While researching which plug I shuold use on my buildup I decided to write a little article for my clubs newsletter. Here it is. Hope it helps!
Understanding the Spark Plug
Spark plugs are probably the most overlooked part of an engine. People tend to swap carburetors, heads, and add high-powered ignition systems while leaving the stock plugs in place. Hopefully this article will help answer some of the spark plug questions you may have.
There are many different spark plugs on the market, but they all have two primary functions: ignite the air/fuel mixture and to remove heat from the combustion chamber. The spark plug is basically a heat exchanger. It pulls thermal energy away from the combustion chamber and transfers it to the cooling system. Heat is transferred away from the tip through the insulator, then to the shell, then to the cylinder head. It must do this by staying cool enough to avoid pre-ignition and run hot enough to burn off combustion deposits or prevent fouling. The spark plugs ability to dissipate heat is determined by its heat range. Whenever an engine is modified and the overall compression ratio is changed a new heat range must be selected. It is important to remember that the heat range of a spark plug does not affect horsepower. The length of the insulator nose determines the heat ranges. A shorter path, means the faster the heat will be dissipated and the cooler the plug. A longer path, means the slower the heat will be dissipated and the hotter the plug.
There is no magic formula for selecting a heat range. But, it is recommended to always start with a colder spark plug. A plug that is too cold can eventually foul and a misfire will occur, which should not hurt the engine. But, if the spark plug is too hot, pre-ignition and detonation can occur which can damage your engine. To make it simple, if you need your engine to run cooler use a colder plug. If you need your engine to run a little hotter use a hotter plug. Remember the warning above and never use too hot of a plug. Also, remember a modified engine with raised compression will add heat, so a colder plug is recommended (retarding timing, richening the fuel level and octane level is also recommended).
A projected nose or extend tip spark plug is preferred in some situations (as long as it will clear the piston!). An extended tip spark plug can act like a hot plug at low rpm's and act like a cold plug at high rpm's. The incoming air/fuel charge that blows by the tip cools the extended plug tip. The plug burns clean without fouling at low engine speeds because of the longer insulator length.
Attention must also be made to the spark plugs gap size since it directly affects the plugs tip temperatures. Some engine combinations can use a larger gap size and others do not run well until the gap is tightened up. Gap sizes can vary from a small .025 to a large .045-. 060. Modified engines with high compression ratios tend to require smaller gaps. The bigger the gap size the more voltage is required to bridge the gap. This is why most people add high power ignition systems so they can run the larger .045-. 060 gaps. But, you still have to be careful. Most of the time you will have to run smaller gaps than you might think. MSD recommends running a .05 larger gap than normal with their systems.
Indexing a spark plug can produce a horsepower increase of 1-5%. Some say indexing is not worth the time and trouble. This means on a 250hp engine, you can gain as much as 12 horsepower. When indexing a plug you are ensuring that the gap is facing towards the valves. To index a plug, use a felt tip pen to mark the location where the ground electrode attaches to the plug body. (DO NOT use a pencil since graphite conducts electricity.) Not every plug will line up correctly, so multiple sets of plugs may be needed or you can use indexing washers. Your mark should be within 20 degrees for your plugs to be indexed correctly.
Spark plugs should also be torqued correctly for them to perform correctly. A spark plug that has been over torqued can harm the insulator, while a plug that has been under torqued will not seat properly and affect heat transfer. Generally, an 18mm plug should be torqued 15-20 ft lbs. in a cast iron or aluminum head. Without a torque wrench, it should be hand tightened, then add a 1/16 turn. Spark plugs should be installed in a clean hole and installed with clean/dry threads.
Understanding the Spark Plug
Spark plugs are probably the most overlooked part of an engine. People tend to swap carburetors, heads, and add high-powered ignition systems while leaving the stock plugs in place. Hopefully this article will help answer some of the spark plug questions you may have.
There are many different spark plugs on the market, but they all have two primary functions: ignite the air/fuel mixture and to remove heat from the combustion chamber. The spark plug is basically a heat exchanger. It pulls thermal energy away from the combustion chamber and transfers it to the cooling system. Heat is transferred away from the tip through the insulator, then to the shell, then to the cylinder head. It must do this by staying cool enough to avoid pre-ignition and run hot enough to burn off combustion deposits or prevent fouling. The spark plugs ability to dissipate heat is determined by its heat range. Whenever an engine is modified and the overall compression ratio is changed a new heat range must be selected. It is important to remember that the heat range of a spark plug does not affect horsepower. The length of the insulator nose determines the heat ranges. A shorter path, means the faster the heat will be dissipated and the cooler the plug. A longer path, means the slower the heat will be dissipated and the hotter the plug.
There is no magic formula for selecting a heat range. But, it is recommended to always start with a colder spark plug. A plug that is too cold can eventually foul and a misfire will occur, which should not hurt the engine. But, if the spark plug is too hot, pre-ignition and detonation can occur which can damage your engine. To make it simple, if you need your engine to run cooler use a colder plug. If you need your engine to run a little hotter use a hotter plug. Remember the warning above and never use too hot of a plug. Also, remember a modified engine with raised compression will add heat, so a colder plug is recommended (retarding timing, richening the fuel level and octane level is also recommended).
A projected nose or extend tip spark plug is preferred in some situations (as long as it will clear the piston!). An extended tip spark plug can act like a hot plug at low rpm's and act like a cold plug at high rpm's. The incoming air/fuel charge that blows by the tip cools the extended plug tip. The plug burns clean without fouling at low engine speeds because of the longer insulator length.
Attention must also be made to the spark plugs gap size since it directly affects the plugs tip temperatures. Some engine combinations can use a larger gap size and others do not run well until the gap is tightened up. Gap sizes can vary from a small .025 to a large .045-. 060. Modified engines with high compression ratios tend to require smaller gaps. The bigger the gap size the more voltage is required to bridge the gap. This is why most people add high power ignition systems so they can run the larger .045-. 060 gaps. But, you still have to be careful. Most of the time you will have to run smaller gaps than you might think. MSD recommends running a .05 larger gap than normal with their systems.
Indexing a spark plug can produce a horsepower increase of 1-5%. Some say indexing is not worth the time and trouble. This means on a 250hp engine, you can gain as much as 12 horsepower. When indexing a plug you are ensuring that the gap is facing towards the valves. To index a plug, use a felt tip pen to mark the location where the ground electrode attaches to the plug body. (DO NOT use a pencil since graphite conducts electricity.) Not every plug will line up correctly, so multiple sets of plugs may be needed or you can use indexing washers. Your mark should be within 20 degrees for your plugs to be indexed correctly.
Spark plugs should also be torqued correctly for them to perform correctly. A spark plug that has been over torqued can harm the insulator, while a plug that has been under torqued will not seat properly and affect heat transfer. Generally, an 18mm plug should be torqued 15-20 ft lbs. in a cast iron or aluminum head. Without a torque wrench, it should be hand tightened, then add a 1/16 turn. Spark plugs should be installed in a clean hole and installed with clean/dry threads.
