Spark Plug Questions

Back in the days when everything had carbs, you used to hear about switching to hotter or colder spark plugs.

What exactly does this mean and when would you move from one end of the scale to another (i.e. running rich or lean, etc.)?

hash include "std_disclaimer.h" I'm a Doctor, not an Automotive Engineer.

Well, heat range refers to how much heat is pushed into the cylinder versus into the head. Colder plugs put more heat into the head instead of the combustion chamber. Colder plugs will foul out easier. Hotter plugs will tend to ping easier. In general, you want the coldest plug that won't foul out. I have use the next heat range hotter to "get by" on a too-fat carb until I could rejet it properly. I've use the next heat range colder for a little bit more power, although how accurate the butt dyno is can certainly be debated.

The easiest way to pick the right heat range is to buy a set of Champions in your stock heat range and in the ones hotter and colder, then swap them out and see which heat range you like. Then go buy that heat range in NGK. The Champions are so cheap, they're practically free. Especially if you have a 2 or 4 cylinder that you're working on.

Also note that different manufacturers have different patterns in their numbering schemes. A 1 might be hotter than a 2 in one manufacturer and colder in another. You have to cross them or find a chart.

The electrode in the center of the plug and where it attaches to the side of the plug determine the heat range:

You want a "cooler" plug when your engine is working hard so that the combustion chamber chamber cools off. You want a "hotter" plug when your combustion chamber never really heats up - say you idle your engine a lot.

This isn't specific to carbs and you can use plugs to "treat" certain engine problems. Say your engine is getting a little oil in the combustion chamber (from weak rings or through the PCV), then you can change to a hotter plug to burn the oil off the electrode and keep it from fouling.

Then, if you're running lean, you'd want a colder plug?

RobL wrote: You want a "cooler" plug when your engine is working hard so that the combustion chamber chamber cools off. You want a "hotter" plug when your combustion chamber never really heats up - say you idle your engine a lot.

so cooler for heavy use..?? would a CRX used only for auto-x's and track days be considered heavy use... ?(also drives to and from those events on it's on)

Yes. Of course, you want to fix the lean condition first...

edit:

and yes, for track day/autocross/drag strip use I would swap to a colder plug and then swap back for normal driving.

Cold plugs = less detonation.

And yes, the wrong plug will make your car no work.