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Spark Plugs may be more important as to engine longevity than we think.
there is a spark plug thread in the Single Turbo Section but the meat of it applies to all our turbo'd 3rd Gen rotaries.
ESPECIALLY WHEN IT COMES TO APEX SEAL WEAR
no doubt you are aware of "warped" apexseals. current best thinking is that rather than warping, the seals are wearing. the wear is a very shallow depression in the crown centering on the... center and extending outward but not quite to the ends of the seal. while it looks like the seal is bowed if you measure it carefully, which i and others have, using a digital micrometer, it is clear there is a thou or two or three of height missing in the center. the metal does not vaporise, rather it is swept off the crown and forms a ledge on the edge of the seal. you can shave w it.
there are mechanical items that cause the wear.
Spark Plug Mountain (SPM)
Exhaust Close Ridge (ECR)
on a new build the apex seals are dead straight and the rotor housings are dead straight.
run the motor and certain areas of the rotor housing become hotter than the neighboring areas. the area around both spark plugs and the closing (top) lip of the exhaust port.
this happens on every motor, it is just a question of degree.
as the heat around the spark plug rises the metal surface adjoining the plug MUST expand. the easiest path is upward and we get SPM. the same at the exhaust port.
all motors after being run will show a lateral carbon line/witness mark at these 3 locations. the only diff being the degree.
the lateral line of carbon means the apex seal is being LIFTED off the surface of the housing. when this happens numerous things happen, all bad.
the seal between 2 adjoining rotor faces is non existant. lost exhaust power, polluted incoming charge air
CHATTER... a big deal. look carefully at a used rotor housing and see chatter marks
apex seal wear
all of the above is existent on all of our motors
the key, and the reason for this post, is that the degree can be significantly reduced.
heat transfer away from the spark plug boss is the name of the game.
COLD PLUGS transfer heat. hot plugs raise SPM.
our cars came w 7 and 9 plugs, as we all know... many have switched to 10s. which is a big help
i recently switched to 11s and i find they run no differently than the 10s.
here is a pic of them from yesterday. note that the annealed portion of the ground strap is slightly more than half the strap length. this indicates that they are not too cold. too cold is when the strap is less than half annealed.
to reiterate, the colder the plug the less wear on your apexseals.
you may notice the other plug... it is a surface gap NGK 11. resistor. $22. it has come to my attention, a few months ago that a non ground strap plug in the trail position is essential at the 500 hp area to prevent pre-ignition and detonation. the trail sees a less compressed incoming mixture which is much more easily (pre)ignited by a glowing groundstrap. that's why Mazda created (along w NGK) the funny looking OE plugs - no ground strap- and also used a colder plug up top.
i have been running them in the trail (10s initially, now 11s) for a month. i have an engine customer that has been running them for 3 months. he sees a 15% decrease in his detonation readings.
i received an email from someone who has been running them in all 4 positions:
"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 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 water at 20psi with no issue (running 4.9ms dwell)."
no worries about a glowing ground strap causing premature ignition.
i have ordered additional 11s and will be running them, next week in all four positions.
specific part numbers are on the Park Plug Section of my site
as we consider our apex seals, let's give them some additional support by carefully considering our spark plug options. perhaps not all of us can run 11s but there are many who are running too hot a plug that could at least switch to all 10s.
most likely if you lose compression and your motor comes apart you will find worn seals and carbon tracks from SPM and ECR.
don't be that guy
Seven Eleven
Last edited by Howard Coleman; 03-11-23 at 05:19 PM.
Very interesting.
To summarize, this is saying that when more mass is present in the spark-plug body, the faster it conducts heat away from its exposed area and the housing area around it. This reduces the adjacent housing temperature and as a result, minimizes the SPM. And, because there is no hot ground strap on the surface gap plug, detonation is also reduced.
I'm not a spark plug expert, but I've read that the distance between the edge of center electrode and the outer (ground) metal is the gap. It looks like the gap is larger than the other plugs, but it looks similar to the gap on the original BUR7EQ plugs.
One nice thing about surface gap plugs is there shouldn't be a question of indexing them or wondering if the spark gap is pointed the correct direction in the combustion chamber. People claim indexing doesn't matter with modern engine and ignition designs, but I haven't seen this sort of test done on a rotary engine.
I like the idea of colder spark plugs. But I've wondered how are people even knowing that one of the spark plugs is fouled when there are two per rotor?
