FD Spark Plug Tech 2013
 

FD engines, being only 80 cubic inches or 1.3 liters are SMALL.

when combined w a turbo system, the unique flow capability of the rotary can make BIG POWER easily.

at the OE power rating, 255 flywheel hp, the engine is making over 3.18 hp per cubic inch!

thinking more in today's metric world:

using the same output per displacement a 2 liter motor would make 387 hp!

and that's stock.

a quick search reveals a recent article re a new Mercedes- Benz AMG 2 liter 4 cyl turbo'd to 26 psi, direct injected etc with an expected power rating of 345 hp flywheel.

345 from 2 liters cranked to the max from MB AMG in 2013.

stock output FD at 2 liters 387.

and that's STOCK.

on our board we think that a GT35 mod is on the modest side...

a Borg Warner S363, a GT500, PT67 is considered medium, and a GT42 or BW 475 is considered large.

so we are thinking

400 to 450 RWhp is modest

500 is medium

600+ large.

and we are still talking about a 1.3 liter 80 cubic inch motor!

Combustion Chamber Pressure (CCP) is pretty linear with output per displacement.

enter the spark plug. not only does it have to FIRE the highly compressed mixture but it also does not get an every other TDC rest as do its piston buddies.

pressure and heat go hand in hand. lots of pressure, lots of heat. the heat needs to get transferred pronto. otherwise you get this:

http://imageshack.us/a/img580/5760/lrpj.jpg

i call the lateral carbon tracks around the lead plug hole "wings" and they are on every motor i dis-assemble. anywhere on the housing where carbon exists there is no apex seal to housing contact. so how come the apex seal is not on the housing?

of course apex seals ride on springs and so they can move in and out. the motivating force is because of Spark Plug Mountain (SPM).

as the heat builds within the sparkplug the surrounding area also becomes hotter than its neighborhood. when heat increases metals expand at the molecular level. and when they expand they need somewhere to go and that direction is always the easiest path which is up to form:

Spark Plug Mountain.

SPM rises from the flat plain of the housing surface and pushes the apex seal off the surface. note the immediate lateral area at the plug is wiped clean of carbon because it rose and WAS in contact w the apex seal.

not good for power and not good because adjoining rotor faces become in communication.

another effect ARE the inevitable cracks that form in the same direction.

so what does this all have to do w the spark plug?

the cooler the plug you run the lower becomes SPM. a cooler plug transfers more heat away from the problem area.

you need to run as cold a plug as you can. 10s are a good start and if your system can handle more, do it.

i chamfer the hole a bit to reduce SPM which does help.

of course running AI is a big help, even in the one bar boost area.

the plug options for our motors are wide ranged. from $4 to $35. some require special sockets.

previously i was in the CD camp running a couple of boxes and monster coils. a couple of years ago Lance Nist (PanteraEFI) and i were chatting and Lance is a really persuasive guy. when it comes to ignitions you'd best tread carefully and take notes.

Lance is an inductive guy... he had a hand in the design of the all conquering Mercury Marine coil which is now re-badged as the AEM Smart Coil, IGN-1A etc etc.

so i rolled the dice and made the switch.

WOW.

inductive, just like Mazda fixtured on the FD, sure is simple and deadly. it also is relatively cheap as no other boxes are needed and nickel plugs, often priced closer to a nickle than $35, work great. no need for the platinum items.

Lance suggested, w my Merc coils, that i try what he termed the "NASCAR" plugs.

turns out that they are an Autolite Racing plug:

AR3932X

i suggest you write that down.

i bought a bunch from my NAPA, around $5 per. slipped them into my motor in March and i haven't removed them other than to do a couple of plug checks. they are a 10 heat range and the rumor is they are closer to 11.

they cold start fine and are well mannered around town. no special wrench required. i am personally happy to be back w Autolite as i used them in my racecar for 17 seasons and considered the Autolite Racing plug to be of superior quality.

how do they run?

here's my dyno chart from saturday...

http://imageshack.us/a/img842/5241/nqh3.jpg

another excellent aspect of the AR3932X is that it does not have a welded ground strap. ground straps have been known to fly thru motors producing an unhappy ending. nothing is going to come off this plug..

http://img31.imageshack.us/img31/7599/dhxy.JPG


so what's in your motor? plugs are especially important.

howard

Do those plugs only work with merc coils or at least some kind of ignition amplifier? Of can it be run on stock coils.

they should work well w the stock setup.

are you saying that changing from oem coils to the AEM coils and autolite plugs will make the carbon area around the plug hole go away?

