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1998 Forester S
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Discussion Starter · #1 ·
I'm curious about opinions on this. It's probably one of those things everyone has an opinion on. I usually try to use Bosch Platinum +4 spark plugs, but just as often end up using the +2 because the auto parts store is often out of the +4. These have worked well for me so I've just stayed with them.

Does anyone have any thoughts as to the pluses or minuses of using iridium spark plugs?
 

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First, I'd go with NGK over those bosch. The multiple point plugs aren't all that great.
Iridium perform better than platinum but at a higher cost. I'd use copper or iridium in the turbo and copper or platinum in the N/A. Coppers perform the best but don't last especially long.
 

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2019 Crosstrek 2018 Forester XT
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First, I'd go with NGK over those bosch. The multiple point plugs aren't all that great.
+1
I don't think Iridium are any better than Plats. Its about being able to charge more. I had the Bosh +4's in a couple of cars but they were nothing special and seat of my pants told me they were not developing the power the previous plugs had given.
 

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These fancy-pants plugs are really no better than a plain old spark plug for the most part. The only advantage (theoretically) to platinum plugs is the longer life. Unless you plan on leaving them in there twice the normal lifespan they are not a good value.

I've used NGK's in darn near everything my entire life and have never had a problem, and they last far beyond the recommended change intervals. If spending more money makes you feel better, hey, go for it.
 

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2015 Forester X 6MT 6-Gear Manual
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Recommendation depend on engine but: The Bosch +2 and +4 are absolute garbage. You have to use the Ir for DOHC with coil-on-plug and use either copper or platinum for SOHC dual-coil system. Dont use Ir on Dual coil! They will have a weaker spark. the Denso K20PRU-11 copper is a good plug. NGK V-Power are junk. NGK Dual Plat (blue script) are OK. I've owned 6 subarus and tried everything. Bosch std copper plugs are welll made and OK, but may not be the correct overall length for the boots to fit correctly on the SOHC and the heat range seems more limited than the NGK or Denso. Autolites are OK but the nuts tend to rust since the coating on the nut isnt as robust as others.
 

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Dont use Ir on Dual coil! They will have a weaker spark.
I use Autolite Ir Plugs in my 07. Never seemed to have a problem. I mean worse comes to worse, you have to drop the gap a bit.
 

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NGK V-Power are junk.
As long as we're going on anecdotal evidence I'll say that NGK V-Power plugs have worked just fine for my four Subaru N/A SOHC engines and I've never had any problem with them lasting over 30k miles with no noticeable misfire. So "junk", they are not.

Guy
 

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2015 Forester X 6MT 6-Gear Manual
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Ive tuned hundered of cars with thousands of plugs. The US made v-power are not the same heatrange as factory - they run 1/2 heat range hotter, the gaps erode quickly. Unless they improved them since I last used them a decade ago - they are junk to me and i dont go near them OR recommend them. I'm no fan of STD NGK either - the gnd straps would erode 50% faster than other units like Chap or ND or Autolite. Same NOT Recommended with Bosch platinum and +2 and +4 garbage. Denso K20 pru-11 is a MUCH MUCH better plug.
 

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I use Autolite Ir Plugs in my 07. Never seemed to have a problem. I mean worse comes to worse, you have to drop the gap a bit.
Dual resonant coils have to fire BOTH directions gnd-to-center and center-to-ground. the Ir are NOT designed to do this. ONLY C.O.P or dist applications. I recommend denso K20pru-11.
 

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1998 Forester S
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Discussion Starter · #11 ·
OK. It looks like platinum for my Legacy, and iridium for my 98 DOHC Forester - NGK or Denso.

I'm a little confused on one thing though - aren't they all dual coil systems? The coil on my Forester looks just like the one on the Legacy.

I came across this in trying to educate myself on the topic:
"All Subaru Legacy, Forester, Impreza models use a ‘direct’ ignition system. This is a distributorless design that features a crankshaft position sensor mounted at the front of the engine behind the crankshaft and a camshaft position sensor mounted behind the left camshaft sprocket.
Secondary ignition is supplied to the spark plugs by two separate coils contained in a common housing. Each coil supplies voltage to two cylinders."
-northursalia(dot)com/techdocs/pdf/electrical/ignition(dot)pdf
(sorry, I don't have enough posts yet to add outside links)
 

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Dual resonant coils have to fire BOTH directions gnd-to-center and center-to-ground. the Ir are NOT designed to do this. ONLY C.O.P or dist applications. I recommend denso K20pru-11.
So what are you saying, that im gonna kill the plugs or my coil? Shes running great.
 

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COIL on PLUG use the Ir. They will work with the dual coil, but the negative firing cyl will fire easier with a larger center electrode since its firing (ionising) ground strap to Center conductor. As long as its running well leave it be, but if you start to notice lots of misfire, then, have at it.
 

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OK. It looks like platinum for my Legacy, and iridium for my 98 DOHC Forester - NGK or Denso.

I'm a little confused on one thing though - aren't they all dual coil systems? The coil on my Forester looks just like the one on the Legacy.

I came across this in trying to educate myself on the topic:
"All Subaru Legacy, Forester, Impreza models use a ‘direct’ ignition system. This is a distributorless design that features a crankshaft position sensor mounted at the front of the engine behind the crankshaft and a camshaft position sensor mounted behind the left camshaft sprocket.
Secondary ignition is supplied to the spark plugs by two separate coils contained in a common housing. Each coil supplies voltage to two cylinders."
-northursalia(dot)com/techdocs/pdf/electrical/ignition(dot)pdf
(sorry, I don't have enough posts yet to add outside links)
Remote dual coil would be best off with a std Copper or dual-platinum in both applications - not Ir. The Ir have very small "pin" like center conductors that facilitate quick ionisation in lean mixture with the tiny coil-on-plug (secondary wireless)
ignition systems. If you have an individual Coil for-and-at each spark plug, and, NO ignition (hightension) wire, then you have COP DLI ignition and should use Ir plugs.
 

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2010 Forester
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You have to use the Ir for DOHC with coil-on-plug and use either copper or platinum for SOHC dual-coil system. Dont use Ir on Dual coil! They will have a weaker spark.
Just wanted to say thanks for bringing this up. I dug around and found this on NGK's Did You Know PDF:

"It is important to know that in a waste spark ignition, both the center firing electrode and ground electrode will see gap erosion. In plugs with a positive polarity, most wear will occur at the center-firing electrode. Plugs with negative polarity will experience more wear at the ground electrode. As a result of this, vehicles equipped with waste spark ignitions often use double precious metal spark plugs, such as the Laser Iridium or Laser Platinum. These dual precious metal plugs feature either iridium or platinum on the center firing electrode and platinum at the ground electrode. Because these electrode materials are denser, gap erosion is reduced, allowing a longer service interval Being that these materials are denser, this will reduce gap erosion, thus leading to a much longer service interval. The use of nickel- alloy or single precious metal spark plugs in vehicles equipped with waste spark ignitions will result in more aggressive gap erosion and shorter spark plug life."

I decided to go with the Laser Platinum NGK.

NGK (5463) FR5AP-11
 
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I recently changed the plugs on my '06 N/A Forester with NGK Iridium IX, and it's running great, huge increase in power and pickup over the old plugs. Of course, the old plugs had 30K+ miles on them.
 

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2019 Crosstrek 2018 Forester XT
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I recently changed the plugs on my '06 N/A Forester with NGK Iridium IX, and it's running great, huge increase in power and pickup over the old plugs. Of course, the old plugs had 30K+ miles on them.
I doubt you would notice any diffference between iridium or the plats.
 
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