The following procedure explains removing and replacing spark plugs on a Subaru Impreza STi. The original instructions below specifically refer to fitting Denso Iridium spark plugs that are one step colder however these procedures are generic for OEM plugs also. Please click the thumbnail pictures below for a full size version.
The author indicates this took around 1 hour to complete the first time, perhaps half that next time. Right, off you go you are being timed!
Remove the battery and windshield washer fluid reservoir (Step 1)
This is simple, just four bolts holding the battery and two bolts (<< seen in step one), a hose and a clip (<< seen in step 1.1) holding the washer reservoir in.
Washer Fluid Reservoir
Removing intake and assembly (Step 2)
Note: the author can break this down further as he doesn't have the stock intake, instead the K&N typhoon is shown. The stock intake is very easy just a couple of bolts, clamps etc. (<< steps 2 and 2.1)
Removing the spark plug coils (Steps 3, 3.1, and 3.2)
One gold colored bolt on each of the coils (they look like little grey boxes). There is one coil for each cylinder (total of 4, two on each side). Remove the four bolts and coils. See Steps 3, 3.1.
Coil Packs - Right
Coil Packs - Left
Once the coils have been removed this is what it should look like (see step 3.2 pic).
Coil Packs Removed
Remove the old spark plugs (Step 4)
You'll need a spark plug socket, with a small extension. Get the socket seated on the spark plug and remove them. It can take quite some torque to get them out. Get all four of them out.
Install the new spark plugs (Steps 5)
Note: Check the new spark plug gaps now - they should be fine, but check anyway
To install the new plugs just put the spark plug onto the sparkplug socket (which is connected to the small extension). Once you get a hold on the spark plug which is in the coils whole, tighten it. It should be very easy to tighten at first (85% of the tightening was done by hand). If it's hard to tighten at first, then your cross threading, back it out and try again. Do this to all 4 spark plugs and make sure there tight.
Installing the coils (Step 6)
This is just like steps 3. Line the coil back over the spark plug and re-install the bolt. Remember on the left rear side (facing car) there is the bracket that needs to connect with the coils bolt. Each coil must connect to it's spark plug. The coils with the shorter wires go to the spark plugs closer to the firewall. Even though when removing these, it felt like they had a clip holding them in, THEY DO NOT CLIP BACK IN, they just seat there and you tighten them down
Tighten Coil Packs
Re-install battery, washer resevoir, and intake (Step 7)
If you forgot how to do this look at the pictures in steps 1 and 2 again.
Your done! Start the car up and go for a ride.
- Spark Plugs Used: Denso Ultra-Fine 0.4mm Iridium Spark Plugs, part# IKH22. These are one step colder over the stock spark plugs
- Price: Around $50 bucks
- Gapping: These spark plugs DO NOT need to be gapped at all. They come pre-gapped for the STI. Just put them in and tighten them down.
- Tightening torque (spark plug): 21 N·m (2.1 kgf-m, 15.2 ft-lb)
- Tightening torque (coil): 16 N·m (1.6 kgf-m, 11.7 ft-lb)
- If a torque wrench is not available, tighten the spark plug until gasket contacts cylinder head: then tighten further 1/4 to 1/2 turns.
- Anti-seize can be used around the plug threads before screwing them in to prevent the spark plug from seizing to the aluminum.
Why colder spark plugs can be better......
Colder spark plugs help prevent knock when running higher boost levels. They also help run lower EGT's. Running to cold could cause pre-ignition. One step colder is recommended once about 75 hp over stock.
Spark Plug Heat Range
A spark plug's heat range has no relationship to the actual voltage transferred though the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.
The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.
Information source - Crewchief from IWSTI forum thread I installed my Denso Iridium (1 step colder) spark plugs today. DIY included