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

[fahr_side]

You have to take the pistons out in order to split the block halves

[xt2005bonbon]

This is the video I saw a loong time ago:

 

[ame=http://www.youtube.com/watch?v=TR0Ed6qTIVQ]pulling pistons out of Subaru block - YouTube[/ame]

 

You have to take the pistons out in order to split the block halves

 

Well, there ya go

[Sgt.Gator]

Because a lot of engine builders suck. And there were a couple high profile / high money (aren't they all?) builds that failed.

 

So the best advice this board has is stay with the OEM shortblock.

 

Look at Boxkita's head rebuild / engine whatever the **** is going on fiasco.

 

After talking with my engine builder I went forged.

 

BUUT look at Sgt.Gator he actually drives a racecar and was on the OEM short block for a long time. Might still be, don't remember how he rebuilt it.

 

Yes, we rebuilt the race car with a factory shortblock. And I'm rebuilding Drew's former Spec B with a factory shortblock too.

[monkeyposeur]

This is the video I saw a loong time ago:

 

 

 

 

Well, there ya go

 

That's a good video. The hooked rod he uses to pull the wrist pins out isn't necessary though. I didn't want to make one so I found a long 1/4" rod and you can push that through the opposite wrist pin access hole and thread it through the block and tap the wrist pin out. It works great.

[magnusonsubie]

Hopefully big update coming tonight. In the shop right now. Let's just say OB2.5XT's pistons were a bit worse off than originally thought.

[Rhitter]

Yay! I like exceeding expectations.

[magnusonsubie]

Exceeding expectations was done. This first picture is the original failure that we knew of and was the plan to observe.....

 

Piston #4 Original failure

 

Then we started cutting it up to create a sample to view under a microscope and a larger chunk of the ringland between the fire ring and the second ring fell out. Looking at it the only reason it still held on was because of the ends keeping it a full circle. There was oil residue behind the part of the ringland that fell out as pictured below.

 

Piston #4 More failure

 

Thats all for now. We did a red dye test on the cuts we did on piston 3 and 4 and 4 showed some cracks. There was a hairline fracture pretty much separating the crown from everything else on the piston. I'll get all the photos I took as well as a little bit of writing about it done tomorrow and probably edit the 1st post to show all that I found.

[snow05gtRI]

sub'd. cool thread. i wonder if I still have my old pistons with my cracked piston 4 as well. big chunk of ring was cracked in two places and loose, but still held in place because of how it cracked. you could move the broken section by touching it but had to pry it to get it out. could pop it back in and it would stay

[Flinkly]

hmm... so is this all due to degredation of the metal from temperature?

 

kinda makes sense since they all don't have the same intake/exhaust/fueling flows and so are not all living in a preferred environment.

 

we need improved fuel routing, ELH, and STI intake manifolds.

 

Can't wait to see the writeup. fantastic work. worth a few free costco dogs for sure!

Edited December 4, 2015 by Flinkly

[Rhitter]

hmm... so is this all due to degredation of the metal from temperature?

 

kinda makes sense since they all don't have the same intake/exhaust/fueling flows and so are not all living in a preferred environment.

 

we need improved fuel routing, ELH, and STI intakes.

 

Can't wait to see the writeup. fantastic work. worth a few free costco dogs for sure!

 

The STI intake manifold?

[StkmltS]

sub'd for awesomeness

[Flinkly]

The STI intake manifold?

Yeah. While the plastic is fine for boost holding and significant power, it isn't as balanced in flow as it could be.

 

Or so i've read. No experiments or data here. [emoji58]

[fahr_side]

Yeah. While the plastic is fine for boost holding and significant power, it isn't as balanced in flow as it could be.

 

Or so i've read. No experiments or data here. [emoji58]

 

 

There's a big thread on nabisco where several manifolds are flow tested, including the plastic one. Flow difference between runners is not that bad.

 

 

Sent from a device using some software.

[Flinkly]

There's a big thread on nabisco where several manifolds are flow tested, including the plastic one. Flow difference between runners is not that bad.

