Sump Sense for Subarus

Every boxer engine has a plumbing problem built into its shape. The cylinder heads hang out to the sides, at or below the level of the crank, and oil that gets pumped up there has to find its way back down to a pan that's sitting under a very wide, very flat engine. Add sustained cornering G, high rpm and a big turbo's worth of blow-by, and you've got the recipe for the oil being everywhere except the pickup.

A dry sump is the motorsport answer. It's also expensive, complicated and loaded with new ways to ruin your weekend if it's done halfway. Let's look at how Subaru dry sump systems work, what they actually fix, what they cost you in complexity, and who should (and shouldn't) run one.

Why Boxers Have It Harder

On an inline or V engine, oil drains from the heads straight down to the sump with gravity's help. On a boxer, gravity isn't nearly as helpful. As we wrote in our bore finish guide: "Boxer engines are a unique design where gravity drainback simply isn't an aid," which puts more workload on the piston rings and crankcase sealing to keep oil consumption down.

Under hard, sustained cornering, oil can pool in the heads instead of returning to the pan. Dailey Engineering designs its Subaru EJ dry sump with "two -10 scavenge inlets for scavenging the cylinder heads keeping oil from collecting in them while cornering." When a dry sump manufacturer adds dedicated plumbing for a problem, that's a pretty good sign the problem is real.

Don't confuse that with the FA20DIT's dedicated scavenger pump. That one exists because the turbo hangs under the engine, too low for its drain oil to get back to the pan by gravity, so the pump pulls it out of a catch tank at the turbo. Separately, when Toyota and Subaru packaged the FA20D for the BRZ/86, Toyota said its engineers "designed the oil pan to be as compact as a dry sump" to get the engine lower, with internal fins to scoop up oil.

What a Wet Sump Does (and Where It Runs Out)

A wet sump stores the oil in the pan under the crank. A pickup sits in that puddle and the engine-driven pump pulls from it. Simple, cheap and reliable for most driving.

The weakness is physics. As Engine Builder put it: "When a race car is cornering hard or accelerating at full throttle, G-forces can push the oil inside a conventional oil pan away from the oil pickup tube. Baffles and a deeper oil pan can reduce the risk of oil starvation, but not entirely."

Add the boxer's habit of holding oil in the heads, and a wet sump EJ on a road course or hill climb can see the pan level drop at exactly the wrong moment. The bearings don't care that it was only for a second.

What we've seen on EJ track cars

Rod bearing wear and oil pressure go hand in hand. When a wet sump Subaru using a large diameter gerotor pump is busy pushing all the volume up & out, surge can creep up, and a few milliseconds of this can break the hydrodynamic wedge. That's where touchdown happens. This is compounded by rods 2 & 3 sharing oiling from main number three.

Combine this surge phenomenon with a cross-drilled crankshaft, high rpm and an engine whose oil rings don't have gravity-assisted drain back, and you have the perfect setting for journal-to-bearing touchdown, which will become rod knock. For what it's worth, we've logged oil pressure as low as 55 psi at high rpm on track without an acute event occurring, even when we're well below the old rule of thumb of 10 psi per 1,000 rpm.

How a Dry Sump Works

A dry sump moves the oil supply out of the pan and into a separate tank. The pan becomes a shallow collection tray, and an external, multi-stage pump does two jobs:

1. Scavenge stages suck oil and air out of the pan (and on a good boxer system, out of the heads) and send it to the tank.
2. A pressure stage draws de-aerated oil from the bottom of the tank and feeds the engine.

The tank isn't just a bucket. Roger Clark Motorsport's EJ installation manual explains that returned oil is "highly aerated especially when oil temps, RPM and cornering forces are high," and that dry sump tanks use a swirling action and a baffle plate to separate the air. RCM's emphasis: tanks need roughly ¾ oil to ¼ air space, and it's easy to overfill one.

What an EJ kit actually looks like

Using RCM's published four-stage EJ20/22/25 system (developed with Auto Verdi Racing) as an example:

• 3 rotor-style scavenge stages + 1 spur-gear pressure stage
• A replacement oil pan fitted with scavenge filters and a windage tray
• A replacement cover where the OEM front oil pump lived
• A combined toothed timing belt idler and oil pump drive pulley, which replaces the OEM toothed idler, and a separate drive belt for the pump
• Plumbing: -12 JIC minimum supply line from the tank (with -16 considered for long runs), -10 pressure line to the engine, -12 return to the tank
• Breathers from both heads and the block into the top of the tank, and the tank breathing to atmosphere
• About 8 liters of oil total with RCM's tank

Dailey's EJ system takes a different approach to packaging. Its latest revision raised the hose ends 1.5" for ground clearance, is shallower than the OEM pan, scavenges the block, and adds the two head scavenge inlets.

