NGK spark plug heat range and part numbers by engine

Half the spark plug orders we take start with a part number someone found on a forum. Sometimes it fits, sometimes it is two steps too cold and the car fouls a plug in the first week of commuting. Heat range is the part that trips people up, and it is simpler than the numbering makes it look.

The short answer

• Heat range describes how quickly a plug pulls heat out of its own firing tip. It has nothing to do with the spark itself.
• With NGK, the bigger the number, the colder the plug. A BKR7EIX runs colder than a BKR6EIX.
• Add real boost and you usually want one step colder than factory. Two steps is for high boost, E85 or a car that lives at full load.
• Jumping three steps colder to be safe is the classic mistake. Cold plugs foul in traffic and the car runs worse, not safer.

How heat range works and what the numbers mean

Every plug has to run hot enough at the tip to burn off oil and fuel deposits, and cool enough at full load that the tip never becomes an ignition source of its own. A hot plug holds heat in the insulator nose. A cold plug moves that heat into the cylinder head faster.

Under boost, cylinder pressure and temperature climb hard. A plug that was correct on a stock engine can start glowing at the tip, and once it does it lights the mixture before the spark does. That is pre-ignition, and it wrecks pistons quickly.

Go too far the other way and the tip never gets hot enough to clean itself. The plug carbons up on a cold morning commute, you get a misfire at light load, and everyone blames the coils.

The letters in a part number are not the heat range. They describe thread size, reach, seat type and electrode design, which is why you cannot swap a BKR for a BCPR just because the number in the middle matches. Get the whole part number right, not just the digit.

A trailing number, like the -11 on a BKR5EIX-11, refers to the gap the plug is supplied at. Two plugs with the same heat range can arrive gapped differently, so check before they go in.

Picking the right plug for your build

Stock to moderate boost

The BKR6EIX is the workhorse of this range. We list it as a set of 6 for RB25DET and 2JZGTE, and as a set of 4 covering SR20DET, EJ20, 3SGTE, B16A and B18C. Singles are available too if you only need to replace one.

Higher boost and E85

Step to the BKR7EIX-11 when the tune is asking for more than a mild street map. On the Mitsubishi side, the BPR7EIX set of 4 suits 4G63, the ILFR7H set covers the 4G63 in an Evo IX, and the ILKR8E6 set is the one for 4B11.

Factory replacement on later engines

Later Subarus take the ILFR6B set of 4. A VQ35DE runs the PLFR5A-11 laser platinum set of 6, and a BP 1800 MX-5 takes the BKR5EIX-11 set of 4.

Rotaries

Rotary engines use different plugs front and rear, so buy the set rather than assembling one. We list a laser iridium set for the RX8 and a laser platinum set for the FD3S RX7.

Race only

The BR8EIX, BR9EIX and BR10EIX are non projected tip plugs built for high boost race engines, and the R6725 and R7420 racing plugs go a step further again. They foul on a street car doing short trips, so only run them if your tuner has asked for them.

Here is the quick version, using only the sets we actually list against an engine.

Browse the full vehicle specific sets if your engine is not in that list, or the individual plugs if you are building a set for something unusual.

What to buy with it

A colder plug asks more of the ignition system. Tired coils that coped with a wide gap and a warm plug will start dropping spark once cylinder pressure goes up, so look at ignition coils if you are already chasing a misfire under boost.

If the car is running an aftermarket ECU, the ignition accessories range covers the leads, ignitors and adaptors that usually go missing during a coil conversion.

Fitting water or methanol injection at the same time? Read the water/methanol injection guide first, because the two decisions interact. A wideband helps here too, and our air fuel ratio gauge guide explains what to watch when you pull a plug to read it.

Where NZ Performance fits

We keep the NGK range in stock in New Zealand, with the full plug catalogue covering iridium, laser platinum and racing plugs. Orders ship from Palmerston North, so a set ordered today is not waiting on a container.

If you are still deciding how much boost the car will see, our turbo size and fuel system guide is worth reading before you commit to a heat range. Pick the plug to suit the tune you are going to run, not the one you have now.

FAQ

Does a higher NGK number mean a hotter or colder plug?

Colder. In NGK numbering the number climbs as the plug gets colder, so a BKR7EIX runs colder than a BKR6EIX and a BR10EIX is colder again. Colder plugs shed heat from the tip faster, which is what a high cylinder pressure engine needs.

How many steps colder should I go when I add boost?

One step covers most street cars running moderate boost on pump fuel. Two steps starts to make sense on high boost, E85 or a car that spends real time at full load. Going colder than the engine needs just fouls plugs in traffic.

What plugs suit an RB25DET or a 2JZ-GTE?

We list a BKR6EIX iridium set of 6 for RB25DET and 2JZGTE. An RB20DET or RB26DETT takes the BCPR6EIX set of 6 instead, so check which block you actually have before ordering.

Do I need to regap iridium plugs?

Check the gap, do not lever on the electrode. Iridium tips are fine and easy to damage, so use a proper tool and adjust the ground strap only. Many tuners close the gap slightly for high boost, so ask whoever is writing your map what they want.

How often should I change plugs on a boosted car?

More often than the factory book says. Boost, ethanol and a hard tune all shorten plug life, and a tired plug shows up as misfire at high load long before it misbehaves at idle. Plenty of our customers carry a spare set for track days.