Some inquiries on the factory plugs

[Coheed]

I was just wondering if the tuners here have any thoughts on why Subaru has utilized a multi-grounded plug in this application.

The factory plugs are iridium, have a 60k change interval, and 3 grounds. Toyota has experimented with 2-ground plugs before and TSBs for fault codes resulted in them putting a platinum single-ground plug in as a fix. Multi-ground plugs can shroud the flame kernal, inhibiting proper flame propagation.

The higher compression of this engine may help with this, or perhaps the engine design has sufficient quench to combat this issue. But i wonder if replacing this factory design with a conventional NGK laser iridium plug could offer some benefits. Has anyone dealt with this or experimented with alternate plug designs?

[arghx7]

[quote=Coheed;1497120]I was just wondering if the tuners here have any thoughts on why Subaru has utilized a multi-grounded plug in this application.

The factory plugs are iridium, have a 60k change interval, and 3 grounds.

Toyota has experimented with 2-ground plugs before and TSBs for fault codes resulted in them putting a platinum single-ground plug in as a fix.

Which engine(s) was this? Got a link to the TSBs?

Multi-ground plugs can shroud the flame kernal, inhibiting proper flame propagation.

Sure, under certain circumstances it could. And under others it could improved. The only way to tell for sure requires a combustion analysis system at the very least, and ideally an optical engine.

The higher compression of this engine may help with this, or perhaps the engine design has sufficient quench to combat this issue.

At the risk of getting bogged down with semantics, I think you are referring to charge motion, specifically squish flow. Quenching is when the flame goes out (often prematurely). But people use quench and squish interchangeably sometimes.

But i wonder if replacing this factory design with a conventional NGK laser iridium plug could offer some benefits. Has anyone dealt with this or experimented with alternate plug designs?

I don't see how you could quantify it in any meaningful way without equipment that costs as much as a fleet of BRZs.

I suspect the plugs are designed that way because at cold start or some very low load points it increased combustion speed. Let's say under about 3000rpm and maybe 5 bar BMEP. That's an area that probably has a significant internal EGR rate due to AVCS tuning. Basically, call the plugs emissions and fuel economy related.

In terms of getting more power out of the spark plugs, yeah you night get a small benefit from running something else on a setup with just boltons. With any kind of forced induction you gotta change the plugs anyway.

TL;DR They had their reasons, but it's probably emissions and you probably don't care about that.

[Coheed]

The Toyota 4 runner is one particular application that used a dual-ground plug. They often had issues with misfires within the specified change interval, and Denso platinum plugs were often the fix. These plugs are very similar to the NGK GP platinum.

Toyota did this, I would guess, as an effort to extend plug life with their waste-spark ignition setup utilized on these engines, while keeping costs down by utilizing a copper plug instead of Platinum/Iridium.

I was reading on here about some people having some issues with coil failure, and it did spark my interest in this very unique plug design.

If this plug is used for emissions reasons, it really begs the question of why other manufacturers are utilizing conventional designs with newer materials for extended change intervals. I have zero doubt an NGK laser iridium would outlast the factory plugs and provide better performance without increasing emissions.

Though I'm no engineer, I do have a BS in Automotive Technology, and I have built several cars/engines used for various forms of motorsport. I have tried multiple designs of multi-grounded plugs: E3, Bosch, Splitfire, HKS, etc. None of which provided any performance increase (and often pretty severe ignition issues) over the good ole conventional design.

[Coheed]

During a search for TSBs related to this subject, I found this video referenced. [ame="http://www.youtube.com/watch?v=53yfHLdn41k&feature=youtube_gdata_player"]Multi - Ground Spark Plugs - NGK Spark Plugs - Tech Video - YouTube[/ame]

It's from NGK themselves and gives a brief overview on my concerns. Perhaps these plugs were spec'd to provide better starting performance, or like you said, better emissions. Though I do find it hard to believe they would use it for emissions purposes, as this plug design will impact overall ignition performance.

However, there have been little gremlins with my BRZ as well. Occasionally the engine has a fail-start. During these failed start attempts, the engine continues to crank over for several seconds before coughing to life again.

On one attempt the engine failed to start completely, and I had to activate the flood control programming to clear the cylinders. After 10 seconds of cranking or so, the engine started. It's only happened on about 6 separate occasions in the last 2 years and 8500 miles. It doesn't really concern me, I know my way around these sorts of problems. But this subject has some cogs turning in my head.

[mad_sb]

I always thought multi ground plugs were simply used for longer service life... Given the manual calls for lifting the engine to change the plugs i would imagine that has a lot to do with it.

My favorite plug series is the ngk race series. I used BR8EG's on my evo, cured all the ignition issues i had with meth and water injection. Could not find an application for the twins last time i looked though.

[arghx7]

Since the video brought up rotaries, which I haven't meantioned yet: NGK makes BUR7EQP and BUR9EQP surface discharge (4 ground) platinum (copper also available) OEM plugs for rotaries. Of course, those plugs were designed in the 80s, and other surface discharge plugs were used prior to that on rotaries. Let's take a look at a 1993 Rx-7 service publication:



So yes, there are anti-fouling reasons to use them. As for how the side electrodes affect combustion, I would still argue that you need combustion analysis equipment or trustworthy 3D combustion modeling to make that judgement. It might make it better (relative to some other plug with similar maintenance life) or maybe it makes it worse. Or maybe it just depends under what condition you are checking.