I was taught to disable each coil trigger signal in the ECU software to double-check that a car's wiring is correct. When you disable Coil1 on a piston engine, that cylinder can be heard misfiring. Disabling Injector 1 + Coil1 is a way to check that both are feeding the same cylinder; if a second cylinder begins misfiring that's a sign the wiring needs to be fixed.
On my car, I can't hear the engine run any different if I disable only the Coil1 (Front Leading) trigger signal because the Front Trailing coil is still igniting the mixture. I need to disable both the Front Leading + Front Trailing coils for me to hear the front rotor misfire at idle or light load. Would the difference enough to feel it at full throttle? Do you need back-to-back dyno pulls to tell the difference in power or smoothness?
I'm not a spark plug expert, but I've read that the distance between the edge of center electrode and the outer (ground) metal is the gap. It looks like the gap is larger than the other plugs, but it looks similar to the gap on the original BUR7EQ plugs.
Makes perfect sense. Was just wondering if there was an exact value and if this gap is idea for a leading plugs in higher boost applications.
Assuming a cold plug or large surface-gap electrode is needed to keep the engine mechanically safe, we might be stuck with whatever gap is available in that safe spark plug design and then needing to build the rest of the ignition system around lighting that gap at high boost. Again I'm not an expert, but surface-gap plugs have that entire big circular area and you could imagine a whole 360 degrees of individual gaps where the spark could jump, with each section having a slightly different air/fuel mixture that makes that section of gap easier or harder to ignite. So if the air/fuel mixture is inconsistent (and it probably is), it only needs to be easy to light in one little area of that circle for a spark to jump.
Personally I'm not building cars that run that much boost, but my job sometimes involves talking to tuners building big-power cars and it's interesting to hear how many ways the same situation can be approached. But I'm very interested if anyone can comment about finding fouled plugs on rotary engines. Do both the leading and trailing plug need to foul before a person will notice anything, or is there a way to know that the trailing plug is too cold and getting fouled?
Also, does anyone know if we can use something like a borescope to check for chatter marks, carbon lines, or witness marks inside the engine of a working car without taking the engine apart? Or does the engine need to be completely disassembled and cleaned before those marks will be visible?
I wish I could find some information on the plugs that came in the stack of single plug/Natural Gas 'pump' engines I've used over the years for cores/e production builds - Kyle Mohan and Mazdatrix had a couple and did a video on them as well.
The ignition/distributor systems on the 'pump' engines were set up perfectly for both centrifugal and vacuum advance to net 25* at peak torque at full throttle and their plugs were very similar in electrode design as the R6601 except they have the same hex size, thread length, and protrusion as the factory BUR plugs.
They're labeled as NGK R T613OE and obviously very cold given how far down the porcelain is however I've never found any info on them outside of throwing them in an engine to see what they did - dropped power using a dizzy on basic street port race engine.
Also, does anyone know if we can use something like a borescope to check for chatter marks, carbon lines, or witness marks inside the engine of a working car without taking the engine apart? Or does the engine need to be completely disassembled and cleaned before those marks will be visible?
we've seen people put cameras and borescopes in the exhaust port over the years, its been IMHO inconclusive at best.
People claim indexing doesn't matter with modern engine and ignition designs
there is some Mercedes engine, a pretty new one, where if you do not use the factory spark plugs it won't be indexed correctly and the pistons will hit the spark plugs....
"Again I'm not an expert, but surface-gap plugs have that entire big circular area and you could imagine a whole 360 degrees of individual gaps where the spark could jump, with each section having a slightly different air/fuel mixture that makes that section of gap easier or harder to ignite. So if the air/fuel mixture is inconsistent (and it probably is), it only needs to be easy to light in one little area of that circle for a spark to jump."
Scotty 305 may not be an "expert" but he raises a very important point re the surface gap plugs. they have an option of any of the 360 degrees to discharge. of course they find the easiest path. gasoline is comprised of a large number of distillates... some are heavy and some are light. no question the path will be thru the easier to ignite lighter distillates. in addition, there is the movement of the mixture which relates to the physical aspects of the fairly crude combustion chamber which also will present a varience as to ease of ignition.
another good point Scotty 305 makes is that if the 11s significantly help as to apex seal wear, considering the costs of worn apex seals, it would be an easy calculation to find it cheaper to upgrade the ignition so as to run 11s.
my 11s for the lead position should arrive Wednesday.
I really wish we could somehow consolidate this and the other informative spark plug thread in the Single Turbo section: Another spark plug thread? 4568 r6601-11
I'll post here in deference to Howard.
