Spark Plugs
Posted: 18 Apr 2001 20:09SPARK PLUGS
HEAT RANGE AND SELECTION
The type of electrode alloy and the length of the core nose determine the heat range of a spark plug.
Hot plugs have a long insulator nose which prevents heat transferring away from the tip into the cylinder head and are used in cold, low performance engines.
Cold plugs have a much shorter nose length which transfers heat more rapidly. These are used in hotter performance engines.
If your engine is modified you may want to go one step colder than stock. If your engine is very modified but still street driven you may want to go two steps colder.
I always find it difficult to determine how hot or cold a spark plug is compared to another. The people selling them never know what they are talking about and there is no useful information on the packaging.
Some have quite a long tip but are still rated quite cold. These extended nose types are very good because they act as a hot plug at low speeds and cool off to act as a cold plug at high rpm. The cooling off happens because of the in rushing air/fuel which blows past the extended tip.
The extended tip also puts the spark into a better position for igniting the mixture. Because they are hotter at idle they tend to not foul up as much as normal plugs.
The main disadvantage is that you may not be able to find one cold enough for your engine but it is worth a try. Also extended tips are not recommended where racing fuels are used or boost is present as with nitrous, blowers or turbos.
Another good plug is the Champion U-series which has a booster gap inside. A gap inside the spark plug is a good idea because the voltage has to build quite high before it can jump the gap and then jump the gap at the tip. Without the booster gap some of the current can leak away before the spark jumps due to deposits around the tip.
Resistor plugs are o.k. if you need them for suppression but your spiral core wires should be enough to prevent radio interference.
Autolite are quite good plugs too.
Stay away from expensive "trick" plugs such as Split-fire and Rapid-fire. These will work well at first because sparks jump from sharp edges more easily and all these plugs are doing is providing more sharp edges. But as they wear they become no more effective than a standard plug. It is cheaper to buy normal plugs and to change them more often. Some 'normal' plugs have sharp edges built in these days anyway.
Remember - plug selection is less critical if you have a good ignition system.
If you ever have the heads off your engine it is worth while checking to see how far the plug threads into the head. You do not want any thread showing in the combustion chamber either on the spark plug or the head as this can cause hot spots. The plug should be just flush on the inside and the side electrode indexed away from the squish area of the chamber by using indexing washers on the plugs although I think this is less critical on mopar engines than other makes because the angle of the plug makes the tip point at the squish area and not the side electrode.
THE PLUG GAP
The ideal plug gap can only be found by trial and error on any individual engine. A lot depends on your style of driving and the frequency of plug replacement.
First make sure that the side electrode is centered and flat across the tip of the plug and not at an angle.
Manufacturers recommended gaps are set to be adequate for the average middle aged driver on a cold day in a car that is in need of a tune-up and will be driven very easy in slow city traffic.
The best way is to increase your gap 10 thou. at a time and see how it goes for a while. If there has been an improvement in power or mileage over the last 1000 miles then try another 10 thou. Increases in plug gap will give improvements up to the optimum gap. After that, further gap increases will cause a deterioration.
To get you in the ball park I have some recommendations I found in a book which might help and are worth a try.
If you only use wide open throttle ( like on a drag car ) then a 10 thou. reduction in the recommended gap will give a 6% improvement in power at about 4,000 rpm, a 7% loss in mileage when driving slowly and a 25% increase in the life of your plugs.
If you decrease the gap by only 4 thou., you can add an extra 5,000 miles between tune-ups and you lose only 4% mileage.
Increasing the gap by 6 thou. over the recommended setting however will give you 4% better mileage, smoother operation and an increase in power of about 2%. But - you will have to change plugs more often and the rest of the ignition ( especially the leads ) will have to be in good condition in order to handle the increase in energy.
The after market 'computer' CD ignition systems allow you to increase your gap by about 25 thou. and will give you benefits in mileage, power and frequency of tune-ups.
HEAT RANGE AND SELECTION
The type of electrode alloy and the length of the core nose determine the heat range of a spark plug.
Hot plugs have a long insulator nose which prevents heat transferring away from the tip into the cylinder head and are used in cold, low performance engines.
Cold plugs have a much shorter nose length which transfers heat more rapidly. These are used in hotter performance engines.
If your engine is modified you may want to go one step colder than stock. If your engine is very modified but still street driven you may want to go two steps colder.
I always find it difficult to determine how hot or cold a spark plug is compared to another. The people selling them never know what they are talking about and there is no useful information on the packaging.
Some have quite a long tip but are still rated quite cold. These extended nose types are very good because they act as a hot plug at low speeds and cool off to act as a cold plug at high rpm. The cooling off happens because of the in rushing air/fuel which blows past the extended tip.
The extended tip also puts the spark into a better position for igniting the mixture. Because they are hotter at idle they tend to not foul up as much as normal plugs.
The main disadvantage is that you may not be able to find one cold enough for your engine but it is worth a try. Also extended tips are not recommended where racing fuels are used or boost is present as with nitrous, blowers or turbos.
Another good plug is the Champion U-series which has a booster gap inside. A gap inside the spark plug is a good idea because the voltage has to build quite high before it can jump the gap and then jump the gap at the tip. Without the booster gap some of the current can leak away before the spark jumps due to deposits around the tip.
Resistor plugs are o.k. if you need them for suppression but your spiral core wires should be enough to prevent radio interference.
Autolite are quite good plugs too.
Stay away from expensive "trick" plugs such as Split-fire and Rapid-fire. These will work well at first because sparks jump from sharp edges more easily and all these plugs are doing is providing more sharp edges. But as they wear they become no more effective than a standard plug. It is cheaper to buy normal plugs and to change them more often. Some 'normal' plugs have sharp edges built in these days anyway.
Remember - plug selection is less critical if you have a good ignition system.
If you ever have the heads off your engine it is worth while checking to see how far the plug threads into the head. You do not want any thread showing in the combustion chamber either on the spark plug or the head as this can cause hot spots. The plug should be just flush on the inside and the side electrode indexed away from the squish area of the chamber by using indexing washers on the plugs although I think this is less critical on mopar engines than other makes because the angle of the plug makes the tip point at the squish area and not the side electrode.
THE PLUG GAP
The ideal plug gap can only be found by trial and error on any individual engine. A lot depends on your style of driving and the frequency of plug replacement.
First make sure that the side electrode is centered and flat across the tip of the plug and not at an angle.
Manufacturers recommended gaps are set to be adequate for the average middle aged driver on a cold day in a car that is in need of a tune-up and will be driven very easy in slow city traffic.
The best way is to increase your gap 10 thou. at a time and see how it goes for a while. If there has been an improvement in power or mileage over the last 1000 miles then try another 10 thou. Increases in plug gap will give improvements up to the optimum gap. After that, further gap increases will cause a deterioration.
To get you in the ball park I have some recommendations I found in a book which might help and are worth a try.
If you only use wide open throttle ( like on a drag car ) then a 10 thou. reduction in the recommended gap will give a 6% improvement in power at about 4,000 rpm, a 7% loss in mileage when driving slowly and a 25% increase in the life of your plugs.
If you decrease the gap by only 4 thou., you can add an extra 5,000 miles between tune-ups and you lose only 4% mileage.
Increasing the gap by 6 thou. over the recommended setting however will give you 4% better mileage, smoother operation and an increase in power of about 2%. But - you will have to change plugs more often and the rest of the ignition ( especially the leads ) will have to be in good condition in order to handle the increase in energy.
The after market 'computer' CD ignition systems allow you to increase your gap by about 25 thou. and will give you benefits in mileage, power and frequency of tune-ups.
