View Full Version : Removing Spark Plug washers

03-24-2002, 08:31 PM
Has anyone tried removing the spark plug sealing washers in their rotary engine? Or tried alternate plugs in an attempt to bring the spark closer to the mix?I ran a stock plug without the washer tightly into the leading hole in a spent 13B rotor housing. At least 30+thousandts remained between the ground and the surface of the housing at the closest point. Has anyone found single ground plugs to work better than the typical 4? I'm wondering if moving the spark closer,using a single groud plug( that would allow more of the mix to engulf the spark )would bring about better combustion.
Any comments?

03-26-2002, 01:08 AM
The RX-7 forum mentioned that as one way to improve spark delivery... Me, I figured Mazda specified the spark plugs with a washer for a reason, and until I've got some track time, there's other things to worry about at the moment.

However, the RX-7 forum guys seem to recommend it.


03-26-2002, 11:45 AM
Brian, if you have a 12A motor take out your spark plugs & with a mirror & flashlight take a look into the plug holes......

Stuff is not always what one thinks it should be. Looking down one of the holes is going to surprise you. http://Forums.ImprovedTouring.com/it/wink.gif Let me know what you see.

Sometimes we over engineer ourselves.

Have Fun


03-26-2002, 10:14 PM
As mentioned in my post, I have a 13B rotor housing on my work bench. I can get a rotor side view of the plug ends. There is room to move the leading plug tip closer to the apex surface of the housing.Even without the sealing washer there is at least 30-40+thousandths to spare. Unlike most combustion chambers, the plug electrodes do not project into the the chamber and mix, but must ignite the fuel mix from a recess as the rotary engines (difficult to efficiently combust) mix whips by at high rpm. I'm thinking that moving the plug end as close as safely possible,and by using a single ground electrode to provide space for a little eddy of mix to engulf the spark . This might allow better initial and thus better overall combustion. Paul Yaw has kicked this subject around on his web site in the past. I've experimented with a stock plug with 3 of the 4 grounding straps removed and noticed a difference, Mazdaspeed sells thinner washers for there racing plugs with a note that moving the plug closer to the mix showed HP gains. I've talked to an IT RX-7 racer in the south that runs a single electrode plug without the washer and swears that this "wakes up this engine" So I think that this is worth pursuing,especially in light of the fact that rotary engines arn't very efficient and usually due to incomplete combustion.Anything that improves this shows up all around the track.
I figure that there are some rotary guys out ther that have played around with this that might share their findings.

03-27-2002, 01:11 PM
Brian, Send me your email address to [email protected] and I can send you some info on side-fire and/or direct-fire ignition conversions I have from the FB(SA) list. No personal experience though.

03-27-2002, 01:35 PM
brian, Im running the single ground NGK GOLD V (B9EV) it does exactly what you want, you will however have to make your own plug wrench by grinding down very thin a 13/16th socket.

Daryl Brightwell
ITA Mazda #77

Speed Raycer
03-27-2002, 04:09 PM
Back when we first started our RX7 project, I pulled a bunch of parts off of a early RX7 (RB header, early 12a intake, etc). I pulled the plugs because at the time we didn't have any, and they were brand new NGK plugs... but they wouldn't fit in our motor because the socket part sat too deep into the hole and we couldn't tighten them down. I went back to the junkyard to look at the motor and whoever built the car had ground down the little "ledge" or eybrow around the spark plug hole...

Now... it's better to have just a single electrode plug than a 3 or 4 pronger???? That makes no scense to this "boinger"!!

It's not what you build... it's how you build it
http://home.swbell.net/srhea66/AllThreeProjectsRealSmall.jpg (http://www.angelfire.com/mo2/radrodder)

PFM Racing (http://www.angelfire.com/mo3/rudder_racing)

03-27-2002, 05:01 PM
My quess is that the 4 grounds are there to prolong the life of the plug. These plugs fire so often! It seems to me my car runs better with 3 of the grounds removed.(I don't have a dyno to confirm this,but I'm convinced) Doing so must reduce compression slightly,which wouldn't be considered a plus normally.So if there is an improvement, it must be due to the mix being exposed to the spark better.

03-28-2002, 01:03 AM
grinding the eyebrow off is illegal in IT,
thats why you grind a 13/16th socket down.