Howard, on my 3 rotor I've got the AEM coils as well and have swapping between the NGK 9s ($7 per plug) and the $35 per plug race ones. Would love to find an option that is better on the wallet and also one that doesn't require swapping out the plugs. I just had the engine pulled apart and didn't have any of the telltale signs of spark plug problems like in the pic you posted.

Would you recommend these for a 3 rotor? Not sure there would be any difference but I thought I'd ask. I take the car up to only 18-20 PSI.

Did you note any ignition issues with your setup and as I recall, you went up to 26 PSI or so with your dyno run? Finally, what is the your dwell time setting for the AEM coils?

"are you saying that changing from oem coils to the AEM coils and autolite plugs will make the carbon area around the plug hole go away?"

no, i am saying that running a cold (10 or higher) plug will help transfer more heat from SPM.

turn SPM into SPm (Spark Plug molehill)....... along w AI which i consider necessary for any of our cars.

Hi David,

sure, a 3 rotor doesn't know it is "different" inside:)

maybe no tracks due to proper tuning and AI?

while the dyno sheet stops just before 8000 due to loss of signal up top most runs were to 8900 with no ignition issues whatsoever. dwell was a conservative 4.5 maximum. the coils can accept more dwell if necessary for short periods.

so i run the oem coils can i run the ngk 10 plugs with no issue?

^Thanks Howard. I'd concur on the good tuning and AI = no spark plug tracks.

I'll give these a shot and see how they perform. I'm due at the dyno in about one month so it will be a good test. I've got a set of the 9s and the 10.5 race plugs so I'll compare them all.

Interesting. I like the price. Looks like they're non-resistor plugs, which seems to have caused EMI issues in some applications. Also, the reach is shorter than that on the R7420s (.750" vs .846").

Have you experimented with other non-resistor plugs? Other short-reach plugs? Issues?

These are quite similar to the Bosch racing series I've been trying out. I don't have enough time to comment on them but I think we had this discussion before in another thread. Good to hear you found something cheap, durable, and most importantly EFFECTIVE!

so what exactly is wrong with stock NGKs in the proper heat range?

The NGKs in 7s and 9s are not transferring enough heat out of the area.

Surely you guys, being "car guys" understand what plug heat ranges are all about? Its the most basic of basic tech. Go read some Hot Rod Magazine if you dont understand how plug heat ranges work.

I always thought that mazda ran such hot plugs to meet emissions. Your grandma's buick would run 7s or a similar heat range as she pootles around town at 20mph. Hot heat range plugs are for the low performance daily driver, because they resist fouling.
Colder plugs are more prone to fouling, but the porcelain design transfers more heat away from the surrounding area. You put 10s in your stock daily FD and it will reduce SPM, but increase the chances of fouled plugs. You dont get something for nothing in the performance world.

^ And I'm basing my plug knowledge on having read about plugs in some Holley tuning books, then gone online and researched it for a while on Car Craft/Hot Rod magazine, and talking to my dad about the subject, and putting it into theory in my previous cars.

Nothing like being 18, and in the middle of a carpark and the car doesnt want to run on all 8. It runs on 7, and half an hour ago you bought some sets of colder plugs, and you got to the point you went too cold and you fouled some and now it runs like a pig.

Following phone conversation goes like:

Me: "Hey Dad, would you mind bringing me that other set of J8 plugs I had on the bench at home? I'm in the carpark of Kmart and the b#$%^& fouled some plugs on me. I went to Kmart to try a colder set, and the things fouled. I need the J8's, not the J9's. I f$%^ed up."

Dad: "Jesus Christ, stop fiddling with that car. It goes well enough as it is. I told you not to go too cold or it would foul. Give me 15 minutes, I'm busy in the lathe."

I'll always remember that part of growing up. I'm 26 now, and I feel like I went through enough of that stage of my life in traffic and at the track, to be able to reasonably talk about plug heat ranges with at least some confidence.

I ran a 440 cubic inch V8 Chrysler engine, and an Australian Hemi 6 cylinder engine, stepping the plugs colder and colder (8's, 9's, 10's) until those engines began fouling plugs. I then went back one warmer, and that was the coldest plug for that engine, in its circumstances.

