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How to Avoid Piston Failure | Subaru Performance Tips

Posted on 24 April 2015

New to the Subaru game, or turbo cars in general? Here are a few tips to make sure you don't end up with a new set of automotive themed paperweights!

Our years of experience in the Subaru aftermarket have shown us a few things. The first is how to make a ton of power via bolt-ons, an engine build, and a performance tune. We've also learned a few things about what not to do - like how to not blow up engines. Unfortunately, the latter seems to occur very often with the EJ25 series of Subaru engines, and even in stock form they have shown to be prone to catastrphic engine failure. There are a few different theories regarding the failure of the pistons, each one as true as the last.

We've assembled this short list of reasons why Subaru STi / WRX pistons and engines fail, as well as a few solutions that every Suburu owner can employ via tuning, installation of a few parts at home in their garage, and remember to do on a daily basis. First off, lets define a few items inside your engine. The piston inside your EJ25 series engine are cast from Aluminum alloys. As you may know, aluminum can be weak if exposed to heat, so pistons that could survive engine combustion temperatures had to be developed. New alloys such as Y alloy and Hiduminium were designed with implicit intentions for use as automotive pistons.

The "ringland" portion of the piston holds the piston rings via channels cut into the piston head. On the Subaru EJ255 and EJ257, there are three ringlands, which each have a separate job in regards to protecting the engine and piston. In some cases, these ringlands have been identified as the weak point on most Subaru engines. The first ringland is an area that typically fails the most. It's most susceptible to damage since it supports the first compression ring, which seals out heat and combustion gases. The second ringland plays defense, blocking any remaining combustion gases from the combustion chamber and blocking oil that has snuck past the third ring. The third ring is special, as it is designed to force oil into the various parts of the piston, which helps lubricate and cool the combustion chamber and piston. Below are a few of the reasons why many believe Subaru Piston ringlands fail.

At the end of the day, the biggest reason is excess heat and pressure generated by engine knock, detonation, or pre-ignition.

Reason #1- The Factory Tune is Junk

While not entirely true, this still holds value as an acceptable reason given the sheer value of the tune. While Subaru did a great job tuning for all altitudes and climates at various boost levels, there has been some notice of grey areas within the engines tune, especially when driven under high load. Stock engines as well as modded engines are at risk here. What do we recommend? Get your car tuned after every mod to ensure you're safe from detonation. If you're stock, consider getting an wideband AFR gauge like the AEM UEGO or other popular brands to make sure your air-fuel-ratio's are on point through the entire power band. For tuning check out the Cobb Accessport!

Reason #2 - Blow By

Many performance companies attribute premature Subaru engine failure to a poorly designed crankcase breather setup. This sends large amounts of oil back through the intake, dilating fuel octane and causing knock. Turbo engines cause lots of engine blow-by, and it gets considerably worse when running higher boost, but other instances add to the amount of blow-by your engine see's. What do we recommend? Run a proper Air-Oil-Separator system that pulls oil from the intake system via a catch can, and recycles clean air back into the intake system. See our post here for best Air-Oil-Separators.

Reason #3 - Over Driven Engines

In our opinion, one of the worst things you can do to your EJ25x is put heavy load on it at a lower RPM. Building 14 – 18 PSI of boost at 3000 RPM is a high load : low RPM situation, that may not have been properly tuned for your vehicle, especially if you have an aftermarket tune. Some tuners don’t bother sticking the car in 5th or 6th gear and test for knock. You also shouldn't drive the car hard unless you absolutely need to. Despite popular consensus, the Subaru EJ25x wasn't designed for repeated beatings, so treat the engine nicely and it will last as long as you need it to! What do we recommend? Treat the engine well with regular maintenance, keep the oil changes and coolant topped off, downshift and get above 3500RPM before you lay the hammer, and don't drive like you're outrunning your ex.

Reason #4 - Weak Pistons in Heavily Modded Environments

This should come as no surprise to most people. First off, these cars are mass produced, and Subaru needs to consider their bottom line with every aspect in the development of the car, and forged pistons aren't exactly cheap to produce. The factory pistons are as strong as they need to be for stock and slightly modded WRX / STi's, but when you start talking about upgraded stock frame turbo's and tons of boost, the reliability of the stock slugs become questionable. What do we recommend? Forged Pistons if you've got more than bolt-ons.

Reason #5 - Combustion Heat

This reason is a bit less common and harder to prove as a direct result to piston failure, but definitely worth mentioning. Subaru Cylinder heads have quench pads that squeeze air and fuel into the combustion chamber. An interesting design, but the pads are prone to hotspots which create pre-ignition and localized heat rise, effectively weakening the piston leading to melting. What do we recommend? Use the correct grade fuel and run an AOS. Hopefully these tips have explained a few things about the Subaru engine, and opened your eyes to how easy it is to prevent premature engine failure in your Subaru WRX / STi. If you have any questions or concerns, feel free to email us, give us a call, or drop us a comment below! Keep an eye on the blog for more awesome Subaru tips, aftermarket parts, and technical installation guides!

