by Alan LaFrance, find him on Google or read more articles at www.subaruwrxsti.org
http://www.vikingspeedshop.com/how-to-avoid-blown-ringlands-in-your-subaru-wrx-or-sti/
The Problem | Ringland Failure on EJ20 and EJ25 Series Engines
While largely limited to the late model WRX and STI with the EJ255 and EJ257 motors, theres been some concern raised about the shear numbers of ringland failures being reported on the forums. Our goal with this article is to address why this is likely occuring and how you can guard yourself from it happening to your Subaru. Some of these may seem like common sense to most, but for many younger or first time turbo car owners it may be new.
The "ringland" is the portion of a piston that supports the ring itself. The standard Subaru EJ piston will have three ringlands per piston. The first is the most susceptable to damage because it is supporting the primary compression ring. This ring is what seals out the majority of the heat and combustion gases from the crankcase. The second ringland supports the secondary compression ring which is shaped differently from the first ring. It is tasked with ensuring that the remaining combustion gases don't get past and also to scrape any oil that has gotten past the third ring off the cylinder walls. The last ring is known as the oil control ring. It is shaped completely different from the two compression rings because it's duty is to force oil into the various parts of the piston to help cool and lubricate it.
Ultimately for one reason or another the ringland is cracking and failing.
Why do the Ringlands Fail?
There are many theories out there as to why the pistons are failing. Many hold weight with common reasons found in other piston failures, some are model specific, and others are a shot in the dark. Ultimately, the excess heat and pressure generated by knock (aka detonation or pre-ignition) is the culprit. The causality of the knock is what is at odds in this theories.
Theory #1 - The OEM Tune is Terrible
Subaru has had continous problems with it's OEM tunes causing knock since the first USDM WRX rolled off showroom floors. To their credit, tuning an engine to run on all sorts of pump gas with 14+ lbs. of boost pressure in all climates is not exactly easy, especially when you have to conform to vague emissions standards for 100K+ miles. There's strong evidence showing that the failure prone 08+ WRX / STI do run exceptionally lean during a high load moment (read: under boost). In fact, a couple dozen completely 100% stock and well maintained WRXs / STIs have had ringland failure within 40,000 miles.
Counter Point - It seems that tuning the cars doesn't make you immune to the failures as people with tuned stock turbo cars have had similar failure rates.
The Fix - Get the car tuned after most mods to take full advantage of the mods, make more power, get better fuel economy, ... JUST DO IT.
Theory #2 - The Crawford Oil Vapor Dilution - AOS Theory
According to Crawford and a few other shops a major source of engine failure can be traced back to the crankcase breather setup backfeeding large amounts of oil through the intake. This in turn dilutes the octane value and knock occurs.
This is a valid theory since there is considerable amounts of blow-by generated by turbo engines in general. Running higher boost, aging turbos, questionable PCV routing, crankshaft oil aeration, and a multitude of sources can feed oil vapor through the PCV. This vapor dilutes octane value, which causes pressure to rise quickly in the chamber. Once the pressure rises the rate of the reaction also quickens, thus you have a large rise in chamber pressure before an ideal crank position which imparts heavy loads onto the piston. Subsequent latent heat from this will cause continuing detonation, even as the ECU begins to pull timing in reaction. The result is detonation until significant heat is pulled from the chamber, ie. lifting, or the reduction in timing is sufficient to reduce hot-spots.
The Fix - Run a proper AOS system that re-routes the PCV and breather systems to a catch-can. Ideally this should be back fed to the intake, vacuum side, but you can also feed it to the downpipe for a blown suction effect. Grimmspeed makes a good and simple unit, but Crawfords V2 AOS is ideal.
Theory #3 - Driver Induced Failure
I believe that a large amount of failure is due to the driver. Even the most cautious of drivers can cause significant amounts of knock on any tune. This isn't to say that the drivers are dumb or have poor skill, it's simply that most drivers are unaware of the load they are putting on the engine and how that correlates to the tune.
Simply put, one of the worst things you can do to your turbo engine is to load it up at a low RPM on the highway in a high gear. Building 14 - 18 PSI of boost at 3000 RPM is a high load : low RPM moment that may not have been properly tuned for your vehicle. Most tuners don't bother sticking the car in 5th or 6th and road force testing it for knock at low RPM. Subaru and Cobb both recommend getting further into the RPM range before going WOT (3500 - 4000 RPM).
A special consideration for GRs is also the fact that our cars have a fueling harmonic issue around 2800 - 3200 RPM.
It also goes without saying that driving the piss out of the car for prolonged periods of time will cause engine failure. The engine is designed for short bursts of speed. Contrary to whatever you might think or have read the engine does not like prolonged racing. If you are at a track event, have fun, but remember your cool down laps and be sure you are using a quality oil and monitoring coolant temps.
If the car is completely heat soaked from sitting in traffic, don't rip it through the gears the first chance you get. Slowly get up to speed and let the radiators / intercooler shed off some of the heat. In 118 degree Texas summers I've logged 180+ degree inlet temps before the turbo with the A/C running. The engine was misfiring even under light throttle until I could get up to speed and bleed off the heat.
The Fix - Downshift ... get up in the RPM, 3500+ before going WOT. You'll see far less knock. Lastly, don't drive like an idiot.
Theory #4 - The Pistons are Weak!
The EPA and Congress have more to do with ringland failures than Subaru. The ever increasing emissions requirements have pushed many OEMs to leave no stone unturned when it comes to reducing emissions. The piston crown has been weakened because the ring pack was moved further up the piston. Subaru did this to avoid fuel from being trapped between the piston and cylinder wall above the first compression ring. This would increase hydrocarbons in the emissions and necessitate additional catalytic converters (and cost and power reduction). Trust me, Subaru would MUCH rather install a nice forged piston with beefy ring lands.
I recommend you guys read up on the Federal emissions testing requirements for OEMs. It's very involved and includes testing the vehicles into 6-digit mileage figures with remote accessible emissions equipment that the EPA can request/access at any point in the test. Failure would set back Subaru not only in time, but in significant costs as well. These costs would be forwarded on to you.
Ultimately, the pistons might be weak, but the real cause is pre-ignition and detonation. Remove the knock and the pistons work just fine.
The Fix - Forged pistons ... unfortunately.
Theory #5 - Latent Combustion Heat
An observation I've made is that nearly all of the failed pistons are failing at the skirt end of the piston. This is also in line with the quench pads on the head which work to squirl and squeeze the air / fuel mixture into the chamber center. However, these pads are also prone to building up hotspots, especially the sharp ridge at the end of the pad. These hot spots can create pre-ignition and localized heat rise. This heat weakens the piston and causes the failure / melting that you see.
Since the gasket is 0.6mm the quench effect is largely reduced, but when you add carbon deposits and rod stretch at high RPM you can start to greatly reduce the distance. Especially on an engine that is consuming poor grade gas and oil vapor.
Failed Piston
These are pictures I took of a customer's piston. We replaced them with some JE forged pistons. Notice the melting (rounded grey metal) as well as the pitting and fast fracture and crushed ring lands. Some serious pressure and heat were at play here.