Then there's the Cosworth setup we've had on an EJ257: a dry sump tank and scavenge pumps, but an OEM-style gear rotor pump still makes the pressure, so it's neither beast nor fowl, just an odd one.

The Pros

1. Oil supply that doesn't care about G-forces

This is the reason to do it. The pressure stage feeds from a tank designed to keep oil at the outlet, not from a puddle in a pan. RCM notes that wet sump systems with surge show low oil pressure "because the oil is full of air bubbles," and that this "foamy oil is not good for bearing life."

2. De-aerated oil

The tank separates air before oil goes back to the bearings. Clean, solid oil supply is what bearings actually need. Pressure on the gauge is just one symptom.

3. Head scavenging, the boxer-specific win

Pulling oil out of the heads directly attacks the problem that makes boxers different. It's a big reason a Subaru dry sump is worth more than "just a race car part."

In the Subaru application, evacuating oil from the valve covers is an advantage for cars running slicks on banked or off-camber tracks. Slicks let the car hold more lateral load for longer, and banking or off-camber corners hold that load in one direction long enough for oil to pile up in the low-side head. Pulling it out of the valve covers keeps it moving back to the tank instead of sitting where the pump can't reach it.

4. Crankcase vacuum and less windage

With enough scavenge capacity, a dry sump can pull vacuum in the crankcase, which reduces windage and oil drag on the rotating assembly. Engine Builder reports a potential gain of 5 to 15 hp or more, but says realizing it requires "at least 8 to 10 inches Hg or more of vacuum," and that higher vacuum mainly pays at very high rpm. They also note that on a low-rpm, long-stroke, high-torque motor, a dry sump "is probably not going to show any measurable power gain." Translation: don't buy one for horsepower.

5. Lower and shallower pan

A shallow pan gets you ground clearance, or the option to mount the engine lower. On a Subaru, ground clearance under the pan is often the practical win.

6. More capacity and better oil temperature control

An external tank holds more oil and makes it easy to put an oil cooler in the circuit. RCM shows the cooler on either the return or the feed line, noting about 0.5 bar less pressure with the cooler on the pressure side. More on where that oil temperature should live in the setup section below.

The Cons

1. Cost

Engine Builder called cost "the main stumbling block." And the pump is only the start: add the tank, lines, fittings, cooler and installation time.

2. Complexity and more places to leak

Lots of hoses and fittings means lots of potential leaks. Suction lines have to resist collapsing under vacuum, pressure lines have to resist bursting, and everything has to be supported away from exhaust heat. Blow a line and you've starved the engine.

3. A belt is now keeping your engine alive

Engine Builder notes that a belt failure on an externally driven pump "could cause a loss of lubrication resulting in engine failure," though they add that most belt drives are very reliable. RCM recommends replacing the pump drive belt once a year, or sooner based on inspection.

4. Maintenance you didn't have before

RCM's maintenance list includes cleaning the pan's pickup filters periodically and inspecting the pump drive pulleys and belt. It's not a set-and-forget system.

5. Packaging the tank

The tank has to live somewhere: engine bay, passenger footwell or trunk, all of which RCM says they've proven out, with correct hose sizing. On a street car that's a real tradeoff.

6. Sealing and the piston cooling question

If you're pulling meaningful crankcase vacuum, the engine has to be sealed tight. Engine Builder recommends pressure testing an assembled engine to around 8 psi and listening for leaks. They also warn that pulling oil out too aggressively can reduce the splash oil that cools pistons and wrist pins, which is why high-output engines often use piston oil squirters.

7. When it does fail, it spreads the mess

A dry sump does suck debris out of the pan instead of recirculating it. Engine Builder also notes the downside: that debris ends up in the tank, cooler and lines, and all of it has to be cleaned or replaced after an engine failure.

