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Subaru Oil System Upgrade

Eliminate oil starvation in your WRX.

Scott Tsuneishi
Jun 2, 2011

If you have a Subaru WRX and are looking to improve or upgrade your engine’s oiling system but don’t want to spend a pretty penny, we have the perfect solution use an STI oil pan. Why upgrade to an STI oil pan you ask? The ’02 to’05 Subaru WRX oil pans are designed with a flat belly surface. Lateral and longitudinal forces from turns, acceleration, and braking can push oil around the floor of the pan and up the sides into the crank assembly, which creates windage, a condition that causes parasitic drag on the crankshaft caused when oil splashes out of the sump and keeps the oil from settling back into the pan. Poor internal oil circulation keeps oil from returning to the sump quickly, causing the oil pickup to surge and in many cases starve the oil pump.

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The ’04 to ’05 STI and ’06+ 2.5L oil pans use a built-in sump; think of it as a built-in reservoir, that concentrates the oil closer to the pickup, making it more stable and less prone to oil flow fluctuations. Another advantage to using an STI oil pan is its more compact design, which offers additional clearance that can be used with a majority of aftermarket equal-length headers versus only a select few that can be used with the WRX pan. Follow along as we show you the basics on swapping out your own pan, installing a baffle plate, and replacing the oil pump pickup.

STI Oiling Parts List:
• ’04 - ’05 oil pan (Subaru PN 11109AA131)
• ’04 - ’05 dipstick (Subaru PN 11140AA120)
• ’04 - ’05 oil pump pickup (Subaru PN 15049AA070)
• ’04 - ’05 oil control tray baffle (Subaru PN 10915AA010)

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1. Upgrading to an STI oil pan setup is easy and can be done in the comforts of your own driveway. Although our engine was already removed when we performed our upgrade, the oil pan can be swapped with an engine still under the hood by removing the header and downpipe, loosening the motor mounts, and disconnecting the pitch stop to lift the engine a few inches off the chassis to gain access to the oil pan bolts.

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2. The oil pickup connects to the bottom of the oil pump as its draws oil up into the engine. Be sure to clean any foreign material inside and from out of the pickup, and that includes the filter screen.

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3. When removing the WRX baffle plate, be careful not to drop any bolts into the block.

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4. Use a razor blade followed by a wire brush to remove the old silicone gasket prior to installing the STI oil pan.

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5. Here’s a close-up of an STI oil pump pickup on left versus the WRX setup on the right. The WRX pickup measured ½-inch shorter than the STI pickup, which must be replaced when upgrading to the STI oil pan.

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6. We recommend replacing the oil pump pickup with every new engine build, especially if the engine experienced a catastrophic failure like spun bearings, blown pistons, or piston ring breakage as bits of metal can get caught in the strainer and flushed into your rebuilt engine. Inspect any used and even new oil pickups for damage or dents, especially surrounding the pickup tube as we have come across a few posts on internet forums claiming their factory welded/brazed oil pickups have cracked at the base (see pointer) of the pickup tube. A dented or damaged pickup can severely inhibit the flow of oil into the engine, causing oil starvation. Aftermarket pickups sold by manufactures such as Killer B Motorsports and Moroso have become a popular upgrade among Subaru owners.

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7. While the WRX pan shown on the top has a wider bottom design and looks to be the better of the two pans, its flat bottomed design gives the oil more room to move away from the pickup while cornering hard. The STI pan on the other hand uses a sump design to keep the oil near the oil pickup tube during hard cornering and is less prone to cause oil starvation, which proves beneficial for track use. The most important role of any oil pan regardless, if it is OEM or aftermarket, is maintaining oil control to allow good oil supply.

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8. Comparing the internal oil pan baffling systems of the WRX (top) to the STI (bottom) you will notice the WRX pan is designed with fewer passages for oil to flow. The WRX pan’s double baffle layout requires oil to cascade down two levels before reaching the bottom of the pan. During extreme g-force situations, lateral loads can cause the oil to sit between the baffles, causing oil to inefficiently drain into the bottom of the pan. The STI oil pan uses a single baffle design with larger openings for oil to flow more efficiently.

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9. We used Photoshop to show the location of the STI oil pickup and internal pan structure.

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10. A side-by-side comparison of a WRX oil baffle plate (windage tray) on the left to an STI unit on the right is significantly different in design. The oil baffle plate works by directing oil from the cylinder heads onto the baffle plate, making its way into the oil pan. This travel helps remove air that mixed with the oil during the lubricating operation before the oil returns back into the pan and rerouted through the oil pickup. Cosworth and Tomei Powered offer upgraded oil baffle plates with one-way valves to prevent oil from reentering the upper crankcase chamber, thereby limiting blow-by and preventing oil starvation during hard driving.

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11. Before installing the new STI oil pump pickup onto the baffle plate, be sure to replace the O-ring (PN 806917080). The O-ring is a key component that initiates a tight seal between the strainer and oil pump to draw oil up into the engine.

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12. Be sure to replace the O-ring gasket, located towards the rear of the pan with a new Subaru gasket (PN 11122AA340).

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13. We used thread locker on the oil pan bolts as a precaution to ensure they won’t come loose from engine vibration.

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14. Since the WRX does not use an oil pan gasket from the factory, you will need to use silicone to properly seal the pan. We used Subaru Fuji bond sparingly around the pan before torquing down the 10mm bolts to a factory specified 3.6 ft-lb.

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15. Use a feeler gauge to make sure that the oil pan’s lip is flat prior to installation. Used oil pans are often bent or damaged due to it being pried loose using a screwdriver. If it’s bent, you’ll have a leak, regardless of how much silicone is used to seal the pan.

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South Coast Subaru
By Scott Tsuneishi
247 Articles



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