If you look at a plug, without knowing the heat range, you can see if its hot or cold, based on the design of the porcelain on the electrode. It will either absorb and extract combustion heat from the chamber, or retain the heat in the chamber and not transmit it out of the surrounding material based on how far up the porcelain extends.

In this phenomena, it is possible to select a colder plug for the 13B-REW, which will absorb more heat out of the surrounding housing material, acting as a heat sink, and transmit it away. Less heat means less deformation of the housing, which leads to Howard's "Spark Plug Mountain" description. Its very simple.

If you wanted to, you could run a 10 plug in the REW, and then if it worked well on the street, in the most testing environment possible (idling in traffic, engine just warm, with some quick boost in traffic) without fouling, you could try a colder plug, move towards an 11. Eventually you will find a plug that runs too cold, and cannot "flash off" the combustion fuel/oil and carbon that can foul the plug tip. You would then go back to the last coldest plug number that worked well and stay there.

Mazda had two considerations when choosing plugs-

1. How easy it would foul (cant have customers bringing the cars back with fouled plugs every month).

2. Emissions. The hotter the plug, the better the combustion and less leftovers for the cat converter to have to deal with.

The factory did not go colder than 9's, because they did not want fouling and customer complaints. Also, they used 7's because they needed a complete hot burn to help emissions. There are other timing considerations that go into it too, but its not worth covering it here.

Also of note, hotter plugs can encourage pre-ignition (the nose of the plug acts as a glowplug) in high performance engines or when running dodgy low octane fuel. Thats why race engines tend to run colder plugs. They will make more power and reduce pre-ignition but they will foul more readily. (Not an issue in a race engine, just whack a new set of plugs in, before you head out to the track again.)

Some racers would read plugs (high rpm, high load, then neutral and shut her off, pull over and check the plugs) but that was always something I did if I was fine tuning jet sizes on a new Holley carburetor.

Some people (piston engine guys) would index the plugs (mark the electrode position on the porcelain, then screw it in and see if the electrode points toward the valve, and if not, add shims til it screws in and points the right way) hoping to make more power by very fine tuning that sort of thing. It is irrelevant in a rotary, because the REW uses a "Surface Discharge" type of plug nose- IE no welded electrode as Howard mentioned, but a flat nose with four slots. No electrode to index.

Disclaimer: I apologize for writing when I'm angry. My tone goes bad very quickly. I am not angry at anything to do with the forum. I just had a bad day.

You could probably run a 10 (one step colder) in a stock FD3S. My feeling is that they foul their plugs enough already that a 10 would be about the limit before you start to get pissed off with it fouling its plugs more frequently in normal use. A "hot" 11 might be okay, but each car needs the heat range tailored, as they are all different, and the environment and driving styles are very different from one owner to the next.

Howard, do you noticed the pronounced "SPM" as you coined it, more on the leading or trailing plug apertures in a stock engine running 9's and 7's?
My feeling is that the 7's will show more pronounced "wing" effect than around the 9's on a stripped REW?

here is a pic of a ~100,000 mile 12A rotor housing. as you can see every wear aspect is worse than the REW, despite the REW making 2.5 times more power.

so youre saying I can run those with my 500+hp on a twin power instead of eating 39+ bux per plug for the NGK?

Interested as well on how these will perform with a twin power and stock coils.

"so you're saying I can run those with my 500+hp on a twin power instead of eating 39+ bux per plug for the NGK?"

i have no experience w the plugs other than what i have set forth. i do consider it a low risk ($20) speculation to give them a try. my guess is that if your current setup, the more expensive plugs, works the nickel plugs should work. i don't see any significant motor risk especially if you turn down the boost a bit and sneak up on it.

i believe others on the board are running them and would welcome additional comments.

"so what exactly is wrong with stock NGKs in the proper heat range?"

nothing.

my post is about a NEW OPTION for the FD. it may be worth noting that some people have lost NGK ground straps. there are no ground straps on the AR3932X. the really stock NGKs are designed like the AR3932X with no ground straps but are only available in a 9 heat range.

howard

good deal.

i had a question on these months ago. there are a few threads about them around here, but not hard data.

like everything else on here everything is hidden away.

Ill ask The same question. How will they work on HKS twinpower, gt35r @17psi

which has got to be the most common single turbo setup on this forum

First off, nice dyno Howard.

I will also '2nd' Autolite plugs as an option for our engines. I have used their plugs for years with good results (in my FC and FD as well as all types of engines).

