Colder Spark Plugs


What are colder grade spark plugs?

A colder grade spark plug is one that transfers the spark plug's heat quicker to the cylinder head.


Fuel and air goes into the cylinder. This is ignited with a spark plug and goes bang. This bang generates heat which is transferred to several components including the spark plug. Several design factors affect how the spark plug "retains" it's heat. If the plug dissipates the heat from itself quicker than another plug, then it is a colder grade plug. If it dissipates the heat slower, then the plug is a hotter plug.

Yeah, so what!

If you modify your engine e.g. increase boost pressures then the engine will run hotter. Therefore, the standard spark plug will run hotter as it can only dissipate so much heat into the cylinder head. Now consider a hot spark plug and a fuel/air mixture waiting to go bang. Normally, the mixture will ignite in a controlled fashion i.e. when the spark plug sparks, but as the tip of the plug may be too hot the mixture may ignite prematurely; resulting in detonation and maybe a broken engine. If you are lucky then only the plug will melt/disappear! (I've seen it happen).

If we take a colder plug then this will conduct heat away from the spark plug tip at a greater rate, therefore the plug will be physically colder, thereby reducing the possibility of detonation.

This all sounds good. How do I know that you know what you are talking about?

Click here - NGK Website

What grade plug do I need to run?

Popular decision is this...

Plug Heat range Boost pressure My 2p...
(Stock 165 platinum plug)
5 ~ 7psi

Okay for stock 165 boost pressure but expensive, consider a BKR6E instead. Note only the 165 uses a Grade 5 plug as standard...

BKR6E 6 8 - 14psi

The stock heat range for a 185 and 205

BKR7E 7 15 - 20psi

I have had no problems running these at 15psi+ for >4 years up to 300hp, on a daily driven car

These plugs will run fine on a mildly modified GT4 and will not foul

BKR8E 8 20psi - ?

Big pressures!

That's a lot of pressure...

This grade of plug will foul in a mildly modified engine, do not fit to a mildly modified engine


Why run copper plugs instead of the recommended platinum plugs?

What about the spark plug gap?

The standard gap I have found is fine - 0.8mm.

Can I go too cold on the plug?

Yes. There is a happy medium that you need to find. the spark plug needs to run hot enough to burn off all the carbon/crap, but cold enough to prevent detonation. As stated above, running a grade 7 in a mildly modified 185 and 205 works fine.

What if I get it wrong and get a plug that's too cold?

You'll get misfires and the spark plug will foul up with crap, this will be occur pretty soon if you do mostly in town driving, or lots of small journeys.  If you have problems then examine the colour/condition of your plugs. It's always a good idea to periodically inspect your plugs to see what's going on... If it does foul then simply remove and clean or just fit another plug.

Anything else?

If you are increasing your boost then the spark will find it harder to jump across the gap. To help it across the gap get a new rotor arm, distributor cap and maybe better leads, however I've found that good condition stock leads will be fine for 300hp+. While you're in the "ignition mood", check the engine earths are in good order to ensure you get a good engine earth return path for the spark.