5. Subaru EJ257
The first major weakpoint in the engine is the stock pistons. The EJ257 has a pretty large bore in stock form, and in an effort to reduce crevice volume and hydrocarbon emissions with a large bore, Subaru engineers moved the top compression ring high on the piston. This is great for clean air but also greatly weakens the ring land- not too good for a turbocharged engine, especially when they are modded for more power on pump gas.
The pistons are cast with a high silicon, hypereutectic alloy, which is very dimensionally stable allowing for tight piston to cylinder clearances, which is good for long term wear and emissions. The drawback is that this alloy is somewhat brittle, and when combined with a high top ring location, you have a piston that is very prone to damage with even a slight amount of detonation. Many an EJ257 has died an early death due to overzealous tuning. A few times in the engine’s production history, errors in factory tuning have caused engine failure and warranty headaches for Subaru as well!
Around 340 hp is the conservative limit for stock bottom end pump gas bolt on power with the stock EJ257 bottom end although some reputable tuners claim to get more. For much more than this, forged pistons and good connecting rods are a must.
The next weak point of the EJ257 is the oiling system. The rod bearings get starved for oil at much over 7500 rpm and can spin. The fix for this is a cross drilled crank, upping the oil pressure by shimming the oil pump relief and using a good, higher capacity pan and baffling. Ideally a drysump should be considered.
As the power approaches 500 whp, the main issue with the EJ257 is that you start to approach the limits of the engine’s structural integrity. The engine cases are not up to the task of containing the power and start to flex. The first manifestation of this problem is the inability to maintain head gasket seal with the open deck of the case. Some builders weld plates to close up the decks and others sleeve the cases which causes other issues with case integrity.
If you have ever picked up a Subaru engine case you can see how lightly constructed they are. It is difficult to machine the bores without several thousandths of an inch distortion due to the flex of the parts caused just by bolting them together, hence any Subaru case should be machined with special tooling to simulate the stresses of being bolted together with the other half of the case and the heads!
At power levels past the mid 500’s, the case halves also flex in the area of the main bearings which causes issues with bearing life even with proper high grade studding of the cases. Some engine builders will dowel pin the cases together to help deal with this.
Despite all of this, some tuners have gotten in excess of 1000 hp from the EJ257, albeit only in drag racing form. It’s possible, but not cheap or easy.
A closing thought is have you ever seen a reliable, Unlimited class Time Attack Subaru?!
We really don’t think any of these engines suck, well maybe the 4AG does, but we had to grab your attention! Like we said in the beginning, all of these engines can be made powerful and reliable but it just takes some careful thought, engineering and money. If you skimp on any of these, you can easily build a bomb. Other engines can be made just as powerful and reliable for less money but it all comes down to how much are you willing to spend to do it right.
Of course everything can be made fast with money….