(Almost) Everything You need to know About Spark Plugs

So I’ve had a question that’s been bugging me for a little while concerning spark plugs. So I know that there are different temperature plugs, but I can never figure out when to know to know what plugs to run. I keep hearing that people are running 08 or 09 or 10 plugs, but I have no idea.

Does anyone know a bit more about Spark Plugs that could help?

1 Like

09 will run hotter than a 10 so you would perhaps use them in cooler wet races.

depending on the brand of plug, one brand 1 is cold and 10 hot. Another brand starts as 1 is the hottest. So numbers from one brand to the next is not apple to apple. ALSO, as HOW I stated that is incorrect if you really want to get to the inner workings of A spark plug. The number is NOT how hot A plug fires at…Its how much HEAT is retained in the plug. A colder plug will disperse/transfer the heat faster to the head keeping it cooler. A hotter will not transfer it as fast And warm up faster and stay there. Go to A parts store and look at too extreme hot and cold next to each other. In front of each catalog autolite or champion they explain what, and how A plug works. It will even tell you how to read them.


NGK are the ones I’ve used whenever I can…

Lower numbers = Hotter plug
Higher numbers = Colder plug

Typically on a TaG you’ll probably be on a 10 unless it’s wet, you’re running suuuper rich or you’re breaking in the motor. In those cases you might run a 9. On really hot days you may use an 11, probably not on spec PRD though.

Another thing. “R” on the NGK plugs does not mean “Racing” it means resistor. In that the plug contains a resistor. Ideally you want a resistor in the plug or the lead, but not both. Check your plug cap to see if it is a resistor cap.

The next thing is the composition/metallurgy of the plugs. For racing you’re looking at EV, EG, EGV, and EIX.
I think EGV is less common in the US though.

I usually have a couple of EVs and EIX’s on hand. The EV’s are cheap as chips and great for plodding around on in practice. Then I’ll probably put in a new EIX when it’s showtime.


NGK Spark Plug Model Decoder


NGK Plug Types and differences.

Note the heat range is not defined by the plug type. Heat range is specified by the number.

1 Like

Information regarding the “R” designation from NGK’s website.


A: NGK “R” or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking.

NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.

They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer.

In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer undesirable side effects such as an erratic idle, high-rpm misfire, engine run-on, power drop off at certain rpm levels and abnormal combustion.


This is a great guide by Gordon Jennings. It’s old, but gold.

1 Like

Spark Plug heat range cross ref chart.


Hi…as per my knowledge to resist cold fouling and avoid overheating the tip of the spark plug should run a temperature between 450 and 800c
Hot spark plugs resist cold fouling but risk plug over heating and pre-ignition Cold spark plugs run cooler. Manufacturers produce a wide “heat range” of spark plugs to produce the most suitable plug for any engine.

printed circuit boards

1 Like

That’s my understanding as well

Colder plug: Transmits more heat to the cylinder head in order to stay cooler.
Hot plug: Retains more heat, transmits less to the head.

Typically you can gauge the relative heat range of the plug by looking at the insulator core.
Longer insulator core, more protruded into the cylinder = hotter plug
Shorter insulator core, less protruded into the cylinder = colder plug

Here’s a comparison of two NGK plugs in the EIX (fine wire iridium) series.

BPR6EIX “Hotter” plug. (Suitable for OHV four stroke output levels)
Note how much of the insulator core (white part at the firing end) protrudes into the cylinder

BPR10EIX 'Colder" plug (Suitable for higher output, KZ, OK and certain 100cc on hot days)
You can see the insulator core barely protrudes at all

So based on this, a Denso IW31 / 5319 plug should be a temperature like for like plug with a NGK B10EGV?

I currently race a senior X30 where both the BR10EG/BR10EIX and the NGK R6252K-105 plugs are allowed. I noticed that the R6252K plug is suggested here, which is usually my go-to site for X30 tuning. Is it actually a better plug than the BR10 or its BR10EIX iridium variant, or is the suggested plug due to the SKUSA regulations?

The latter. The BR10EG is allowed, but only during rain conditions.

What about the gap? How can i identify what is the right one

What plug is usually run for senior KA100?

These are the only legal plugs for KA:
NGK B10EG, NGK BR10EG, NGK R6252K-105 & NGK 6254-105

1 Like

Saw that in the IAME rules. Seems like they missed the “E” in the 6254 model, since I don’t see that plug anywhere.

Would somebody explain to me the purpose of making spark plugs a tech item?

Maybe, we’ve got KA’s from 3 different builders and the only plugs any of them have used have been the B10EG or BR10EG

Heat range could slightly effect performance and a longer throw on a plug could extend into the head and consume combustion chamber volume increasing compression and performance at the same time. That’s why they standardize what the plug needs to be.