Setup Details That Make or Break It

• Prime it before it ever fires. RCM's procedure: with the drive belt off and the system plumbed and filled, spin the pump with a drill or air ratchet (not an impact) until oil returns to the tank, then recheck the level.
• Set the pressure with hot oil and the oil you'll actually run. RCM's recommendation is a minimum of 5 bar and maximum of 7 bar at 7,000 rpm with oil around 90°C, with most applications targeting 6 bar. It also notes that bearing clearances, piston oil jets and oil-controlled VVT change what's right for a given engine.
• Fill to the hot running level. Not to the top of the tank.
• Size the lines. Undersized supply lines starve the pressure stage before the engine ever gets a chance.

How we set one up

Get the largest feed fittings you think are reasonable, and then go bigger! -16 or 1" will minimize suction losses on the feed side, provided you avoid tight radii on the inlet path. Before first startup we'll often pull the belt & give everything a good prime, or better yet, use a pressure pot.

I may be biased, but we've found Bill & Patrick Dailey of Dailey Engineering great resources in this arena. We've worked with some Peterson Fluid Systems stuff as well as a couple of other systems. I like using fitting screens and inline cartridge filters before the tank & cooler. In most systems we start with the manufacturer's recommended relief settings until we see hot (210°+) idle. Then we set our hot idle relief floor pressure and carry on.

Oil temperature: three things to remember

1. Oil likes to be above 100°C (212°F). That's where moisture boils, and moisture causes oxidation & corrosion.
2. Oil doesn't like to reach its smoke point or flash point. These temps vary based on what oil you're running, which on the internet can get you into a fight quickly, so I'll say regular mineral oil is going to start oxidizing at 245°F–265°F. Proper synthetics can survive 300°F+, with smoke points nearing 390–400°F on proper ester base stock.
3. Keep the oil in the sweet spot. Keep in mind, the bigger the tank, the longer it takes to warm up, and the looooonger it takes to cool back down if you're using an air heat exchanger.

Who Should Run a Dry Sump?

Probably yes:

• Dedicated track, time attack, hill climb or rally cars that see sustained high G and high rpm
• High-dollar engines where one oil pressure dip is a very expensive lesson
• Builds where pan depth or ground clearance is a real constraint

Probably not (yet):

• Street cars and occasional autocross. A well-baffled wet sump, a correct oil level and good oil temperature control usually make more sense.
• Builds where the budget would leave the rest of the engine under-done. A dry sump won't save a crooked main tunnel or a bad tune.

Do this first, either way: get the basics right. That means correct bearing clearances, true main bores, a sensible oil, oil temperature monitoring and an oil pressure gauge or log you actually look at.

FAQ

Do Subarus have oil starvation problems? Boxers are more prone to oil control trouble under sustained cornering, because oil can collect in the heads and surge away from the pickup in the pan. That's why EJ dry sump kits like Dailey's add head scavenge inlets.

Will a dry sump add horsepower to my Subaru? Maybe a little, with enough crankcase vacuum and rpm. Engine Builder cites 5–15 hp potential when pulling 8–10 inHg or more, and little to no gain on low-rpm, torque-oriented engines. Buy it for oil control, not power.

How many stages does a Subaru dry sump need? It depends on the system and goals. RCM's EJ kit uses three scavenge stages and one pressure stage. Dailey's EJ design scavenges the block plus both heads. More scavenge capacity generally means more ability to pull vacuum.

How much oil does a Subaru dry sump hold? It depends on the tank. RCM says its EJ system with its tank typically takes around 8 liters total, with the tank filled to about ¾ oil and ¼ air.

Is a dry sump reliable on a street car? It can be, if it's properly installed and maintained. But it adds belts, lines, fittings and a tank to look after, and for most street cars the cost and complexity aren't justified.

Let's Talk Oil Control for Your Build

If your Subaru is seeing real track time and you're wondering whether a dry sump, a better wet sump setup or just better oil management is the right call, send us the details: how the car is used, power level, tires and any oil pressure logs you have. We'll help you decide before you buy, and we'll build the engine to suit. Call 970-242-0919 or drop a note through the contact form on our engine machining page.

Sources

• Dailey Engineering, Subaru EJ dry sump
• Roger Clark Motorsport, Installation Instructions for RCM Dry Sump System, Subaru EJ20/22/25, V4 14/11/2023
• Engine Builder, "Dry Sump Oiling Systems," Larry Carley, Nov 2012
• Australian Car.Reviews, FA20E/FA20F
• COBB Tuning support, "VA WRX FA20 Turbo Removal"
• GCG Turbos, "Don't forget the scavenge pump"
• Toyota Europe, GT86 press kit
• ASF Machine, bore finish guide