Your point about power density and the correct heat ranges is also well taken; moreover, the importance of AI and why it should be standard equipment.

Ever the engineer, I wanted to ask you for clarification as to the mechanism causing the SPM, as you call it:

The reason I ask, is that a hotter plug actually transfers less heat to the shell--and thus, the surrounding chamber (and heat is transferred deeper in the shell, somewhat further from the combustion chamber surface). The plug retains more heat in the core, which is why it is characterized as hotter. The shell of the plug is not generally characterized in the heat range and is going to be close to the temp of the material it's screwed into (in this case, the housings).

Or put another way, a cold plug should actually transfer more heat into the housing.

So what else could be causing the SPM effect?

I don't doubt that a correlation to heat range may exist; others have made similar observations. I am just not convinced thermal conduction is your mechanism.

A couple of other things that also enter into the equation:

Aluminum's coefficient of expansion is much greater than steel;

Aluminum's thermal conductivity is much greater than steel; and,

when a material expands, the holes in the material expand larger.

I recall reading a mazda paper, posted somewhere on the forum, that says they chose a aluminum for the housing material because cast iron could not conduct heat away from some areas (spark plug?) fast enough, resulting in localized overheating. Aluminum, with its superior thermal conductivity, apparently mitigated these concerns--but with the astronomical power densities that we can achieve--perhaps we're just hitting the limitations of the materials and/or design.

Your thoughts?

So whats the deal with the shorter reach and non - resistor. What exactly does that mean in terms of performance and compatibility with HKS twin power and Power FC if anything?

"cold plug should actually transfer more heat into the housing."

yes and therefore the cold plug, itself, remains... colder. a hot plug retains more heat within the plug and therefore the immediate aluminum contact. as you mentioned aluminum is one of the top substances for heat transfer.

hold one end of a 1/8th inch aluminum welding rod, hit the other end w a torch and you will almost immediately not be able to hold it.

do the same w a stainless steel rod or even a mild steel rod and you will feel almost no increase in heat on your end.

aluminum is crazy good w re to heat transfer.... and it works w the rotary, transferring (more) heat away from SPM.

since a cold plug transfers more heat to the aluminum housing that heat spreads quite well over the housing and into the coolant creating less immediate heat at SPM.

let's assume the heat total is a constant... it is spread out over a larger area w the cold plug which reduces the uplift at SPM. that's why a cold plug runs at a colder temp, as the heat is better conducted away from the spark plug boss.

immediate heat around the spark plug boss IS a challenge for the rotary and cannot be completely eliminated but can be reduced.

i can say that taking apart an engine that has run cold plugs does support my, uh, theory:)

another interesting off topic consideration is the effect of SPM on the apex seal. (not good).


"shorter reach and non - resistor"

all i can say is they work well for me. i realize there have been a lot of questions re how the plugs work w a more typical setup and i do know that a bunch of people on our board are running them. maybe we will get some comments.

howard

So you have a sample size of 1?

about this plug hole problem. reminds me of:

http://www.pettitracing.com/wp-conte...lureTheory.pdf

"So you have a sample size of 1?"

more like 50. "an engine" just refers to a representative sample. of course.

thanks for the link 00SPEC.

here's the Pettit SPM Mod:

http://imageshack.us/a/img41/1496/y6b9.jpg

while it doesn't clearly show, Cam chamfers the top off of the spark plug boss.

in addition to causing loss of compression and non-linear apex seal wear the lifting of the apex seal is the primary reason for chatter marks on the housing.

Cam and i discussed SPM a couple of years ago and he suggested the chamfer mod which i adopted.

lowering SPM starts w running colder plugs and AI.

howard

"An engine" is singular tense. Of course. Nonetheless thanks for clarifying.

I understand the info but at what point does this really make a difference though? I've seen enough mildly modded turbo rotaries last 100-150+k miles. Is this more of an issue with high HP motors?

Also, is the issue heat range or spark plug design? It isn't clear to me in your original post. I'm inferring heat range but wanted to verify.

Thanks.

"Is this more of an issue with high HP motors? "

i do think as CCP and CCH rise w output SPM gains elevation.