03-29-2002, 08:56 PM
Brian, humor me a little & remove the trailing plug from your 13B motor. (Or look at it on the bench.) Then with mirror & light look down at the bottom of the hole. I would presume that 12A & 13B plug hole bottoms are the same.
I have never heard/viewed a discussion on what is at the bottom of the trailing plug hole. I have an idea what the configuration is for. I have never viewed any photos/pictures anywhere of the rotor side of the plug holes.

It sure is not like the leading plug hole.

Come on 7's folks join the hole discussion. http://Forums.ImprovedTouring.com/it/biggrin.gif


03-29-2002, 09:38 PM

[This message has been edited by 7'sRracing (edited March 30, 2002).]

03-29-2002, 10:50 PM
The trailing plug hole(13B) bottoms with a 3/16 ths hole in the center. From the rotor side it appears as though the electrode of the plug is centered on this hole about an1/8th in. I believe the trailing plug is used to provide a secondary spark to help complete the combustion process,however the way it's set up it looks like it wouldn't be very effective. The spark is back in a small recessed hole. Removing the sealing washer from the trailing plug would help this.
The leading plug hole is the same diameter as the plug itself. This is where i think the biggest improvements can be made.
I picked up an NGK B9EV plug an put it in the 13B housing, but found it to be as far back as a stock plug with a washer. If Daryl has found an improvement with this plug ,my guess is that it's the single electrode thing.Jake emailed me some plug #s but have found them to be obsolete.
As of now I think I'll experiment with the stock plug without the washer and probably 3 ground straps removed. I might try having 20-30 thousandths removed from the sealing surface of the plug to move it even closer. This is getting alittle too close to apex seals for comfort, but if I index the plug in the housing and remove the 3 leading side grounds, this will give me an additional margin of safety due to the curved shape of the housing(plug hole gets deeper on it's trailing side) across the leading plug hole.

Dave Damouth
03-30-2002, 11:00 AM
Try searching Mazspeed.com for "side fire" plugs to get the part numbers. Side fire vs. 4 electrode (assuming msd on both), I measured 1 hp at peak hp. Well within the realm of dyno error, so its not a huge deal. Lotta people ranting about how great plug xyz feels in the car maybe had old plugs, maybe had a mixture problem. Some folks are now swearing by Iridium plugs. At $120 a set, I doubt you'll see more than the increase from a new set of plugs. I run modified side fire NGK's because they're cheaper than 4 strap ones. The groundstrap doesn't last long, but I don't really need them to.

03-30-2002, 01:47 PM
Dave, how long do you run them before changing them, and I agree 10 bucks for side fires beats 35 for four prongers.

03-30-2002, 02:30 PM
Brian, I appreciate your thought process & your taking a look at the trailing plug hole.
I do not claim to be motor knowledgeable. I respect that Dave Damouth has the experience to be motor knowledgeable. A couple years ago I quized one of the guys at Mazda Comp with reference to the trailing chamber as I will call it & he was very confussed. No one talks about it anywhere. Brian my take is that the trailing chamber has it's own little HOT burn that escapes the chamber & promotes a better secondary burn than without the chamber.

Dave, without your experience & dyno data I would have expected something similar to your post. Thanks

The four ground, the B?EV type, the "V" groove type, & other usable NGK plugs all have the same distance from the mounting surface to the far side of the ground. On Monday I am going to do some checking with NGK on their angled ground racing plug. It is more open & may move the spark closer to the mixture. Similar to what Mazspeed shows. Dave you tried to evaluate moving the plug closer to the mixture ? The whole deal seems to be to get a crisp predictable repeatable spark.

& now back to my 12A stock motor. I enjoy learning & some thing tried might help in some little way. If I could turn laps as consistant in the 7 as I can (could 91 through 95)in a Kart then one can tell some differences.

Daryl, I thought for a moment you were changing your handle. http://Forums.ImprovedTouring.com/it/wink.gif & no not to edit.
You should have work in the garage... http://Forums.ImprovedTouring.com/it/frown.gif

Continue with a http://Forums.ImprovedTouring.com/it/biggrin.gif


03-30-2002, 05:54 PM
The following info is straight off the MazdaMotorsports online catalog.

