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Old 04-24-2012, 09:28 PM   #1
BeastianSTI
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Default Spark Plug Gapping - 330WHP

Hey guys, I've got a 2005 STI, which is running at 327whp. It's been running a bit rough lately, and i've sorted out some vac/boost leaks, and it's still not super happy with me! In lieu of this, i'm replacing the plugs. It was tuned for 20PSI, and has this list of mods:


TurboXS Top mount
Deschtwerks (w.e..) Injectors - Not sure on CC..
APS Hard Inlet
TGV Deletes
APS Cold Air Intake
Grimmspeed AOS

It was protuned by Scott Seigel, but i've heard he is hard to get ahold of nowadays (was tuned for previous owner, not myself).

The question is, should I leave these NGK2309 (1 step colder plugs) gapped as they came, or should I adjust them? Also, is there a general rule of thumb to tighten these without a torque wrench? I made the mistake of lightly anti-seizing the first plug I replaced, but I tightened it down until it felt relatively tight without putting any weight or muscle into it, using a 6-inch socket wrench.


Cheers guys!
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Old 04-24-2012, 10:06 PM   #2
Uncle Scotty
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even if you know EXACTLY how to PROPERLY gap iridium spark plugs...you will destroy about 1 in 5 doing it

if you think you can just slam-bam gap them like you do regular plugs...you WILL...without anypossible doubt...DESTROY them, every time

DO YOU KNOW HOW TO PROPERLY GAP IRIDIUM PLUGS???
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Old 04-24-2012, 11:11 PM   #3
mowgli29
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Ideally, you want to run the largest gap you can, without misfiring.


Why do we want the largest gap possible?


Because the larger the gap, the more "exposed" the spark is, and the easier it is to ignite the fuel/air mixture. This reduces the likelihood of knock and should allow your tune to be slightly more aggressive.


Why can't we just increase plug gap as much as we want?


Two reasons:
1. As you increase plug gap, the amount energy required to jump that gap increases.
2. As you increase power, you essentially increase the density of the intake charge. The denser the charge, the more difficult it is for the spark to "jump through" it.


How do I figure out what gap to run?

1. Gap your plugs to a commonly accepted gap. IIRC, .026 to .03 is typical on turbo Subarus.

2. Drive the car.

3a. If you get misfiring at WOT/full boost, close the gap by .002" and drive again. Repeat until the misfiring stops.

3b. If it drives fine, open the gap by .002", drive, and repeat until it misfires... then close it back down by .002".


What if it runs fine and I'm too lazy to figure out the largest gap I can run?
Go ahead, leave it, slacker.


Also, keep in mind: as spark plugs age/wear, the gap increases.
If your car suddenly starts to develop some misfiring under WOT/full boost, it might be time for new plugs, or at least a re-gap.
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Old 04-25-2012, 09:47 AM   #4
Bacon117
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What Scotty said. Use a pin style gap checker, not the inclined plane type or feeler gauges.

If you have access to an oscilloscope and a 1:1000 passive voltage probe (Tektronix P6015A or equivalent), I can explain how to do this the proper way.
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Old 04-25-2012, 01:19 PM   #5
BeastianSTI
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True. I looked between the ones that were in there previously, and they were NGK's already. The new ones are gapped the same, it appears. I spoke with the previous owner, and he said he never changed the plugs (70k miles has past since the last change).

I installed two so far, and i'm not sure if this is normal, but i'm hearing a popping noise (afterfire?) from the exhaust while idling. It'll happen every few seconds, but i'm not sure if it is misfiring, or is this normal?
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Old 04-25-2012, 01:28 PM   #6
Bacon117
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Quote:
Originally Posted by BeastianSTI View Post
True. I looked between the ones that were in there previously, and they were NGK's already. The new ones are gapped the same, it appears. I spoke with the previous owner, and he said he never changed the plugs (70k miles has past since the last change).

I installed two so far, and i'm not sure if this is normal, but i'm hearing a popping noise (afterfire?) from the exhaust while idling. It'll happen every few seconds, but i'm not sure if it is misfiring, or is this normal?
I replace them every 70k. I use Iridium's. I got to assemble some plugs myself during my training in Japan, but I'm not sure I will use them.

I would go with a smaller gap. Close it a little from stock, 0.4 mm at most. This will decrease performance slightly, but will allow your plugs to last longer, and decrease your chances of misfire.
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Old 04-25-2012, 01:43 PM   #7
BeastianSTI
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I think it's at .3 mm, stock gap. I turned the plugs as far as I could, with my hands, and then turned the plug another 1/4 turn. It still seemed like they weren't really tight. Is there really any way to tell?
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Old 04-25-2012, 02:33 PM   #8
Bacon117
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stock gap should be around 0.8mm. 0.3mm is too small for stock.

https://docs.google.com/open?id=0B8l...I2YjBjMWMwY2Vm
Owners manual should have torque specs. Check out page 19 of that document I just linked. It will get you where you need to be. If you don't have a torque wrench, use the angular tightening method. it's 1/2 to 2/3 of a turn for a new plug. (remember you are crushing the gasket for a good seal).
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Old 04-27-2012, 10:20 PM   #9
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my 06 impreza fsm shows 15.2ft-lbs for torque on plugs for the dotc engine

i have a 1/4" drive, 20 - 200in-lb torque wrench that I bought from harbor freight just to do....i use 185in-lbs and call it all good
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Old 04-30-2012, 09:01 AM   #10
Bacon117
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The min NGK recommended torque is 18 ft-lbs. I don't know why Subaru recommends lower torque. There might be some good reason, but since I only work with domestic OEM's, I have no idea what it is.

It's better to stay with what the factory recommends, since that's the whole system, and not just the plug. Maybe the threads in the head are crap... I dunno.
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