Science behind cracked plugs?

[ihatecats78]

So I've seen a lot of talk on here about cracked plugs caused by increased boost conditions. There seems to be some debate with regard to whether colder plugs (HKS) are needed, or our stock plugs can be used as long as they've been re-gapped.

What I'm curious about is the science here. Why do plugs crack when they misfire/blowout? Is there any advantage to using a colder plug as long as we aren't increasing boost beyond what most of the piggybacks are pushing?

 

[WhiteStinger33]

Cat hater, very good question. My understanding is that the various levels of heat dissapation designed in to the plugs dictates the heat level. Higher heat dissapation = colder plug design. It's my understanding that you would want plugs hot enough to burn up residue left on the tips of the plugs. If it's too cold there is a risk of crap building up.

I believe depending on the AFR of your tune and the specifics of the performance of your engine, you would need to select the heat range of your plug. This is done empirically by engineers at auto manufacturers and then through experiment and logic by people like you and me.


If I can add another science question to the community about plug modification:

What effect does a smaller electrode gap have in real terms when operating under lower boost in our cars? Do we need to sacrifice engine performance in some way?

I will be receiving the GTS Black chip in a few days and plan to gap the OEM plugs to .026"-.028" tomorrow so if nobody else has an answer I can provide some experience.

Sent from my Pixel 2 using Tapatalk

 

[FIM-92]

When i turned up the boost (in my Lancer Ralliart Sportback) from 18psi to 23psi i had to go two colder heat range on the sparkplugs. Same gap.

I have not seen the "cracking" OP speaks of...
Is it the ceramic near the tip (inside the engine) or the ceramic outside the engine?
Do you have pix?

 

[mldavis2]

A lot of spark plug "science" is trial and error. The plugs transfer heat into the heads by thread contact. If you increase the boost pressure, it gives more oxygen for more fuel, and hence more power. But a more dense fuel/air mix requires more coil power to fire the spark across the plug gap in a boosted engine. When the coil pushes current to the plug and the mixture is so dense that it strains the electrical charge, the charge looks for the easiest path to ground which can become along the outside of the ceramic top. This creates a carbon track which can then can cause cracking of the plug tower. Or it can back up and damage the coils.

This is not so much a problem in a racing engine -- you simply increase the coil capacity, run "colder" plugs and find that magic gap that works best. On a DD, you can have problems because you want a nice, smooth idle with no misfiring as well as increased performance from additional boost when you ask for it. If you run colder plugs to accommodate the additional boost, you risk fouling at normal driving speeds. The extra boost also puts a strain on the coils and can cause them to burn out as well.