"does this really make a difference"

a good question.

i suppose it depends on how you wish to view it. often i receive a motor that runs and has put down 100,000 miles... the owner wants a refresh and more output. without exception i find the carbon wings along w chatter marks. the chatter comes from SPM as do the wings.

in many cases the motor could continue to provide useful service.

i will admit when i see the housings i feel for them.

nowadays we have vendors that provide really good housing renewal so most often the housings, which are crazy priced new around $800+ these days, can be made dead straight and almost as good as new. and that includes leveling SPM.

another SPM issue is that any time the apex seal is not in contact w the housing surface it rapidly gains heat. the apex seal transfers significant heat into the rotor housing face but not when it is up in the air.

generally the product of seals not being in contact w the housing is a flat shoulder and a sharp trailing edge. i see it often.

i doubt that by running colder plugs, AI and chamfering the boss that SPM can be totally conquered. it certainly can be reduced and i have seen clear evidence of this with my motor and the Pettit picture adds support.

as to heat range and plug design... heat range probably trumps design in most instances. in higher hp apps design becomes relatively more important. i do like the lack of a ground strap as power rises. there are numerous recorded instances where a strap was lost in a high output motor.

i am not selling spark plugs:), i just thought i would share my first hand experience w the AR3932X... prior to Lance mentioning to me them pretty close to nobody was running them and they seem to be a solid rotary option.

howard

Anybody else using these plugs yet? I'm going to order some up as replacements being as they are only $4.59 and the plugs site.

I will be purchasing a set for my new motor . in a couple of weeks . I thoguth these were found at local autoparts stores which was one of the benefits, compared to having to order the plugs of a website .

I plan on using the 10 equivalent. I think this will be a good chance to test howmuch abuse they will take

There are found at local auto parts, at least at my closest napa they were.

I installed a set today.

And since im such a baller..... I bought another set just cause I can. Total. .. $47..

Lol. Spent $140 on my denso iridiums..

That made me laugh.

Do you have some sort of opposition against the 'AR3932X' option?

Nope. Just asking for details so I can understand the information Howard used to come to his conclusion and use that to make a decision on my own.

i'll just leave this here :)
 

AR3932X (new) vs NGK-R7420-11 (used)

https://www.rx7club.com/attachment.p...1&d=1380136985

https://www.rx7club.com/attachment.p...1&d=1380136985

https://www.rx7club.com/attachment.p...1&d=1380136985

Barry,

i knew you'd come up w some sort of paradigm shift.

Howard

Laser plugs..... Now we're talking!

interesting , continue on with said laser plugs??? could they be a direct replacement ? or what would need to be modded in order to use them?

do they require more power to well shoot laser beams at our rotors?

Maybe its irrelevant maybe not , but these are my housings spark plug holes Attachment 660500

as you can see , NO signs of the wings . the housings themselves had 0 signs of carbon.

the engine was a JDM engine which I bought and drove for 12k miles . before it went boom .
The rotors on the other hand had plenty of carbon on them .

And the housings I just got with 20k miles on them also have the wings .

so why do mine not have wings? since I got the car I've been running NGK9's and I've been running AI ( aux injections ) some proof for the nay sayers that colder plugs + meth do help with the cleaning

Mazda research shows that the closer the plug tip is to the housing surface the more healthy the ignition sequence.

The real beauty of the laser is that it can focus the ignition point out into the rotor's pocket.

General News Parts - Lasers move closer to replacing spark plugs | GoAuto

What's the difference between an Autolite AR3932 and an AR3932X? What does the "X" mean?
Thanks.

Per my local Napa store, they said the entire west coast doesn't carry the AR3932X. It's special order. Local O'Reilly does carry the ASP AR3932 without the "X" which is why I was asking what the "X" meant. Thanks....... and by the way great stuff here Howard and others!

the X plug has four grounds that are part of the spark plug body. similar to the OE FD plug.

the non X 3932 has the traditional ground strap and like the "X" is a racing plug. it does have the advantage of being able to be gapped to your spec,

the plugs are available from NAPA. they have to order them from a NAPA warehouse or source located in Detroit.

not a biggie.

howard

sparkplugs.com

update re AR3932x

"upon further review" of the dyno sheet i see squiggles from around 5500 on. it has been pretty much proved that these are a result of too much spark plug gap. the 3932X is not adjustable as to gap.

additional evidence of misfire is the DynoJet losing the RPM signal further up the RPM plot.... around 7800. this is probably because some of the spark energy was leaking on the exterior of the plug to ground since there was too much CCP.

i am in debt to Cam Worth for bring this to our attention. Thanks Cam. i talked to Lance on it and he was in general agreement and offered an additional fix which was longer, almost impossible to install/remove spark plug boots. both Cam and Luke just focused on gap.

so we grabbed my NGK 6725 plugs and gapped them at .02. (did not have a set of AR3932 immediately avail)

the dyno retained the RPM signal immediately to redline.

