How far can you push a stock STI block and keep it reliable? This is a question that comes up fairly often, so I wanted to layout what the limitations are of the stock EJ257 first, and then work through the progression of points where stock components in the STI block would need to be replaced.


Before we dive into the details, I want to mention that no matter the engine build, a good tune is a required starting point to keep your car running reliably. The rest of this discussion is going to assume that a quality tune is being used.


I will also mention that we put together a video a while back called “to build or not to build your Subaru engine” and that would be a good resource to take a look at too.


Okay, so with that out of the way, let’s get into it. If you have a stock EJ257 engine (or EJ255 for that matter), the first stock component that is likely to fail as you turn up the power is the pistons. The common failure point are the ringlands which hold the piston rings in place. The most common cause of ringland failure are tuning issues and engine running issues that cause lean running conditions like vacuum leaks. But even if all of that is not an issue, above about 350 wheel horse power, the stock ring lands can crack from the pressure they see even in normal combustion events.


This doesn’t happen the first time that you hit 350 whp, but the more time that you spend around that level of beyond, the greater the likeliness that you are going to crack a ringland. If you do have a ringland failure the symptoms to keep an eye out for are quickly increased oil consumption or additional pressure in the crank case.


But can you push a stock past 350 whp without it blowing up? Yes you can, but reliability will go down somewhat quickly. For example, the first engine in our Pikes Peak WRX Race car ran a stock EJ255 at about 400 whp for a number of events that added up to around 3,000 track miles. And actually, 3,000 track/race miles on an engine is not too bad at all. But if your car is a daily driver and you want to get closer to 100k miles our of an engine, than we would not recommend trying to push to that point.


Now, if you want to push past this first limit, forged pistons are going to be the first part that needs to be replaced. The stock rods would be the next one. They will hold up to 400 whp (again as shown in our PPIHC car), but if you want to push up to or beyond that power level, a set of aftermarket H-Beam rods would be recommended.


Beyond that, if you replace the pistons and rods, the next weak link would really be the cylinder walls themselves. At around 500 – 550 whp, and/or if you are making over about 24 psi of boost or more, the stock cylinders would need to be reinforced to hold that power reliably.

This is where the closed deck blocks from companies like IAG with their Stage 2.5 and up come in. If you are wanting to push past that point, take a look at our blog going through all of the IAG Staged blocks to see where you can go once you sure up those stock cylinders.


- Jon Cooley