Don Terrill’s Speed-Talk

subaru ej255 engines (DOHC) seem to suffer a similar common problem.... broken ring lands in cylinder 4 when they are lightly modified.

they don't have to be making anything north of 300whp for this to happen.

anyone familiar with these engines have any insight as to what is causing the broken pistons?


I'll be taking one apart this week and the first thing I will be looking for is the ends of the rings butting together in that cylinder, but I doubt that is the issue here.


fire away fellas, nice to have the board back up.


CONDOLENCES to Don's family, RIP Don.

It's all consequences of trying to get an old design to pass emissions. Hypereutectic pistons are brittle but seal better, moving the top ring up reduces crevice volume but means the top land is weaker, tune that stays closed loop longer helps emissions tests but means it's closer to the border of detonation... also #4 is the hottest cylinder, also closest to the turbo, and the knock sensor is on the other bank.

Glass pistons, 1st thing you do with a Subi is open up the bearing clearances & fit a set of drop in forged pistons.

I know some Aussie Subaru guys who claim it's our gas too. Dunno. There's a lot of stuff piled on the things over here though.

Ring ends butting?

Yep like Tony said glass pistons and the are the for the reason mekilljoydammit said.
When I worked at MMAL we went from 3.5 V6 with an ACL piston. To a 3.8 V6 with a Japanese piston to meet euro3 emissions. MMAL intended to build the 3.8 locally and consulted ACL about piston production and there reply was they don't make pistons out of such a brittle material.
The manufacturer needs that brittle material and that top ring high to meet emissions, by getting that crevice volume really small . If you want to push up the power level up,change the pistons, simple. Don't blame the piston for failing when you asked it to do something it wasn't designed to do.
I don't thinks our fuel, I think we expect to much. I have broken those 3.8 piston N/A they just don't handle knock at all. I really feel for those stock turbo pistons.

In looking at the spark plug from that cyclinder where does the color change over point on the plugs electrode take place, more towards the plugs body then in the middle?

KnightEngines wrote: ↑Mon Feb 11, 2019 7:55 pm Glass pistons, 1st thing you do with a Subi is open up the bearing clearances & fit a set of drop in forged pistons.

Tony what kind of bearing clearances are you talking about??

It's the fuel delivery system. Because of the two fuel rails joined by a narrow pipe, the last injector to get fuel has notably lower fuel pressure compared to the first. You'll find the regulator is by cylinder #4 and the feed is on the opposite bank.

You need to swap one fuel rail so you can run a split feed into either two factory regulators rails with a joined return or two non regulator rails t'd into a single aftermarket regulator. This will even out fuel pressure across all four cylinders.

"Split fuel rail mod" is the google term

Doubtless there are other factors as well, but this is s known problem and if you have uneven fuel delivery by an amount you can't tune for because you don't have individual lambda, it's easier to fix this and improve the situation a lot.

For a basic motor up to about 500hp you want .0018" minimum on big ends, up to .0022" is fine.
Mains minimum .002" & up to .0026"
Tighter than that & it'll hurt bearings.

I've pulled down brand new sti short motors & measured big end clearance at .0008"

back to the top.

how are the ring lands cracking in multiples places with ZERO evidence of distress to the piston tops or upper big end bearing ?

KnightEngines wrote: ↑Tue Feb 12, 2019 7:42 am For a basic motor up to about 500hp you want .0018" minimum on big ends, up to .0022" is fine.
Mains minimum .002" & up to .0026"
Tighter than that & it'll hurt bearings.

I've pulled down brand new sti short motors & measured big end clearance at .0008"

far out ! Not much room there for anything let alone error!!

Why did humpty dumpty break to pieces when he fell off the wall? Cause he's an egg man.
That engines a glass house. Brittle pistons with very thin ring lands. They are engineered close to the edge and it doesn't take much, backed into a corner with that design, stroke and bore maxed out for the platform, so there is a very limited space for ring lands and skirts, they run the skirts zero clearance to control slap. Which must require a very scuff proof alloy, which do tend to be brittle.
it's a known problem they even had recalls on some models for doing that. There isn't any huge difference to pistons on those that didn't get recalled,, maybe a slightly different alloy or a few thou more thickness. i can't even tell looking.

Now granted, why #4? yes there probably is some reason, that that one goes first, but IMo it's just the one to go first.
If you solve whatever slight difference is causing that then the weak link will just move, probably not a huge amount to gain, although....still would be interesting to know. I'm not really sure what the BEST solution would be, if that's what your looking for. IMO reducing the bore and stroke would be the right direction to make MORE power, but nobody wants to hear that.....and, that's ok, it's not impossible that you could find your own solution nobody thought of yet. High performance subarus are VERY impressive IMO, and a hard game to play, only thing that bothers me about it is most people seem to think it's easy.

^ true dat.
Everyone can build & tune a big hp Subaru, until it undergoes spontaneous disassembly.
It's a tightrope walk.

I've built my share of EJs; 205s, 207s, 208s, 257s, 22s etc.

Tony is correct- I've torn down brand new EJ257s that had .0008 on the mains and rods, were filthy dirty, etc.

The USDM EJs have junk pistons for performance applications, the engines are tuned terribly (for emissions), prone to knock (with camshafts, pcv system, piston crowns and combustion chambers to suit it further).

The JDM EJ207/8s I've built, both stock and modified with bigger HP in mind did not share these issues.