Special thanks to Philip Palmer for helping me compile this document.
Spark Plugs Q & A for the FD (3rd Generation) RX-7
Q. Why did we put this document together?
A. There appears to be some misinformation out there on what type of Spark Plug to run in your (non-racing) "standard" 3rd Generation RX-7. So we thought it would be a good idea to put this together. Of course - everyone has their opinions on spark plugs, however there is some good information here, so you may or may not change your mind by the time you get to the end of it.
Spark Plug recommendations from the Mazda Factory Manual:
|P - Denotes "Platinum" - which is the standard plug|
* - Standard Plug
Note the "L" and "T" underneath the plug number on the pics. above
Spark Plug "Temperature Ranges" and what they mean:
The lower the number, the "warmer" the plug. The higher the number the "colder" the plug. I.e. A heat range "7" plug, will run much hotter than a heat range "9" plug.
What does this mean?
A "warmer" plug cleans itself during normal running by burning off the excess carbon etc.. It will start cleaning itself quicker on start-up and can reduce the occurrence of the infamous \'Flooding\' problem.
A BUR7EQ plug has the electrode much higher up inside the plug, thus it runs far "warmer" than a BUR9EQ.
A "colder" plug is not able clean itself as well as the carbon build up conducts the high voltage, and the spark will jump down the insulator instead of the electrode - causing little or no ignition of the fuel under compression where the resistance\'s are much greater.
Note how much further up inside the spark plug the insulator is. This makes it run cooler than the "7" due to it being further up the inside of the plug protecting it from the heat.
Q. What do the Trailing spark plugs do?
A. These are to ignite any extra fuel that was not burnt from the Leading plug, on the Le Mans winning 26B 4rotor there were 3 plugs per chamber - this also reduces fuel consumption, and emissions.
Q. Why are there "colder" plugs in the Trailing holes?
A. The reason the trailing hole needs a colder plug is because the Leading has already ignited the fuel therefore the trailing is much hotter in the combustion chamber.
Q. How often should I replace the plugs?
A. This really depends on the way you drive your car. Generally speaking the rotary can be a heavy user of spark plugs, we would recommend changing them every 6,000miles at most.
Q. Why run a colder plug for racing?UPDATE TO THIS QUESTION 28/10/2003 - I had kind of covered this by the following answer here, but I thought I had better add in some information about high speed running. For high speed running even with 9\'s in the trailing and leading the conditions will be too extreme for the plugs and engine. If you are doing high speed running quite often then it is recommend you run really cold plugs - even up to 11.5\'s, in all four holes. But these of course can cause problems if you also do a lot of city driving as the cold plugs can foul (also see section below). You can also run 9\'s or 10\'s in the leading and 10\'s or 11\'s in the trailing (consecutively).
A. When you are racing the engine all the time, the engine itself is hotter - therefore it will be cleaning itself already.
You can certainly use 9\'s in all four holes is if you are doing 1/4 mile runs with a modified engine or racing round the track, in both cases the engine will be running far hotter than just normal road/motorway driving. Yes most people run 9\'s and yes they will say they don\'t have any problems but it doesn\'t mean it is correct does it? It will work and the car will boost fine as the plugs won\'t really make much difference to the actual boost unless they are badly misfiring.
Ultimately you should start-up an engine on warmer plugs if it is stone cold, then when warm put the colder plugs in. You will often see people doing this at a racetrack at the very beginning of the day.
Q. What is the best way to clean Spark Plugs?:
A. Use Carburettor cleaner - do not use a "Sand blast" type cleaner as sand will lodge itself in the plug.
Q. Problems with flooding?
A. Using the NGK BUR7EQP/BUR7EQ or BUR6EQP/BUR6EQ in the Leading spark plug holes is \'VERY\' likely to cure any flooding problems. Please note however, if your engine is low on compression you will still have starting problems - but running the warmer plug should still help.
Q. Are spark plugs country specific?
Yes - they can be, as colder and warmer climates effect the starting of the engine. However from reading the Mazda Factory Manual\'s for Australia, and the USA, and going on what a new FD RX-7 comes installed with from Japan (7\'s in the leading and 9\'s in the trailing) there is no difference to what is shown in the Table above.
Checking to see how your plugs are:
You can check to see how your plugs are by removing their plugs after a gentle drive round the block to see if the insulator is black with carbon or clean white. I\'m sure you will find the 9\'s really quite black and dirty, and ready to start to fouling up or even non start (Flood!).
Q. What does a fouled Spark Plug look like?
Pictured above is a fouled BUR9EQ from an (incorrect) Leading hole. Note: the "black" ( normally "white" ) insulator within the electrode. To check if a plug is fouled - this is the part you need to be looking at, not the outer (metal) part of the plug.
From the US and Australian "Factory Workshop Manuals":
From the UK "Factory Workshop Manual":
Below is a picture from the 1992 Mazda Factory Manual, the diagram on the left-hand side clearly indicates to use a BUR9EQP in the Trailing, and a BUR7EQP in the Leading. However on the right-hand side (in the table), it shows to use BUR9EQP in the Leading and BUR8EQP in the Trailing. We are 100% confident that this is incorrect as it is telling you to run a Warmer Plug in the Trailing - WRONG WRONG WRONG! (Must have been a misprint my Mazda - bit scary!)
Mazda and NGK have spent many many years developing Spark Plugs for the Rotary, and for day to day (non-racing) use - it certainly makes sense to go with what they recommend. Afterall - look what it reads on the box, and the plug itself (T=Trailing, L=Leading)!: