Who wants to see some tests done on the material properties of OEM Subaru pistons?

[BrandonspecB]

This is the most reasonable explanation for ringland failure i have seen other than the excessive knock event reason SOA is fond of using. I suspect the truth lies somewhere in between....think about what forces are in play when a ring gap closes up...probably high load sustained like climbing a mountain pass for example. If the gap is just barely closed up probably not a huge deal as long as the is no knock event present, but add a knock event and the ring flexes but not in the way it should and so does the piston crown. Add the two together and you get instant engine damage. Or the extra heat from the knock event pushes the marginally closed up ring into binding mode in the cylinder causing instant ring land failure. I have every reason to believe the rings are several orders of magnitude stronger than the ring lands...any thoughts on this?

 

I think what you said is exactly what happens. Detonation itself can cause ring lands to break. It also causes overheating of pistons and rings. Knock disrupts the boundary layer of gases that insulates the piston from the full heat of combustion. A knocking engine's pistons and rings will soak up more heat than a non knocking engine. Open up the ring gap and you give yourself a lot more headroom.

 

Source: Allen Cline, developed GM Northstar engine. http://www.contactmagazine.com/Issue54/EngineBasics.html

 

I look at the LS engines that people pull from junkyards and all they do is open the ring gap and make 1000 WHP. They're not doing that with a .008" ring gap. They're also not doing it on pump gas with a shitty tune.

 

It seems to me that there is likely no one good number for ring end gaps. The gap needs to be progressively larger as your engines horsepower capacity. If you are going to be consistently running high boost, high load then the ring end gaps will need to be larger than for a stage 2 street engine that sees just a few seconds of full throttle and then mostly just cruising along for relatively long periods of time. This is all conjecture on my part. So does anyone have experience enough to say what those end gaps would need to be for an engine that sees regular track use for example, and what be appropriate for a street engine?

 

100% right. The ideal situation would be a variable ring gap that maintains the same gap under all conditions. This isn't possible as of yet, so there has to be a compromise. The gap must be set for the harshest conditions.

 

There are rules of thumb for ring gap based on inches of bore, power level, etc. The data sheet that comes with new pistons and rings gives you some guidelines. Ultimately it comes down to what your engine builder believes will work best for how you're using the engine.

Edited April 8, 2021 by BrandonspecB

[BrandonspecB]

If you build for your 90% usage, you'll be ok for the rest. If you cheap out, you'll get a chance to tell your builder the truth about your usage plan.

 

This 100%.

[boxkita]

motoiq did a yt on building a high hp subaru engine. worth watching. the grin on his face is enough to make you want to build your own engine using his processes

[xt2005bonbon]

Link?

[BrandonspecB]

 

I really like the 2.0 stroker idea. Get the thicker cylinders of the 2.0 and pickup a bit of low end power from the longer stroke. If I build an EJ again this is what it will be.

[boxkita]

Keep in mind, he has 1,000's in special processes on that engine. And it's not a daily driver.

 

For pretty much everyone who posts here, this is a 0.1% use case engine.

[SAABaruu]

motoiq did a yt on building a high hp subaru engine. worth watching. the grin on his face is enough to make you want to build your own engine using his processes

 

 

That was a great video. I am hesitant on his cam shaft choice, interested to see the results.

 

I'm quite amazed Subaru has built up such a bad reputation with their engines when the JDM EJ207 is arguably the greatest JDM engine ever made. I've pushed plenty of these into the 125-130mph and 10s 1/4s with no problems. Stock headgaskets and headstuds too!

 

I don't see the need to build engines unless it's a racecar.

 

Break in a factory shortblock and you'll find almost no metallic bits in your oil on break in. On a built motor, it's glitter coming out for first 3-4 changes. I think this contributes to the shortened life of a built motor.

Edited April 16, 2021 by SAABaruu

[boxkita]

Most of the poor rep came from builders giving ppl what they paid for without considering longevity.

 

I was visiting builders looking for "the" shop to craft my engine. The owner was talking about a recent 600hp build that he just delivered. While we were talking, the new owner came in dripping oil and banging noises from the engine. I asked if the car was delivered with a breakin tune or the full power. When he said full power, we thanked them and walked out. That build lasted 24 hours.... the shop is no longer in business. go figure.

[awfulwaffle]

 

Somewhere there's a thread that shows ring ends that have mushroomed from expanding into each other. The piston failed with ring land issues.

 

You mean like this? #4 piston which experienced ringland failure in my Type RA block. Interestingly enough, the intermediate ring butted on every cylinder (flare worst on #4 shown here), but the top rings were all fine. The top ring has a tighter gap from the factory, but I noticed when filing rings later on that the intermediate ring cuts WAAAAAY faster than the top ring. I wonder if there's any correlation to thermal coefficient coefficient - ie the intermediate ring expands more for a given temperature change. Or, the material is softer and the flare due to butting is more visible to the naked eye.

[livinon2wheels]

This 100%.

 

 

Thanks for the affirmation Brandon. As i have pondered this problem since my last post, my thinking is as follows, if you are building a hi-po street motor, stay with stock pistons and regap the rings like you would if you were building a track engine, and then use a conservative tune. Not as conservative as the factory tune (duh!) but conservative enough to minimize knock events and then enjoy it. `It will tolerate being driven like you stole it for short periods of time most likely without ever giving a problem. If you are building something for the track, don't screw around, put forged pistons in it, with conservative ring gaps and tune it properly and then go have fun, and it will probably hold up for a couple of hard racing seasons if you treat it right. This is probably oversimplified, but at this point in the day I can't wrap my head around anything more detailed than that.

[boxkita]

track builds get rebuilt way more often than street. shralp's experience is an outlier not the norm. Track engines that run hot tend to become flaming piles of shrapnel really quickly, so most of these issues aren't. ring gap is important from a heat perspective and also from drag. too tight in the cylinder leads to excessive drag and power loss; too loose and you get blowby and lower compression.

 

 

The occasional romp through the twisties is ok on any car. look at rental cars. they masquerade as F1 and baja 1000 buggies their entire life. Your car is no different unless your rebuild uses cheap parts/poor assembly habits/crappy tune.

 

track builds tend to be driven hard for 30 minutes at a time, parked for 30 minutes to an hour and then flogged again. Having one was stupid fun. I recommend it to everyone. Just don't expect long life with minimal maintenance.