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.
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)
Refer to your owners manual for recommendations. Alternately, you can visit an auto parts store or online retailer for recommendations on suitable spark plugs designed for your vehicle. Major manufacturers are:
Who are the specialty spark plug manufacturers? These manufacturers make specialty plugs that have unique compositions or designs that claim increases over traditional plugs. They are listed for advanced users or those with interest.
b. Beru (specifically the Silverstones found here)
d. PREP spark plugs
e. E3 spark plugs
f. Pulstar plugs
What types are there? There are really three main types:
a. conventional nickel alloy (commonly referred to as “copper”)
Which type should I use? That depends on how often you are interested in changing the spark plugs. Conventional spark plugs generally last one year. Platinum or iridium can last, depending on manufacturer specifications, up to seven years.
What’s some good background spark plug information?
Materials: The three main types of spark plug materials are nickel alloy, iridium, and platinum. Copper can be used in the core all plugs.
All ground electrodes are made of nickel. The use of Platinum and Iridium, which are stronger, allow for much finer CENTER electrodes (the ground electrode is still Nickel). These finer electrodes do not quench the flame core as much as a conventional style plug. This increases ignitability, therefore increasing HP. It’s not a huge gain, but cylinder pressures are measurably higher.
Platinum or iridium can be used as a thin pad which is laser welded on the ground electrode (the “J” strap), this serves to increase the life of the plug.