What Determines a Hotter or Colder Spark Plug

[CadenceScion]

For those of you out there that don't know, every spark plug has a rating. If you look at the serial number on the spark plug there will be a series of letters and numbers. To determine whether a spark plug is colder or hotter is simply this: The higher the last number is a hotter spark plug, and the lower the last number is a colder spark plug. Our Scion tC's (OEM) recommend NGK Spark Plugs and the spark plug gap is .044.

NGK's Website:

NGK Spark Plugs

[univision]

What are the consequences of putting in a hotter or colder spark plug than the recommended rating?

[CadenceScion]

More than likely the engine won't run correctly, burnt pistons/valves, backfiring, CEL's. I have never come across this, nor do I want to find out what happens either. I know a lot of useless info, but I am far from any expert. I want to say if you have a F/I application you'll want a step colder plug. To further elaborate on this matter would be asking for Navy's input.

[connman429]

In the days before computer controls, one would change the temp of the plug for efficiency purposes. Hotter plugs would ensure a more thorough burn in constant stop and go with low rpms, like a delivery vehicle. If you were doing mostly highway driving, in the upper half of your rpm band, you would want a cooler spark. Getting too hot would cause pre-ignition, lots of knocking, and eventually burn your valves. Too cold would get you unburnt fuel. Our ECM manages the mixture, so we have no need to go for a hotter spark, even if we never get past 3k rpms. You may see some benefit from running a colder plug if you are constantly running close to redline, but very few of us are doing 500 mile races in our tC's. My advice would be to stick with the OEM temperature range, unless you have modded your tC to the point of not really being one anymore.

[CadenceScion]

^ Thank you for your knowledge, in my years I have never found it necessary to run a hotter/colder plug in my vehicles, always stuck with OEM.

[navylife59]

Anytime that you raise the cylinder pressures via NOx, higher compression, or FI; you will need to run a colder plug to keep ignition combustion temp in check with the fuel quality used to prevent Ping and eventual knock.

[Zerobackup]

Maybe I should change mine on my tC. Will it affect it if I have a cold air intake?

[CadenceScion]

Unless your F/I or S/C, I wouldn't change anything. Although you can upgrade to a NGK Iridium Plug.