what plugs?

340mopar said
Changing the heat range of a plug does not change how well the fuel is burned. The heat range of a plug is related to how well it transfers heat from the tip to the body of the plug then cylinder head. A hotter plug has a longer electrode tip so it keeps the tip hot enough to keep it clean. A colder plug has a shorter tip so it transfers heat easier to the plug body then to the head so it runs cooler in the same given motor. It is recommened as a general rule that with every 75 - 100 change in horsepower you run one step colder plug. The reason is that a higher HP motor will generate more heat and will overheat a "stock" heat range plug. The colder plug will allow the plug tip to run at the correct heat level without overheating it.

Like stated earlier too hot of a plug can cause engine damage from detonation. If you use too cold of a plug the plug tip won't get to the right heat level to keep it clean so it will foul prematurely. So it is much better to start with too cold of a plug and work your way up until the fouling is minimized.

I run a plug 4 heat ranges colder in my 340 because I am at about 550hp.

Chuck

No and yes. The premature fouling you mention is caused by the poor or incomplete burning of the fuel that I mentioned leaving deposits on the plug which is a result of the plug being too cold as I mentioned and not getting to the proper temp as you mentioned. What I said about too cold a plug affecting the burning of fuel was indeed correct as is what you said about the result. This is also true with a plug that is too hot which causes premature burning of the fuel so heat range (how well a plug dissipates heat, a plug does not generate heat) does indeed affect the burning of fuel. That is in fact part of it's purpose- to control the temperature at which the fuel is burning by dissipating some of that heat through itself to the head as you said. The optimum plug is the one that keeps the fuel burning at the temperature that results in the most efficient, i.e complete, burning of that particular fuel in that particular engine. If you change back and forth between pump gas and race gas you will have to run two different plugs depending on which gas you are running because they do not burn best at the same temperature. Pump gas burns hotter. That is why it is important for those of us who mix the two (as I do) to be consistant with the mix. Bottom line is that the best way to know which plug is best is to read the plugs. The "two thread" method I use is something I learned from a local MOPAR tech who has been building race engines since the 60's. FBO uses the same method in the article that mullinax95 posted. As you said, it is best to start too cold because that does not run the risk of damage that starting too hot does.