Spark Plug Thread


So make things a bit simpler for everyone and help first timers, I thought we could compile a list of Rotary-Friendly spark plugs, with information about their cost and downsides. Any info anyone has to add to this of their experiences please pipe up! ;)
General spark plug information;
Heat range: (Normally 9, 10, 10.5, 11, 11.5) The heat range number refers to how well the spark plug absorbs heat away from the plug and into the surrounding engine. A low number plug (e.g. BUR9EQ) will keep a lot of heat in the plug, this is good for burning off deposits that build up on the electrodes, aswell as making an engine easier to start. A high number plug (e.g B11EGV) draws a lot more heat away from the plug and as a result can handle much higher running temperatures (high revs/boost/power) without cracking and causing damage or ignition breakup, but does not get hot enough when cruising about to keep itself clean, leading to fouling and poor starting.
The best way to choose what plug you need is to decide on your power goals and how the car will be driven. The APPROX POWER LEVEL LIMIT below should give you an idea of when you need a colder (higher number) spark plug, but the real deciding factor is how long and how hard the car is a driven. Other factors like ignition system, water injection, boost level, etc contribute.

n.b. Different brands have different ways of measuring heat ranges, for this thread we are presuming we are dealing with NGK plugs,
although hopefully there may be other brands of high performance suitable plugs people have tried?
Staggered Spark Plugs: It is also quite common, and in many cases recommended, to run a hotter (lower number) plug in the Leading spark plug holes (That's L for LOWER and for LEADING) and a cooler (higher number) plug in the Trailing spark plug holes (That's T for TOP and TRAILING). For example, if you have a 360hp RX7 that you drive to work everyday, but then also take to the track and thrash every weekend, then you might want to run 9-Rated plugs in the Lower (Leading) spark plug holes, and 10-Rated plugs in the Top (Trailing) spark plug holes. This would give you the driveability and reliable starting from the 9 Rated plugs on the cold pootle to work, and the 10 Rated plugs to help draw the heat away and not cause issues when things get hot on the track.

Gapping: Gapping refers to the gap between the electrodes on the end of the spark plug. These often need to be adjusted depending on the application/vehicle to allow the spark to properly ignite the fuel/air mix. The ELECTRODE GAP column below will give you an idea of what to set the gap for best performance on a rotary. See HERE for more information about properly setting spark gap.
n.b. Standard ROTARY NGK plugs are flat-faced, fixed electrode type plugs (on the left), the conventional 'side-electrode' plugs (on the right) can be used in a rotary, but must be carefully chosen to avoid them poking too far into the engine -which is what this thread deals with!
so here's the format;
STOCK NGK ROTARY -These are the flat faced, fixed electrode plugs designed exclusively for the Mazda Rotary engine
BUR9EQ - Standard flat face mazda rotary plugs - N/A (gap is not adjustable) - 350hp - £7.94
BUR9EQP - ^ but Platinum and with a longer tip reach(21.5mm over 19mm). Not to be used with Methanol/Race fuel - N/A (gap is not adjustable) - 350hp - £9.36
- A motorcycle/kart iridium tipped plug. resistor (R), iridium tipped (I), Booster Gap (X). Varied reviews, reports of poor durability; insulator cracking and electrode damage occuring in rotaries? Delivers less spark to the charge because of resistor, but provides a clearer signal for ECU's/Ignition Amps. Poor lifespan
BR9EIX - ... - .030 to .040 - 100hp!? - £5.90
BR10EIX - ... - .025 to .040 - 100hp!? - £5.90
- Designed for snowmobiles/motorbikes/karts. Doesn't use a resistor so gives a hotter spark, but recommended against using with Ignition amps, and can cause bad rpm readings on standalone ecus (although many reports seem to suggest no issues). Has a Gold palladium centre electrode for racing use (GV). Can be a pain for removal/installation - requires thin walled or modified spark plug socket. Average lifespan
B9EGV - ... - .022 gap - 350hp - £7.94
B10EGV - ... - .022 gap - 400hp - £7.94
B105EGV - ... - .022 gap - 450hp - £7.94
B11EGV - ... - .022 gap - 500hp - £7.94
- Similar to EGV range but recessed electrode, platinum tipped and allegedly x4 lifespan. Very similar in design to Greddy/NGK Racing plugs.
B9EGP - Remove washer to make more power? - .??? - 350hp - £15.27
B10EGP - Remove washer to make more power - .??? - 400hp - £15.27
B11EGP - Remove washer to make more power? - .??? - 500hp - £15.27
NGK V-Power Racing Plugs
- A more expensive plug recommended for (high) boost engines. Because they are designed with rotaries in mind (unlike the EGV's, EIX), they give better efficiency and power. Due to the design and Iridium element it is possible to use a slightly colder plug for extra safety (e.g. a R7420-10, where you would have had to use a BUR9EQ before) Durability is good with these plugs so (provided proper tuning) plug changes are less frequent. Contain resistor, so are safe for use with Ignition amps.
R6725-9 - Older style NGK racing plugs - .??? - 350hp - £21.98
R6725-10 - Older style NGK racing plugs - .??? - 400hp - £21.98
R6725-105 - Older style NGK racing plugs - .??? - 450hp - £21.98
R6725-11 - Older style NGK racing plugs - .??? - 500hp - £21.98
R7420-9 - Rebranded Greddy Racing Iridium Spark Plug Pro - .??? - 350hp - £21.98
R7420-10 - Rebranded Greddy Racing Iridium Spark Plug Pro - .??? - 400hp - £21.98
R7420-11 - Rebranded Greddy Racing Iridium Spark Plug Pro - .??? - 500hp - £21.98
DENSO Iridium Racing Plug Range
- These plugs feature thin electrodes (0.4mm + 0.8mm) to minimise erosion and carbon fouling, and an inbuilt resistor for cleaner electronics feedback. Direct competitor to Greddy Racing Iridium plugs. Same as rebranded HKS M-series Super Fire Iridium Racing plugs??
IRE01-27 (NGK 9 Eqv.) - ... - .028 - 350hp - £26.62
IRE01-31 (NGK 10) - ... - .028 - 400hp - £26.62
IRE01-32 (NGK 10.5) - ... - .028 - 450hp - £26.62
IRE01-34 (NGK 11) - ... - .028 - 500hp - £26.62
IRE01-35 (NGK 11.5) - ... - .028 - 550hp - £26.62