 

 

Sent from a device using some software.

I guess i need to search harder on nastysock. There have been some pretty incredible threads known to spring up over there. Well, if you don't hear from me in a week, i've drunk the koolaide.

[StkmltS]

While you're there say hi to Tris

[magnusonsubie]

First post updated. I took a ton more pictures than are included in the original post. I was really hoping I could just upload a PDF of it but it was too large for the system. I will upload all the others pictures to flickr in the same album if anyone wants to look at more of the cross section microscopy and other parts of the testing we did.

 

Link to flickr photo album: https://flic.kr/s/aHskqAFgn5

Edited December 9, 2015 by magnusonsubie

[StkmltS]

Nicely done.

[boxkita]

Because a lot of engine builders suck. And there were a couple high profile / high money (aren't they all?) builds that failed.

 

So the best advice this board has is stay with the OEM shortblock.

 

Look at Boxkita's head rebuild / engine whatever the **** is going on fiasco.

 

After talking with my engine builder I went forged.

 

BUUT look at Sgt.Gator he actually drives a racecar and was on the OEM short block for a long time. Might still be, don't remember how he rebuilt it.

 

to be fair, my outfront closed deck block with forged everything would still be running if outback auto's monkey (dumbass mechanic) had removed the shop rag from the cooling system before starting it up. That it ran 1000 miles with part of the rag lodged in the HG is a bit amazing, especially given the way I drove it.

 

If you are doing a custom build, the workmanship and attention to detail of the person doing the work will have a large impact on how your engine. Granted if you use crap parts, there's not much hope.

[alex0856]

Can't believe I just found this. I'm going to HMS tomorrow to have them look at all of the oil that's on the bottom side of my car. I'll see if they by chance the old pistons from my car aren't too buried in their scrap pile.

[Flinkly]

so does this mean factory pistons might/could last longer with a piston top heat coating like TBC from Swaintech?

 

i've always thought that for a DD, that most coatings are redic, and i still think most are (cause the longer a cold start is, the more wear to the bearings, so combustion chamber and exhaust runner coatings, i believe, are not desired for DD), but this one might be a grand idea.

 

coating the crank and rods for oil repulsion would probably be good too, or piston skirt friction coatings. mostly i'm against protecting the head/block from heat, cause a DD needs that heat to warm up in a reasonable time.

[cSlenker802]

Well I'm about 2 years late to this party but as a first year MET student I find this thread unbelievably interesting. I'm currently learning about all the same concepts applied here in my metals lab/class, which makes this even cooler. I still haven't taken thermo (first year student) so if anyone does read this please correct me if I'm wrong, but could this at all correlate to cylinder 4 experiencing repeated excessive heat cycles causing a portion of the piston to "temper" and become softer in just that one region? The excessive heat being caused by the runner design/unevenly flowing injectors after 100k+ miles/garbage OEM tunes; or a combination of them all? Especially if an injector starts to go once the car reaches higher mileage and doesn't flow right. I could be very wrong in saying this but couldn't an uneven flowing injector cause a lean condition in one region of the piston?

 

Regardless finding this thread made my night. Glad I'm not the only Subaru obsessed engineering student around lol

[SAABaruu]

The true source of the ringland issue is the piston ring gap. An OEM shortblock that is regapped and spec'd is totally reliable. ASF machine has redone several now and I know of one that is pushing 400whp for 50,000 miles trouble free.

[livinon2wheels]

The true source of the ringland issue is the piston ring gap. An OEM shortblock that is regapped and spec'd is totally reliable. ASF machine has redone several now and I know of one that is pushing 400whp for 50,000 miles trouble free.

 

 

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?

[livinon2wheels]

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?

[boxkita]

When my engine was built, I told the machine shop it would see sustained 7000rpm. He said "I hear that all the time, but never see the cars anywhere near a track". I said "My engine failed at the track racing a Porsche". When this engine failed, it wasn't because ringland issues.

 

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

 

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.