So, I broke a piston...
This is cylinder #7.




I'm not sure exactly why it broke. Bad gas maybe? I heard detonation when it happened, but my data log doesn't show any knock.
I'll add the log later when I'm on the laptop.

Best octane we can get around here is 91, and I've been adding a bottle of octane boost with every fill-up.
I'm at about 8lbs of boost on a T-Trim.
LS3, stock bottom end, BTR Supercharger cam, ARP head studs, dual springs, Johnson lifters, etc...

Was this a fluke (bad gas)? If so, should I just slap another stock piston in it and go?
Would forged pistons hold-up any better in the event of bad gas?
Is it worth the extra $ to put a set of forged pistons in there?
Any advantage to doing rods too?
I'd love to turn it up a little bit more, but since 91 is the best I can get, I don't see how forged components will make any difference.
Meth is probably the only option for more power.
Maybe a dished piston will help (safer)?

Would love to hear some thoughts...
Gotta get the head pulled-off soon to see how bad it is in there.

 

I had a piston come apart at 9lbs on E85 last fall, seems like it’s just one of those things no matter what you do. They make drop in rods and pistons for relatively cheap that would allow you to turn it up along with meth.

 

A couple of engine guys told me not to bother opening-up the ring gap, because the LS3s don't have issues like the earlier LS motors, so I didn't.
I wonder if that would've helped at all in this situation.
It kinda looks like the ring bound-up and chipped the top of the piston.

What's everyone's experience with ring gap on the LS3s?

 

Detonation indeed.

Take a pic from your plugs.

Do you have any fuel-related things that rely on boost-reference? BAP perhaps?

 

Knock sensors do not always catch knock.

 

The second fuel pump kicks on at 2psi...
Unfortunately my fuel pressure gauge quit last year, and I haven't gotten a new one yet.
Wideband says fueling is good, and I'm pretty sure the wideband is graphed on the log.

I'll double check those things though...

 

Yeah, I'd say that's definitely true. Which sucks.

 

What spark plugs are you running? If you are on stock plugs maybe going to the C6 ZR1 plugs (cooler heat range) could also help.

 

I'm running TR6s gapped to .035"

 

Well does guys are idiots. Most top ring damages comes from the rings pushing in to each other. Is there other things that can contribute to a broken piston yes but that percentage is extremely low. All ls engine are about gapped the same from factory.. So i don't know why he's tragating ls3 engines or maybe i don't know something.

Good ring gap to start at is around .025 ish. And factory is no where around that. JMO

 

@sydneyACE
http://blog.wiseco.com/everything-yo...about-ring-gap

 

So maybe just slap another factory piston in there and set all the ring gaps a little bigger?
You guys think bigger ring gap would have saved this one if it detonated for a few seconds from bad gas?

Forged pistons are tempting because it's SO much work to get the upper oil pan off anyway...
BUT I feel like the rotating assembly should probably be balanced if I'm putting forged pistons in there... and I really don't want to take the engine (or transmission) out.

 

Agree, that damage is typical of piston ring expansion until there is no more gap. There is a minimum and maximum gap tolerance and your LS3 is likely on the lower side of that (maybe just that cylinder). LS3 is normally fine running that level of boost so like I said I just think you were unlucky. It would be interesting to see what that cylinder's gap is assuming the ring is not distorted.

Forged pistons are never a bad idea if you are willing to spend the money for them. But still mainly important to get the piston ring gaps correct.

Might not be a bad idea to pull each stock piston out, measure the ring gaps and open them up a bit for some insurance. If the engine is already apart...

 

If you are opening up the motor to replace a piston you are going to be checking everything so i would highly recommend putting some aftermarket pistons and rods. Depending on how the look main and cam bearings too. Aftermarket pistons and rods wont hold up to events like this 100% but they will be a little more resistant. I would consider running E85 if you can or adding some meth injection for extra fueling and cooling of IATs.

Also would be a great time to replace all rings that are properly gapped for boost. Typically LS3 stock is fine but may have been a factor

 

Got the cylinder head off... Looks about like I expected.
I think I'm just going to change the #7 piston this time around.
I'll plan on gapping the rings a little extra.
Also, I've heard of guys running a colder plug in #7 since it gets hotter than the others. Will that help at all to keep it safe?

 

I'm not going to take the motor all the way out... I just got done doing a clutch a few months back. I just don't have it in me to tear into it that deep again.
I wish we could get E85 around here, but sadly that's not an option. I'm wanting to do meth (ha ha!) eventually, but haven't gotten to that yet.

 

Highly advised to pull that engine and disassemble. You don't know the true damage. For instance, how do you know the chunks of that piston didn't make it's way to and through the bearings on the bottom end? Could be living on borrowed time. I feel your pain dude, trust me. I did that to two pistons at the same time. The bores needed a few thou to clean up, yours looks no different. Bad news man...but just putting a new slug and rings in there is making a large gamble. Just my opinion. YMMV.