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59 comments

  • Fred Montgomery: April 30, 2018

    I have a 2018 WRX with 1800 miles on it. When do I go for all bolt on, and what do you recommend?

  • Jake Reese: November 15, 2017

    I agree with Jordan Gaghan, who mentioned adding “How to avoid bearing failure.” I have a 2012 WRX with minor bolt on mods, SPT exhaust and Cobb catted down pipe plus a protune from Cobb Surgline and still had rod bearing failure. I changed the oil every 3500k miles, checked it and topped it off every time I got gas but still had the rod bearing failure. I’m not sure how to correct the issue since it appears to be bad design from Subaru but I have heard that upgrading to a better oil pump and oil pan setup, like Killer B, helps so I had that installed as well as got a built short block with forged internals since I want my WRX to actually be reliable.

  • Chris D: November 10, 2017

    Is it common to drop in forged pistons into a stock EJ block?

  • wrxish: January 20, 2017

    I think the better solution is to forget turbocharging and put the 3.6R engine in the wrx. That would be quick enough.

  • david larkworthy: January 03, 2017

    Thanks for sharing.
    I have a 09 Forester xt & though it was part of a stop sale, Subaru did not stop the sale, switch out the engine or honor it when it failed. After researching how this could have possibly happened I learned of a stop sale issued from SOA that included my VIN. I escalated it to the President of Subaru North America and they would not stand behind their car, even though they put a stop sale on because of silver bearings they installed without changing the tolerance. My car did last nearly 100k of highway miles but there were no mods and it was driven carefully. This was not a isolated incident. There are enormous numbers of Subaru cars that have had “catastrophic engine failure”. Anyone who doubts this just needs to type catastrophic engine failure in and they will see Subaru come up first. I love the idea of the cars, the engineering but I also know SOA is aware of these issues.

    Despite the poor support from Subaru I love the idea of a well made, awd, reliable car so I went to an independent tuner who installed a “much improved” 2014 sti motor. Less than 35K miles later, it had a ring land failure. It then turns out that there is a class action lawsuit against Subaru for this issue in 2014. I have basic mods on the car based on this and other forums that discuss and recommend mods you should incorporate in order to have a stronger engine, less likely to fail. I have changed the oil religiously every 3000 miles, use Shell 93 octane gas and never drive it near it’s potential but I have another “catastrophic engine failure”.

    It’s unfortunate that a car with such potential has some inherent design and manufacturing flaws that prevent it from being reliable. I agree with many others on this thread that it is a much bigger issue for the turbo cars but if you are trying to market your company as a reliable car company you should stand behind your cars and improve the designs to minimize “catastrophic engine failure”.

  • gastonmontminy: November 10, 2015

    These are good things to know if you own a turbo Subaru. The pictures do a good job of showing what you don’t want to happen to your car. I appreciate that you offered good solutions for each of the 5 possible problems. It’s true that a bit of regular upkeep, paired with smart driving can help a lot. Thanks for sharing!

  • Pawel Stefaniak: November 10, 2015

    Krzysiu Baciar

  • Alexander Anderson: November 10, 2015

    don’t buy subaru ;)

  • Bong Lovestobike: November 10, 2015

    Damnbarus

  • Jordan Gaghan: November 10, 2015

    How to avoid bearing failure maybe lol

  • Steven White: November 10, 2015

    Psshhhtttt obviously!

  • Erik Emens: November 10, 2015

    Its been a long time since I’ve disassembled a 255/257 and didnt find cracked ring lands. But then again, its typically a tuner’a fault or people who are cheap as shit and put the lowest grade octane in their vehicle.

  • Robert Harridge: November 10, 2015

    Dont think EJ20’s are immune. They can fail in the same ways, but are less susceptible due to the lower displacement allowing for the rings to be more properly placed. Great article for ALL Subaru owners to read.

  • Tyler Hanson: November 10, 2015

    I break all subie motors for sure

  • Justin Longcrow: November 10, 2015

    Tyler Hanson is available for all testing. 4 motors.

  • Keith Fisher: November 10, 2015

    Steven White

  • Kenneth Prewitt: November 10, 2015

    Nic Brasher

  • James Prouse: November 10, 2015

    Well, my pistons may have notches on the top, but after 157,000 miles, my 4G63 is healthier that other 05 STi’s with less miles

  • Nick Moore: November 10, 2015

    Taylor Boll. You need to read this

  • Joel Mashack: November 10, 2015

    Also, check your oil. I have soooo many Subarus come in with less than a quart of oil left. That poor crankcase breather design causes some heavy oil consumption.

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