Simply stated, the ground strap on the trailing spark plug can get overheated and act as a glow plug. Gasoline auto ignites at 450 F. Backtracking to the comment re surface gap plugs recall that it was about motors making in the 500 or more category. Most single turbo setups, even the EFR8374 and the BW SXE62 are capable of making this power level around 20 psi boost. If you are running 400 to 450 and you run lean just a bit your motor will be at risk. If you have a bit of knock, this hugely elevates combustion chamber heat putting your motor at risk..
There is a reason Mazda went with the weird plugs... to eliminate small ground strap mass which eliminates pre-ignition and decreases knock.
The good news is I eventually found the correct plug.
Did you ever wonder why our cars came with this weird looking sparkplug?
Chances are if you are driving a somewhat modded FD the weird looking plugs are long gone, replaced if you are doing it right with a more traditional looking plug in the $30 price range. It will be colder, around 10 or 11 versus 7 and 9 and it will have a "regular" single ground strap. You may have gotten the part numbers off this site.
"settled science" or so i thought. All that changed a few months ago upon the receipt of an email from someone whom i consider to be one of the foremost rotary guys on our planet.
it read:
With any decent inductive ignition you need to run a full surface discharge plug in the trailing. If you don’t, you will get knock over approx 500hp.
I have listened to this person since 2012. He was highly accomplished in 2012 and nothing has changed in the 10 plus following years. Being a highly curious person i wanted to understand the "why" of it as well as zero in on the specific surface gap plug.
i am no stranger to surface gap plugs. When i took delivery from Roger Mandeville of his two season winning IMSA RX3SP he gave me a box of insanely expensive special (surface gap) NGK plugs. We initially used warm up plugs and then switched to the surface gap plugs for the race.
i attempted to get the actual (FD) plug numbers with a couple of additional emails but no reply. Racers generally don't like to give away the store. After a few calls to others and drawing blanks i queeried NGK. According to my response, there were no surface gaps for the rotary.
Google came back with a Champion but it was a 12 heat range race plug with no resistor. Autolite has a similar plug but, again, no resistor. No resistor, no deal as my logging Laptop (or Surface) locks around 5800 unless the sparkplug has a resistor.
In the middle of all this I asked another person I hold in high regard. He was quite interested and mentioned he always sends his motors out with the OE plug in the trail position as it is a "dangerous" plug location... hmmm, now we are into the "why."
Trail position dangerous?
The trail plug is the first to be exposed to the charge air. The key is the trail is exposed to a less compressed (than the lead plug) mixture. Less compressed mixtures ignite much easier than highly compressed mixtures.
As we make more power the ground strap gets hotter and can act as a glow plug. Our motors have no cooling cycle as do 4 cycle motors. Surface gap plugs have no ground straps to create pre-ignition.
If the easily (pre) ignited mixture is prematurely ignited huge pressure is created. Powerful expanding gas against a decreasing mechanical volume. Something has to give and it is the rotor or the front iron. Dented rotors come from pre-ignition. For instance:
SPARK PLUGS
I just fired my uprated motor and have only put a few break in miles on it with the new trail plugs. I do have an engine customer that has been running them for a bit around 500 and he reports they run no differently but show about a 15% knock reduction. The plugs are a 5/8 hex V the OE 13/16 21 mm. Thread length is 19 mm V 21 MM OE. This may explain why NGK didn't find them as a match. The 2 mm difference is not significant. Price is around $20.
UPDATE:
I ran the 10s in my trail position for a few weeks with zero observed changes as to behavior. i then received a very interesting email re the plugs:
"I just happened across your spark plug article. I actually stumbled across these plugs in an old Mazdacomp catalog a few years ago. They recommended the 10 or 11 heat range version for road racing FD RX7s in both leading and trailing. I actually use them in the leading and the trailing on my rotary swapped 808 wagon. I can say I feel that they work extremely well in the leading position. Being a surface discharge plug, they never seem to foul. Objectively, they crisped up the throttle response off idle as well as at lower rpm. I am running 20psi with water injection on a S366SXE on a half studded, (street ported), S4 FC block. I have had zero issues even while tuning the car myself and occasionally making silly mistakes. I also noticed a power difference using these plugs. I have no way to quantify it but, the butt dyno says so. I believe this is due to the way the spark projects outward from the surface of the plug and exposes more of the spark surface area to the fuel/air charge with no ground strap on the way. I am actually shocked that my lowly LS2 coils are able to fire these plugs off with (700cc of) water at 20psi with no issue (running 4.9ms dwell). To make up for the fact that the plugs are 2mm shorter, I remove the washer on the leading plugs. I checked this on a junk rotor housing I have and there is still plenty of clearance. I find these plugs to be much superior to the race plugs most people recommend for rotaries; R7420 R6725. I personally found these to not idle as well and foul much more easily.
I researched spark plugs a lot while building my car. I found the BUR9s in all 4 holes worked quite well up to 15psi. I figured if I was going to run more than that, I needed colder plugs and water. I was unimpressed with how the R7420-10 plugs worked. I found they would foul fairly easily if idling for any length of time. I also felt the made less power than the BUR9s at the same boost..... Butt dyno feel anyway. So I tried these R6601s and haven't looked back. They are relatively cheap and Ihave been running the same set for 2 years. My car gets about 3000km per year and most of them are hard, abusive, kilometers.... With a few street legal drags thrown in for good measure."
I found the comments quite interesting and decided to run the 11 heat range 6601s in all four positions. I removed the .05 thick crush washers so as to allow the plug to sit a bit deeper into the spark plug boss. After a few weeks i removed the plugs to check for any leakage and there was none.
It has been reported by some that they have experienced a more efficient burn, having to remove a bit of fuel from their map. I found that also to be true. Some have theorized that since the spark has a 360 degree choice as to finding ground it chooses the path of least resistance.
I currently have about 200 miles and 20 or so 3rd gear pulls on the plugs around the 500 hp area.
"With any decent inductive ignition you need to run a full surface discharge plug in the trailing. If you don’t, you will get knock over approx 500hp."