 

95-02 3.0L Supra engines will 800rwhp using OEM cast pistons for 250,000 miles
95-02 2.0L Silvia engines will 450rwhp using OEM cast pistons for 250,000 miles
04-07 Select 4.8/5.3/6.0L Chevrolet engines will 800 to 1200rwhp using OEM Cast pistons for 250,000 miles

I've tuned hundreds of cast piston engines and I'm familiar with how to keep them alive for basically forever.
I created a checklist
<=[Cast piston health checklist]=>
1. Air Filtration quality, filter near 1uM if possible
2. Maintain Oil Temp 212-220*F ranges, close to coolant temp
3. Maintain coolant temp 200-222*F
4. Monitor crankcase pressure (2-bar map) & establish a WOT pressure below atmospheric
5. Maintain reasonable IAT for gasoline i.e. below 114*F Maintain reasonable EGT i.e. ~1260*F Turbine Outlet
6. Spray Small amounts of evenly distributed 100% distilled water as close to the pistons as possible, generally within a few inches of intake manifold.
High quality air filtering!!!

The biggest threat to cast piston is temperature. They get hot, they fail. Factory 3.0L Supra engine and 2.0L Silvia engines and similar (RB26 skyline engines...) All incorporate Piston oil squirts to help control piston temperature. A Chevrolet engine has similar pistons (brittle fracture failure due to shock) but no oil squirts, thus it is our job to aux cool the piston somehow. This is done using Alcohol, methanol, water injection, injected close as possible to the intake plenum so it does not evaporate before it hits the cylinder or close to the cylinder, e.g. E85 flex fuel overhead valve injection.
Another serious issue facing everybody is crankcase pressure. There are very few people monitoring and controlling the crankcase PCV system properly and it plays a pivotal role in the performance of the oil system and longevity of the engine, oil quality, oil leaking, seal performance.
Piston rings will not function well when the crankcase is building pressure, leaking is increased (more blowby) and pressure force exerted to area below the piston during power stroke may contribute to various ring based behaviors such as flutter or failure to swap position, leading to oil consumption and decreased oil life, piston ring carbon buildup, may prevent piston ring rotation (they get sticky/stuck).

other info
You cannot depend or rely on knock sensors. Knock sensors are not tuning tools for wide open throttle.
The Chevrolet 4.8L engine can be had nearly free from junkyards
The Chevrolet L33 engine is ideal for 1000rwhp swap because light weight and gen3 electronics with gen4 internals

 

I have 2 friends that had that happen on #7 hole.... one was an SC LS3 the other an LT4..... both were due to running to lean at WOT is what they said...... that has to be tuning I'm thinking........

 

It's hard to say for certain, but I suspect bad gas. When this happened, I was driving out of state and filled-up at a gas station in WA that I'm not familiar with.
I've been running essentially the same tune for 2 years (probably about 25k miles), and haven't had any issues with knock.
As I was rolling down the interstate a Tesla "raced" me when I didn't know we were racing. I got up next to him afterwards, and let him know that I was ACTUALLY ready to race... so we did it again.
I heard a little bit of noise right as I was about to shift out of 3rd... I grabbed 4th and heard some definite rattle (detonation) as I got close to redline in 4th.

Good news; I found out that I can gap a P100D from a 60mph roll on 7 cylinders. Bad news; My #7 piston wasn't as impressed as I was.

I wasn't sure what kind of damage was done... I picked up a misfire (with accompanying backfire). I was able to isolate it to #7 by unplugging fuel injectors one at a time until the backfire went away.
I had no tools and was at some gas station in the middle of nowhere with somewhere to be, so I drove it to Spokane on 7 cylinders. Thanks be to O-Reilly Auto Parts for being open on the weekends...
Pulled the plug on #7 to find it zero-gapped... which I thought was really weird. The young kid behind the counter was utterly convinced that my piston must have hit it. (Too much time spent working on 4-cyl perhaps...)
I tried to explain to him that that is utterly impossible on an LS engine, after which he started to look at me like I was the dumb one... Oh well... (I am in fact kinda dumb... but not for that particular reason.)
Borrowed their compression tester, and #7 still had 155psi of compression. Running compression check was good too.
Put a new plug in it and it ran like nothing was ever wrong... drove it another 4hrs to get home.
I knew there had to be something up so I bore-scoped it when I got back to the shop... to find what we see at the top of this thread.

Anyway, the long back-story was probably unnecessary, but I thought you guys might enjoy a little tale.

I'm 80% sure I got some shitty gas... my data log doesn't show it going lean... but it's hard to say for sure.
I'm cheating my wideband into HP Tuners through another channel, and the resolution isn't great, so it might not have picked it up.