Thread: HOT spark plugs

yep, I put it down to the newer fuels.

series II and III engines are about the same.

I know nothing about plug heat.

What impact on the engine does it have going hotter plugs or colder plugs?

When would you use hotter plugs in an engine - likewise when do you use cooler plugs.

Thanks

Garry

Originally Posted by garrycol

I know nothing about plug heat.



What impact on the engine does it have going hotter plugs or colder plugs?

When would you use hotter plugs in an engine - likewise when do you use cooler plugs.

Thanks

Garry

Basically the insulator nose of a plug must be clean and non-conductive during operation, so:

Cold plugs do not easily burn off carbon deposits caused during cold starts. They also have trouble burning off oil. It's called "self cleaning" and modern engines rely on this happening quickly for good emissions. LPG engines like cold plugs as the fuel/air mixture burns slower and causes plug temps to go up. They don't generally need to self-clean if started on gas as gas doesn't run rich when cold. High performance engines run cold plugs happily as they don't spend much time idling warm or in traffic. When they do they foul up the plugs.

Hot plugs will cause pre-ignition in a big way when pushed hard as they will actually get to ignition temperature. Older engines need hot plugs to run well at all otherwise they foul up with combustion products. Leaded fuel ran hotter.

Most plugs have a "heat range" in which they run happily, but no one plug will suit all operating conditions. The heat range of plugs is generally determined by the length of the path for heat to travel from the insulator nose to the cylinder head via the seat. A poorly tightened plug will therefore act as a hotter plug, sometimes up to to 2 steps hotter on the plug chart. This usually leads to thread seizure as this is where the heat transfers to the head...

Multi-gap (surface-air gap) plugs can have a wider heat range application as fouling of the insulator will cause a smaller spark gap as the spark runs across the insulator nose. This heats the insulator nose more quickly. I have found them to be of little use with LPG though as their gap is usually not adjustable (down).

the rough rule of thumb is

as the ambient temperature/pressure goes down the plug heat goes up.

as examples if you go up over the snow line you should go up on the temperature rating of the plug because the air is both cooler and thinner so the tip of the plug needs to be hotter to initiate combustion more readily.

similarly if you come down the hill into summer in the desert where the air is hot dry and thick you need to come down the temperature range to help prevent pre-ignition.


Changing the cooling system may also require a change of plug temperature range.

There are some good tuts on the net for reading plugs but be aware many are old or written for racers running leaded or exotic fuels. Unleaded fuel is a lot harder to read fuel mixture. A powerfull magnifier with light is a must.

Heat range is pretty easy to read by looking at colour change on the earth strap (midway between tip/above electrode and the base/top thread is optimum). If your motor setup (timing and fueling) is stock then plug manufacturers recomendations are pretty close. However if not stock check plugs carefully. For instance if your running more advance (= more heat) you need a cooler plug ie for NGK use a #6 or cooler not a #5. Note: Colder plugs have shorter insulator noses. Be aware manufacturers use different codings ie NGK the lower the ref# the hotter the plug and Bosch the higher the ref# the hotter the plug.

I prefer buying cheaper good quality plugs and looking at them / changing them more often than using fancy plugs costing 4X more for no real gain (they last longer for emiision reasons and suit many modern engines that changing plugs takes far longer and PIA).

I have 4 WR7DC super plus plugs left over from our Volvo 240 that was written off by a texting teen early this year. They are the same size thread etc as the B6ES NGK plugs I have been running in my 2 litre series 1. I see they are 1 step colder than the B6ES but is there any reason why I shouldn't use them in my landy?
Mike