Know Your Subaru WRX or STi Oil Pan
The details to follow are specific to USDM (United States Domestic Models) part numbers, as well as common names and descriptions. The models outlined here may not have the same descriptions and/or follow the same models. For example: A USDM Turbo EJ25 STi Oil Pan (part number 11109AA131) may also be found on EUDM (European Domestic Market) or JDM (Japanese Domestic Market) Turbo EJ20 STi Spec-C (Twin Scroll). This information also ONLY covers WRX and STi models. While other models (Forester, Outback, Legacy, etc., and this overseas versions) may use the same engines/turbo configuration, the oil pan part numbers do not always follow the WRX and STi part numbers.
Subaru oiling systems have evolved over the time since the introduction of the EJ engine in 1989. There have been improvements made from a performance perspective, and a general statement can be made that more recent versions offer a performance advantage over older versions. This follows the performance progression over time as WRX and STi models have increased performance. As far as which oil pan would fit which engine, ALL EJ engines are the same from the bottom. They all share the same oil pan (and oil pickup) mating surface geometry and bolt patterns. If changing to a newer version oil pan, you must always use the oil pickup made for that pan, not for that year engine. Same holds true for the dipstick and dipstick tube (if you want to retain OEM level accuracy; you an always scribe your new ‘full’ mark on an old dipstick). In other words, the oil pickup part number to be used needs to be for the oil pan it is made to be used with.
The three most popular OEM oil pans and our own Killer B Motorsport performance oil pan are shown here. As can be seen, they all have the same flange bolt pattern, dipstick location, and orientation. They all use an M20 drain plug and they all use liquid adhesive gaskets.
The Exterior Details
Here you can see that some pans have what’s commonly called a kicked-in, or tubbed, while the older versions have a flat bottom design. The flat bottom version generally holds a little more oil than the newer versions, it also does not control the oil as well. The flat bottom style allows the oil to move around the bottom of the pan a lot more vs the newer versions that keep the oil deeper and near the oil pickup. The flat bottom oil pans have more ground clearance. The other advantage to the newer tubbed versions is that they allow for the higher performance twin scroll equal length OEM manifold, or aftermarket equal length headers to be used.
There are some stark differences in the baffles with these oil pans. Older versions use two horizontal baffles and newer versions use a more direct flow-thru path. The horizontal baffles help with oil control a lower cornering speeds and allow for increased ground clearance, but they have performance downsides. Under more severe cornering and accelerating, the oil can pool on the horizontal baffles preventing good flow to the bottom of the pan and in oil pickup inlet. Conversely, the tubbed pan designs allow for more direct oil flow to the bottom of the oil pan. This is why early generation WRXs tended to have more oiling related issues, especially on track.
The 11109AA131 and 11109AA151 (newer) are very similar in design. The tub section on the 151 version does not have a step, a portion of the baffle is integrated into the tub side and some other subtle changes in the baffle profile. The drain port also does not have a baffle; allowing more oil to be drained at oil changes. These changes should not provide any better performance, but are very likely done to reduce material, part count, and manufacturing times; cost.
At some level of performance, the OEM pan will not be able to provide adequate oil supply to the engine. A Killer B Motorsport oil pan will provide a reliable supply of oil under the most severe track conditions. The additional +30% helps keep oil temps in check and maintain good oil condition. The second NPT drain port can be used for oil temperature monitoring or a quick change oil valve. The Killer B Motorsport Performance Oil Pan is welded aluminum providing a stronger and lighter part than OEM, and completely Made in the U.S.A.