Anyways, I'm planning to compare the R7420 and R6601 plugs in Trailing positions when I get back to the rollers (hello, Shane, Shane, come back Shane?!?). I may try also try them in the Leadings. When I got the 6601's, a synapse fired that hadn't in more a 20 years... I had similar old RE plugs in my Old Parts Bin (OPB). Sure enough, I still have a box of eight NGK SD11a surface discharge plugs... so here is a blast from the NGK RE past!
NGK R7420-11 R6601-11 SD11A Spark Plugs - the business end
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NGK R7420-11 R6601-11 SD11A Spark Plugs - overhead
The NLA plugs are 21mm like the R7420 and so 2mm longer than the R6601 plug. The R6601 are tiny compared to the other plugs; it makes me wonder how tightly the plug boot will seal... Howard? Also, the center electrode on the SD11A's project out ever so slightly from a porcelain prominence. I assume this may have contributed to the fouling, which purportedly made these obsolete. While I'm assuming, this must've been THE surface discharge plug for RE applications back in the day. HEck, now I'm trying to remember if these may have come from Mazda Comp of old...
Last edited by Carlos Iglesias; 03-14-23 at 04:22 AM.
super high output coils output... super high voltage. voltage always takes the easiest path to ground. if it can sneak thru the boot and avoid having to fight its way thru the highlly compressed mix in the combustion chamber it will. so you need tight boots. which are a pain to remove unless you use dielectric grease on the plug. win win... a good electric seal and easy to remove. my Magnecor boots, that fit well on my R7420s, fit fine on the 6601s.
you should also pick up a MSD plug wire seperator kit. no plug wires should be within 1.5 inches of each other.
here's a pic of my Magnecor KV85 wires, prior to cleaning
Last edited by Howard Coleman; 03-14-23 at 11:54 AM.
those SD11A plugs were the factory install on an FC for a hot minute, May to December 1986.
the cuts in the BUREQ plugs were added later to let the fuel drain out and the cars flood less
you should also pick up a MSD plug wire seperator kit. no plug wires should be within 1.5 inches of each other
its really hard to quantify, but there are three phenomena with plug wires we need to be concerned about.
the first is "inductive cross fire". when a plug fires there is a wave of energy, and if there is another plug wire nearby it can fire that plug too. very common in V8's with cylinders that are both next to each other in the firing order and physically. for that engine its like advancing the timing 90 degrees on that one cylinder (rotary is like 180). which will break it. fix is either to keep the wires apart, the FD kind of does this from the factory, because they knew what they were doing. or to cross the wires at 90 degrees, which cancels the wave (like a stock FC turbo)
Second is Radio Frequency Interference, RFI. this is when the electrical noise from the spark jumping starts to interfere with other stuff, like the ECU or the stereo. its a bigger problem on planes and corvettes, because they aren't steel, but typically the plug wire is sheathed and grounded.
Third. Spark Leakage. when combustion chamber pressure goes up, the energy required to jump the plug gap goes up as well. Electricity will find the easiest path to ground. at some point the spark energy will leak out of the wire/boot and skip the plug. loose plug boots are a no go here....
I wish I could find some information on the plugs that came in the stack of single plug/Natural Gas 'pump' engines I've used over the years for cores/e production builds - Kyle Mohan and Mazdatrix had a couple and did a video on them as well.
not gonna name any names, that was the preferred engine to convert into a high revving, bigpowa 13B PP …
i received my second set of 6601 11s which will go in the leads tomorrow. i decided to take a look at how all our plugs fit in the housings...
the OE plugs are .11 from the rotor surface and the center electrode is even with the faces of the four grounding surfaces. the second picture is an R7420. the plug is also .11 from the surface and the ground strap is .07.
(note: it is mis-marked as .7). the third pic is the 6601. i have removed the crush washer which is .05 when crushed. not only will that move the plug closer to the action but it will also remove a barrier to heat transfer, the washer. the center electrode protrudes out of the plug by just over .01
Looks like a surface gap plug with a heat range equivalent to the NGK 10s, but with the same length as OEM plugs. The gap is bigger than the R6601-11 but is still right around OEM spec.
Looks like a surface gap plug with a heat range equivalent to the NGK 10s, but with the same length as OEM plugs. The gap is bigger than the R6601-11 but is still right around OEM spec.
cool, same 0.055" (1.4mm) gap as oem BUR9EQP but with 21.5 mm reach. Will have to be test fitted in an open housing like @Howard Coleman did above to make sure it doesn't go into the chamber. Less pricey but all nickel so will not last very long.
03-11-23, 08:40 AM
