I have a 2015 Subaru Forester 2.5i with a M/T.

Can standard plugs be used in place of the (overpriced) platinum/iridium OEM's?

I tried platinum in two previous cars and they gave less mpg and wore out in barely 20k miles. Autolite & AC Delco both gave the longest life and high mpg.

  • Platinum plugs last longer than copper, so if you are worried about longevity, stay with platinum. The less mpg thing... eh, that's a rabbit hole that I'm not going down. Get your micro vortex intake and magnetic fuel line add-on to help there. /s
    – cory
    Aug 18 '16 at 18:06
  • Use the reccomended plugs. On a recent vehicle I worked on (A Honda Ridgeline, I think) the customer changed to cheap copper Autolights instead of the $12/each irridium bespoke NGK plugs. The check engine light would come on constantly with various P03xx codes. The plugs were specific to that vehicle, and the ECM did some magic in measuring spark efficiency which required the specialized plugs.
    – SteveRacer
    Aug 19 '16 at 5:27

Your car calls for iridium plugs

The manufacturer wants you to put in iridium spark plugs. Iridium plugs can last longer than non-iridium due to their metallurgical properties associated with electricity. They will tend to take longer to oxidize and begin to break down than other metal types.

I would simply go with the manufacturer's recommendations regarding your spark plugs and will probably find that the mileage numbers will be closer to expectations than not.

  • Agreed. The extra money you put into the tougher plugs which are called for, will last longer saving you money in the long run. I would bet the engine is turbocharged, which is probably why they went iridium in the first place. If it's tuned for the iridium, use the iridium ... you cannot go wrong. Aug 18 '16 at 21:45
  • @Pᴀᴜʟsᴛᴇʀ2 interesting, I never knew engines were tuned to plugs. I guess it's all part of the timing, how fast the discharge arc propagates and how large the arc is and all that good stuff. Sounds like there's variability in these matters.
    – jxramos
    Aug 18 '16 at 23:23
  • @Pᴀᴜʟsᴛᴇʀ2 As well, I believe, there is a component about reduced resistance due to the use of iridium. Better spark with same voltage. Aug 18 '16 at 23:25
  • @jxramos - What I was getting at is the fact an ignition system designed to use an iridium plug will throw a certain amount of energy through the system which the iridium plug can handle, but yet a lesser one would not stand up to over time. I hope that makes sense. Aug 19 '16 at 0:17

The 2015 Forester needs Iridium plugs, so you should use them.

Also, they'll last longer than Copper. On most cars, that's not really an issue. With a Forester, having a boxer engine, the plugs are on the sides of the engine. It's not known for mechanics to use a crowbar to move the engine to get the plugs in (or even to undo the engine mounts, which is what I do). You don't want to be changing plugs too regularly; plugs are NOT a trivial job on any Subaru, so use the recommended ones.

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