Sparkplug replacements are definitely "funner" on boxers. This is my 4th time doing spark plugs on this car, was testing different plugs/gaps, thus starting to get better at it. You should expect to set aside around an hour of time when doing the plugs on these cars. Luckily the Legacy has a wider engine bay, thus doing spark plugs on it is actually easier then on the Forester. I don't have to remove the battery to get to the driver side plugs, which saves time and the headache of resetting clocks/radio stations.
Driver side plug removal/installation with battery in place
Unplug the coil packs
Use a 12mm narrow socket to unbolt the coil packs, the front one comes out easily as is, but the rear one requires a trick to pull out. As you can see the frame is in the way.
Flip it upside down and it will come out much easier, follow the same procedure when installing
I use a 5/8" magnetized spark plug socket and a 90* bendable extension, standard small extensions are too long sadly. Pull out the plug, install new one and reinstall the coilpacks.
Passenger side one is a bit easier, it does require you to pull the stock airbox out, but with it out you have a ton of room to breathe.
Once again pull out the coilpacks, you will have to flip the rear one upside down again too.
This time I can use a longer extension that lets you have better leverage to unscrew the plug by hand
Reinstall and plug everything back up
If you got OEM plugs and don't care to do anything else that's it. I personally sidegap all of my spark plugs, even platinum/iridium tipped plugs. I've seen power gains on the dyno and have seen gas mileage improvement by going to side gaped plugs, also it seems like you can run a bigger gap without blowout. On my 04 Forester XT sidegaping the same exact plugs raised my city gas mileage from 14-15 to 18-19. On my P11 G20, my highway MPG went up from 28-29 to 32-33mpg.
Iridium plug as it came, these were -11 gap stock. You can see the group strap still overhangs the tip. I used a sharpie to mark how much to cut.
You can see how much to cut and how the ground strap covers the tip
I like fine tipped plugs, you don't have to cut as much from the ground strap, thus the plug life should still be good.
Another trick is to index the plugs, you want the open end of the plug facing the fresh air/fuel mixture. Now on boosted applications this can promote spark blowout. Using a sharpie mark the top of the porcelain on where the ground strap is located at, then make sure that mark is not pointing directly to intake valves.
I know it's almost impossible to index plugs on the Subaru, but with some mirrors it is possible, weather on or not it's worth your time is up to you.
As for the gap, after you sidegap you will have a rather large gap (doesn't help that these plugs were 1.1mm gaped). I use the wiretool now days, it's more accurate then traditional gaping tool. It's simple, if the wire fits, your at X gap, if it doesn't, your below it.
OEM gap is .030, so carefully gap them down. If you reduce the gap too much, carefully bend the ground strap back up, don't damage the center tip!
Now dust your hands off and go have fun with the car, don't forget to reset the ECU! I'll have some Virtual Dyno graphs and MPG results later.