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Here are the images that were referenced in the original thread from the old FDUK forum - showing the difference between fixed electrode (rotary only) and adjustable (rotary suitable) spark plugs;


..and the different plug heat ranges compared across the brands;

Nice one Craig for bringing it across(y) Be interesting to see if anyone has more info to add about recommended spark plug gaps?
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Just thought I'd add a couple more plugs I've come across/missed since the original post way back in 2010.. Please add any of your experience or gap settings with these (y)(y)o_Oo_O

NGK BR-##-EG RANGE - Similar to other EG plugs but crucially with a 5kohm resistor, making it safer to use with ignition amps, smart coils, etc. Only goes up to 10 rating.
BR9EG - Remove washer to make more power? - 0.020 to 0.024" - 350hp - £6.66
BR10EG - Remove washer to make more power - 0.020 to 0.024" - 400hp+ - £6.66
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AUTOLITE AR393#X RANGE - Appearance similar to rotary-specific plugs - no ground protruding that can fall off into the engine (as has been reported with EIX and some other piston plugs). Well proven in many racing series in the US.
They don't come with in-build resistors so may cause noise/interference issues. Ideally not used with ignition amps (Twin Power).
*Potentially may feature too large a spark gap for big power/heavy WI builds - as the electrodes are fixed there is no way to adjust it so the non 'X' version may be more suitable.


AR3934X (NGK 8 heat range) - Rotary-style fixed electrode - ... - <300hp - US IMPORT
AR3933X (NGK 9 heat range) - Rotary-style fixed electrode - ... - 350hp - US IMPORT
AR3932X (NGK 10 heat range) - Rotary-style fixed electrode - ... - 450hp - US IMPORT (Pack of 8 around £65 inc. shipping + import duty)
AR3931X (NGK 11 heat range) - Rotary-style fixed electrode - ... - <500hp* - US IMPORT
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AUTOLITE AR393# RANGE - Non 'X' model, traditional protruding, adjustable electrode. Seemingly comparable quality to NGK racing plugs, some concerns with more insulation (porcelain) exposure causing fragmenation risk during a detonation event.. Does not feature a resistor so may cause noise issues or interference with ignition amps such as Twin Power.

AR3934 (NGK 8 heat range) - ... - 0.022 to 0.028" - <300hp - US IMPORT (Pack of 8 around £45 inc. shipping + import duty)
AR3933 (NGK 9 heat range) - ... - 0.022 to 0.026" - 350hp - US IMPORT
AR3932 (NGK 10 heat range) - ... - 0.020 to 0.024" - 450hp - US IMPORT (Pack of 8 around £45 inc. shipping + import duty)
AR3931 (NGK 11 heat range) - ... - 0.020 to 0.022" - 550hp - US IMPORT

AUTOLITE AR392# - RESISTOR RANGE - As the non 'X' model above, but featuring a resistor for cleaner signals and better combability. Sadly only available in warmer heat ranges.

AR3924 (NGK 8 heat range) - ... - 0.022 to 0.028" - <300hp - US IMPORT
AR3923 (NGK 9 heat range) - ... - 0.022 to 0.028" - 350hp - US IMPORT
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Ngk 5671a-10 are a good plug same reach and thread and dont foul easily either and I think you can get them in heat 9 and 11's also and theyre nice and cheap compared to some racing plugs


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Ignition problems and fuel problems destroy engines. Would not risk them autolite cheap plugs in my car.

I use the densos 100 quid a set. So be it. Cheaper then a engine rebuilding.