OTOH, Luke was hearing noise from peak torque up in the Link Knock Amp Headphones.

the headphones are awesome.... they keep you ahead of serious knock. we acquired them in January and Luke has adopted them for all his tunes.

here we are indebted to Richard Green (NZ) for the tip... here's an email from him a couple of days ago:

"Isn’t the Link knock block great. You can hear you’ve got a problem before you do damage – an early warning device if you will. Imagine how many tuners are needlessly destroying engines without this tool. Interestingly the engine I tuned today was quite noisy for the first 10 or so low boost pulls but then it cleaned up (sounded like worn apex seal grooves). It’s because the engine had zero miles on it before today and all the seals needed to bed in, even though they were new (most aftermarket apex seals seem to be supplied with a slight bow in them). So I suppose the noise was also like mildly bent seals… until they bed themselves in. And I guess mildly bent seals can cause pre-ignition…. which can be picked up through the headphones. Engines with flattened corner seal springs sound the same."

it appears that what we were hearing a week ago from the 6725s was a combination of a bigtime inductive ignition system (IGN 1A) and fine wire, long strap plugs being turned into a glow plug. this did happen to Richard last year and he switched to the thicker nickel/copper plugs which cured the noise.

we could not come up w another explanation. i think the 6725 plugs work very well in a CDI environment but may not work as well w inductive. the much longer spark duration being the issue.

so we swapped out the 6725 (11) and in installed AR3932s.

bingo. no problems.

for many apps the X may work fine. we are in the 550+ power area and it looks like the larger gap X plug may not be the ticket.

the AR3932 is a non resistor plug.

Autolite Racing does offer a similar plug that does have a resistor. it comes in (only) a one heat range warmer edition and is the AR3923.

howard

Howard,
Great report from the trenches.
I want to try the Link Knock Amp Headphones.

Thanks of the update.
Barry

So Howard, which versions do you recommend? The "non X" versions which ARE gap able or the "X" versions that are not? I would like to see how they work on my NA 20b with LS1 coils. The non "X" versions are in stock at my local Advance auto. :) One last thing, are you recommending these plugs in both leading and trailing locations?

i don't have a specific recommendation and am simply sharing what i have observed.

spark plugs are very important to get right since our motors are capable of easily generating lots of CCP and CCH.

while i was sitting in the passenger seat watching the logs as we made 575 at 25 psi i had zero indication that we were experiencing primary voltage leaks at the plugs.

zero.

looking at the dyno power chart at zero smoothing a different picture emerged.

the squiggles on the power line above 5200, (we were making over 400 SAE at 5000 so we had big time CCP at that RPM), the noise on the headphones and the dyno rpm signal being squelched at 7800 all tell the tale.

too much plug gap w the un-gappable AR3932X.

in went the only set of plugs we had that we could gap, NGK 6725 11 heat range at .02.

all misfire symptoms vanished for the first two pulls. no dyno RPM shut down, almost no noise and a smoother power plot. on the third pull power was down about 30, and the noise re-appeared.

after acquiring a set of AR3932 plugs gapped .02, no further symptoms occurred. we made numerous runs and experienced no noise, no RPM signal loss and the power returned to previous levels.

there are probably two important factors working here.

gap is important and even w a big ignition needs to be in the .02 neighborhood at this output level.

skinny long ground straps when forced to pass a powerful LONG DURATION inductive current may turn in to glow plugs. this is a speculative conclusion but is exactly what happened to Richard Green a year ago. he had the same noise and settled on the same cure: nickel large ground strap (cheap) plugs.

i do think if you are running CDI the NGK 6725s may be excellent as the spark duration is short.

if you are running INDUCTIVE, such as the OE coils, the OE coils and a Twin Power, FC coils or the Mercury Marine/ IGN-1A re-badge/AEM re-badge w a single turbo i would suggest you consider a nickel large ground strap plug. (the AR plugs are designed w a shorter "cutback" ground strap to reduce the chance of glowplugitus.)

there are NGK options in this area. i have always been a fan of Autolite Racing plugs and the 3932s seem to work well for my app. if you like the X design they may work very well at lower power levels.

check w board member Chris Ludwig for a FD friendly Mercury coil solution.

howard