, so it is difficult to create a universal heat range guide of sorts. However, the easiest way we've found to help with heat range application is to use Autolite plug values (part number goes up as the plug gets hotter) and utilize their heat range cross reference chart for Champion and NGK brands respectively.
It is commonly believed that increasing the gap of a spark plug will create a larger spark and ultimately make more power. However, the truth is that a spark plug’s gap will be dictated by the secondary ignition system’s voltage output, combustion chamber pressures, compression ratio, power adders, and more. There is no one “wonder gap” that works for everything. Yes, increasing the plug gap can create a smoother running engine, to a point, as it generally only helps at low speed/light throttle use. Be honest, how many of us drive our hot rods, muscle cars, classic trucks, or race cars in that manner? Exactly! Having too large a gap, especially in performance applications, means at full throttle the plug gap is too wide, creating a weak spark that does not fully ignite the air/fuel mixture. For these applications, a smaller plug gap is required to allow the plug to maintain the spark across the electrodes with the increased cylinder pressure. This is especially true in boosted applications where a turbo or supercharger’s boost pressure can blow a wider gapped spark plug out like a birthday candle! Much like determining your best spark plug heat range, spark plug gap is something that is best determined through testing your combination.
Ignition considerations for plug gap
- Points ignition systems can use up to .030” to .035” spark plug gap for stock or mild performance applications
- High energy electronic ignitions such as GM HEI, Ford Duraspark, Ford TFI, and Mopar electronic ignitions can use up to .040” to .045” spark plug gap for stock or mild performance applications
- Aftermarket CD ignitions such as MSD, Pertronix, Fast, Accel, etc. can use up to .050” to .060” spark plug gap for stock or mild performance applications
- Coil on plug or coil near plug ignitions can use up to .050 to 060” spark plug gap for stock or mild performance applications
Engine considerations for plug gap
- Low compression stock engines or mildly modified engines can tolerate the most spark plug gap
- High compression engines with extensive performance modification will require less spark plug gap than stock engines due to increased horsepower output and thus higher cylinder pressure
- Boosted applications such as turbo or supercharged engines will require the least amount of spark plug gap due to incredibly high horsepower output and cylinder pressure
Vehicle use considerations for plug gap
- Street driven vehicles will benefit the most from increased spark plug gap since idle quality and part throttle response/drivability is typically better with more gap
- Performance vehicles that see some street miles will need to find a good balance between increased plug gap for drivability vs. reduced plug gap for wide open throttle performance
- Racing applications and extreme performance street vehicles are better off using a spark plug gap that is suitable for the power level and disregard any idle quality issues due to the reduced plug gap
You have read our spark plug guide and you have concluded that tuning your engine’s spark plugs for optimum performance is just the edge you need to make the most power and have your performance engine run its best. So how do you tune your plugs? When it comes to heat range and plug gap the best tool to have in your corner is a spark plug viewer. This handy little tool is essentially a lighted magnifier that allows you to “read” a spark plug’s insulator and tip and see just what the plug is doing after some wide-open-throttle runs to get the plugs up to temp. Once you have read the plugs you can more easily determine if you need a hotter or colder heat range or a wider or narrower plug gap. This document on plug reading and tuning is a nice, brief overview.
Another great tip from the racing community that has been used for decades with racing spark plugs is spark plug indexing. Indexing spark plugs allows a clear path for the air/fuel mixture to “meet” the spark from the plug is good for a few horsepower on high performance engines. Indexing of plugs is also sometimes required with domed pistons to prevent damaging the plug by having the piston crush the ground electrode into the plug face. Using a spark plug indexing kit, you mark the plug and then install the appropriate thickness washer to rotate the plug for indexing in the cylinder head, usually with the open side of the plug gap towards the intake valve. We offer a great plug indexing kit for 14mm tapered plugs with indexing tool and even a plug thread chaser! If you need indexing washers for 14mm gasket seat plugs, we have those as well. Spark plug sockets come in just about every tool kit ever sold, but one tool we do recommend is our nifty spark plug boot puller. This tool makes quick work of wire removal without damaging the plug wire’s boot. Lastly, do not forget some anti-seize for the plug threads, especially when using aluminum heads! With these tools you can easily tune your plugs for max performance on the street or the track.
Products Featured in this Article
Longacre® 52-50886 Spark Plug Viewer with HolderView$49.99Compare
Spark Plug Indexing Washers, 14mm Flat Seat, 30 PkView$18.99Compare