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@vini_i gave a great answer to my previous question here (What is spark plug heat range)

Part of that answer is that the reason for spark plugs having different heat ranges is that if it runs to hot you can get pre-ignition, and too cold you get deposits on the plug which are supposed to burn off. So thinking about it, some other questions came up:

  • What symptoms would you see (just driving the car) if you are running to cold a plug?
  • What would the spark plugs look like if you are getting either pre-ignition or deposits?
  • What does a plug running in the butter zone look like for a car engine, and also for a small two stroke engine like a snow blower or lawn mower?
  • Different engines require plugs in different heat ranges. What characteristics of those engines create the need for the different ranges?

Thanks in advance for any info!

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If working on a stock vehicle or small engine just replace the plug with what it calls for. Someone has already done all the testing and research for you for the correct plugs. Your not going to strap a super charger to your lawn mower or string trimmer.

You should only think about changing away from the standard plugs if your car is modified. Also you should mess with the heat range only after the fuel mixture has been figured out. This is because a fuel mixture that is off will make it look like there is a heat range problem.

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  • Very simply if a plug is too cold it will foul. It will look black with soot. You will notice hard starting or misfires under load. A plug that is fouled from too rich of a mixture will look very similar to a plug that is too cold.
  • A plug that is too hot will look white, very white. In very sever cases the preignition will actually damage the plug, melt the center or side electrode, or even shatter the porcelain. The simptoms will sound like ignition knock or pining, what every you want too call it. If the fuel mixture is too lean the plug will also look white because a lean mixture burns hotter.
  • If the heat range is just right the the plug will look mat or tan in color.

    The head design is the biggest factor in what the heat range of the plug will be. The thermal design of the head will dictate how quickly it can wick away that pesky heat. For example if two engines are exactly the same with one having a cast iron head and the other with an aluminum head, the aluminum head will need a hotter plug. Aluminum conduct heat faster than cast iron. The coolant flow through the head can also affect the heat range. A head with better flow requires hotter plugs.

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  • Never a good thing to say things like "strap a blower on your lawn mower"... You never know... I'm smiling just thinking about doing that.. :) A pointless waste of money, but so much fun to make it work.. And probably blow past the rings and pop the engine in the process, but details, details. – cdunn May 24 '17 at 21:48
  • @cdunn But where are you going to find a baby super charger? – vini_i May 25 '17 at 12:52
  • Details, maybe uys time for a lawnmower with a straight 4 engine :) I'm not about to spend money on this, but it's a fun thought – cdunn May 25 '17 at 16:06
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You asked

What symptoms would you see (just driving the car) if you are running to cold a plug?

You would begin to get carbon on the insulator of the plug. Due to plug dispelling heat at a faster rate than a hot plug the insulator would not stay hot enough to keep the carbon from building up. Eventually the plug might foul if the range was too cold. Fouling occurs when carbon coats the insulator portion of the sparkplug from the electrode in the insulator down the insulator to the threaded body inside the sparkplug. When the plug receives power from the coil the electricity runs across the top of the insulator in the carbon which is a conductor. At this point, there is no visible spark when the plug 'fires'

What would the spark plugs look like if you are getting either pre-ignition or deposits?

The plug could get a buildup of various deposits or have the electrode melted away based upon the cause of the pre-ignition. Cross-firing through plug wires can cause dramatic after effects on spark plugs after extended use under those conditions.

enter image description here

What does a plug running in the butter zone look like for a car engine, and also for a small two stroke engine like a snow blower or lawn mower?

It should like tan and the electrodes should have an intact look to them. In other words, physically they should have the same appearance as new but the slight combustion buildup and color are the give away that it's a used plug.

enter image description here

Different engines require plugs in different heat ranges. What characteristics of those engines create the need for the different ranges?

What drives sparkplug heat range selection

Characteristics of the engine such as cam lobe shape, crank type and amount of cylinders don't typically drive plug heat range selection. Things that do drive heat range selection are frequently on the list below.

  • Forced induction engines with a turbo or supercharger will drive a higher volumetric efficiency which can account for a colder plug selection to dispense heat more rapidly from the combustion chamber.

  • Naturally aspirated vehicles with a reduced quantity of fuel and oxygen could justify a cooler plug due the decreased need to dispense with heat due to reduced oxygen and fuel in the overall combustion charge.

  • High RPM engines that run in extreme rev ranges will have more combustion cycles per minute than a low RPM engine, due to increased heat a hotter plug would be required to, again, dispense with heat being converted from more air and fuel in a shorter period of time than a low RPM engine.

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