"Notes: NGK R6725-105 Racing plugs. Recommended for all rotary engines used in racing applications. Use with thinner spark plug washer to move flash point closer and increase power, 0000-10-9101. NGK recommends that you start your vehicle and bring up to operating temperature with stock heat range plugs, then switch to the racing plugs. "

This would seem to suggest some benefit to moving the plug closer to the chamber. They also provide some cautions about the EGV plugs because they do not go into the chamber as far.

For what it's worth.


Dave Damouth
03-31-2002, 12:15 AM
I run about 2 weekends on a set of side fire. The ground strap wears pretty fast. I believe the strap has a plating on it, and when you cut it you expose the bare metal which burns away faster. The 4 strap stock plugs last forever, but they seem to loose a little power after a few weekends. I run the stock plugs in the trailing position, change them every year or two.

No idea why the trailing is shrouded. If you wanted to buy new rotor housings frequently, you could build a center electrode plug that fits up close and uses the rotor housings as the ground strap. I imagine aluminum would wear pretty quickly though....

Dave Damouth
03-31-2002, 03:33 PM
The NGK numbers:

R5672A (10 heat)=Stock # 7942

R5672A (9 heat)= Stock # 7405

Remember you need to modify the ground strap. If you thread them in without modification, you'll intefere with the apex seals.

I would think twice about using plugs without the washer. It should work, but its not always going to work. If you have an old rotor housing around, thread one in without the washer and tighten it. Then look at how many threads are left. There should be about 1/4 thread left. I would not guarantee that that 1/4 thread is going to be there on every housing in everyones motor. If its not there, or not fully formed, then the plug will bottom on the threads, not the flat. That can't be good.

Allen Brown
04-01-2002, 01:02 PM
I'm wondering about one thing that nobidy has talked about. I'm sure the spark plug location has a certain amount to do with noted improvements. But...when the plug is moved closer, the compression also will see a small increase. Typically, more compression will help the hp.

Just a thought...


04-01-2002, 08:50 PM
Do you run these plugs with the side fire modification with the sealing washers or without? I'm curious as to what these plugs measure from the sealing shoulder above the washer to the tip of the unmodified ground strap?And are these plugs resistor type?

Dave Damouth
04-01-2002, 10:38 PM
I run them with the washer. I checked the depth on the threads before assembly, and these housings could run them without. I wouldn't run them without the washer unless you had the engine open and verified the depth of the thread.

Side fire. The plugs are the exact same length as the stock plugs, when modified. Prior to modification, they're a bunch longer. They won't fit unmodified. Unless you want a new motor, don't try. Really. Imagine a 50 thou gap and another 70 thou of ground strap trying to fill that 30 thou opening. Plus, even if it looks like it might fit, remember that metal tends to grow and move around at high temps...

Non Resistor NGK racing plugs, if I remember correct. Might want to check the NGK catalog if resistor is a big deal to you, I'm kinda fuzzy on why the part # starts with "R".

04-02-2002, 12:45 AM
I got an old housing to measure . It's a 2gen 13B. The plug washers are typically .080 thick. A stock plug,less washer, doesn't bottom on the thread,but has about .050+/- to spare. The stock plug threaded in tight without the washer, leaves about .050 clearance to the trochoidal surface at the closest point. I believe that the stock plug or the modified side fire can be run safely without the washer, without bottoming on the threads(provide the side fire is threaded like the stock plug). Losing the washer moves the plug approx. .080 closer to the mix.
Seeing is believing ,so check out a housing with an old plug.
The trochoidal surface moves away from the plug end. It's closest at the leading edge, and about .020 further away at the trailing side of the plug hole. Indexing the plug in the housing with the ground to the trailing side of the plug hole offers an additional margin of clearance.
So take that washer off,and let me know how you make out http://Forums.ImprovedTouring.com/it/wink.gif

04-02-2002, 01:31 AM
Non Resistor plugs can throw off some "electronic noise" that can mess with the on board computer.With a stock BUR9EQ plug the R signifies a resistor plug.

Dave Damouth
04-02-2002, 07:18 PM
No problem running them modified, just don't go for unmodified. And don't assume that all housings ever built were threaded to the exact same depth. Even Toyota couldn't pull that off....

Computer? Us 1st gen guys don't need no stinking computer. We're just one generation removed from the hand crank....

04-02-2002, 07:39 PM
Allen, you mentioned compresion=hp, I think it was paul yaw that said it dosnt apply to rotarys, is that right Dave?