The EJ25 engine has been plagued with ringland issues for years. We finally have a solution to fix the issue. Now, this isn’t meant to replace a forged engine, but more so another option for the daily driver or pump gas cars out there. We have successfully tested this over the course of the last two and a half years in cars making 300HP-500HP on 93 octane and haven’t seen any failures.
You hear all sorts of things as to why they crack and the most common thing you hear is detonation. While this can be true, we do not see that as the number one cause. The number one cause is heat. Which comes in the form of torque, boost, fuel type, lambda targets, etc. What we need to really pay attention to is the in-cylinder temps that specifically come with using pump gas. Compared to ethanol, pump gas will have much higher in-cylinder temps which transfers heat into the top of the piston. This is important to recognize because almost all OEM cast pistons are hypereutectic which will allow more of the heat from the combustion process to absorb into the piston and into the top ring of the piston. It’s great for tight piston to wall clearance since the piston won’t expand and contract as much, but it becomes more brittle with high temperatures. When all that heat is going into the top ring, it expands and the ends touch and put pressure on the ringland. Subaru’s spec for a nearly 4” bore aren’t even remotely close to what they “should” be. Here below there is a comparison between the RA block and an older 2.5l non turbo engine say that was in a 2005 outback where you can see they are basically the same exact gap between a non turbo car and a turbo car…
RA STi block
Outback 2.5l non-turbo
Here is a visual on the size of the gap on the RA block how it comes from subaru measured at .009”:
Just looking at that, you can imagine how little heat it would take for that to touch. Our solution is to open up the ring gaps on the first and second ring to something that they “should” be for a cast piston/turbo/pump gas car. Over the course of the two and a half years of testing we have found great success that hasn’t cracked any of the pistons and also has no increase in crankcase pressure/blowby. We even had tested 40 pulls on the dyno with no oil observed in the test catch can after