Survived the dyno, then broke a piston

Hello HP forum!

My name is Brian, & I own a small shop in VT, USA called Rogue Rotaries & I specialize in... rotaries. I also have an AWD Dyno. I've worked as a few shops "tuner" in the past, found I have a knack that developed into a passion which lead me to where I am now. That's the super short & quick version. Still need to start the Twitter & Facebooks once I have a logo.

A shop across the river took a junkyard 5.7 LS1, a factory LSA Supercharger, adapted it together, put an auto behind it & is controlling everything with a Holley Terminator. Not my first choice. I run MoTeC on my personal rotaries, a Haltech for my own learning, I really like the Link ECU's, what I'm getting at is I'm comfortable tuning a variety of platforms, obviously have my favs though, as does everyone

Car shows up the first time, the ignition cylinder breaks & we have to unstrap it & send it on it's way. It comes back a week later with new column parts but this time it pukes powersteering fluid everywhere and gets unstrapped & sent home again. Third time is the charm & with the shop owner there he's directing me running the dyno while he makes some tuning changes. He finds 523hp @ 165kpa running .87L & either 20 or 22* of timing. He tells me to have fun see where I land.

The first thing I do is start adding fuel, I feel that .87L is too lean for a supercharged LS. Please keep in mind my PISTON tuning experience is only about 5% in the last 15 years. I've been a rotary nut since 1997... yikes... I know...

SO... as the tuning progresses I keep adding & adding fuel & ultimately end up with peak power around .76L.

Now we start to play with the timing by retarding it. Take a few degrees out, make more power. Ultimately finding peak torque with 14.4* of timing. She made 614hp @ .76L & 14.4* I took .7* of timing out (5%) expecting somewhere in the 595-600 hp range & it made 598. With the shop owner there we looked at each other, nodded unstrapped her & went for a ride to verify everything on the street. Aside from the accel enrichment being too much everything was great.

A week later I get a text saying that "Apparently you can run an LS too rich & too retarded.... Broke a piston."

Where did I go wrong? I suspect a piston land is broken & if it happened, I feel it would've happened with his leaner more advanced calibrations. 90hp from start to finish tells me... & told me I was going in the right direction.

Thoughts, suggestions? Would like to hear from experienced tuners as well as shop owners

Cheers,

Brian

Rogue Rotaries

What percentage of the failure is due to a prior more aggressive and knocking tune is hard to guess, but greater than 0.

Once an engine gets beat up, it's just a matter of time before it fails regardless of whether it's running well afterwards or not. A new tune can't undo old damage.

There would be a great deal more detail in a proper diagnosis, but if your fuel target is stoich properly ramping down towards 0.76 as load increases to max observed, then that does not sound damagingly rich.

It's possible the timing used was still a little too much at full load, or perhaps it needed more timing reduction at high charge temps or in other load regions not encountered during the dyno session, and that could cause harm.

However, if the timing advance was actually a bit conservative and a few degrees below the knock limit, that does not break a piston.

Thanks for the response Mike.

I managed to see a few pics on the 'scope that they had taken. The piston is broken & I can see rings. The heads should be off before I leave for the weekend tomorrow, hopefully post a few pics. From what we know right now there doesn't appear to be damage in any of the other pistons, there's no evidence of knock that's been found. The shop owners theory is that it was over fueled which allowed fuel to get under the rings & ran so hot from the retarded timing it combusted unburt fuel between the piston rings popping the piston top off. Now, this seems like a stretch to me but I suppose anything is possible. I asked him what the answer was, to run it @ .87L? He says he's seen that plenty in his extensive drag racing days which is experience I don't have.

Fuel target was 1L idle & cruising quickly getting to .85L @ 100kpa & ending @ .76L by 140-150kpa. Found peak power @ .76L before pulling timing back to making it's peak power @ 14.4*, Subtracted 5% from that, set the compensations to reduce timing by 2* for every 10F* of Air Temp above 100F* (all of the runs started at 85F* & ended around 100F*) Ambient temps in the 60F's

I agree that there is no tune that is going to undo damage & once something is damaged it's only a matter of time before damage turns into broken. If it did happen on the dyno, I suspect it happened with .87L & 20 or 22* of timing. From the start that seemed to aggressive for me, especially the AFR's. So when I had my turn, the first thing I did was bring the L down & instantly started making more power until I got under .75L then it really started to get upset. .76L was where it made peak torque.

Then I did the same with the timing. Started bringing that down. Took 2* out, picked up 25hp, took 2* more out, picked up 25hp again. Telling me the timing was too far advanced. Found peak torque @ 14.4*, took 5% out, murdered the comp tables down if things get hot & went for a test drive. Oddly I could hear the left side had a note that was slightly different @ idle & light cruise & that's where the broken piston is. This is a '78 Corvette with the obligatory sidepipes & not for nothing, but no pieces of piston came out

The history of the engine is unknown, as a practice I try to compression test every rotary I put on the dyno before doing any tuning to hopefully catch issues before making things worse.

For the future I'd say that much enrichment at 100 kpa is concerning because with the blower the car likely cruises around near there, and that level of enrichment shouldn't be required under those conditions. Over time that could significantly contaminate the oil with fuel, possibly foul plugs, and build up piston deposits that cause hot spots which can cause damaging preignition.

Enrichment at high engine load is brief and there's more flow present to burn and/or move fuel out of the engine as compared to cruising.

If the engine got upset at .74L then I'd say .76L is perhaps too close. When you say upset, I'm not sure exactly what you mean, but if the engine was misfiring at that point, I'd move further away from the condition. It's unlikely a single dyno session will net you fueling accuracy such that you're never more than 0.01L off target. On the ignition side of things, spark control is highly repeatable, but the fueling side has far more variables in terms of air mass present and fuel mass delivered. For example, I don't know if that ECU even compensates for variation in differential fuel pressure, and there are many other factors.

Did you correct the accel enrichment issue you found on the test drive before the vehicle left?

My 5c worth.

Reading the description of the timeline, I understand the owner/other "tuner" was initially running excessive ignition timing when compared to MBT? If so, there is an excellent chance it was running into, at least, light detonation that could have weakened the top lands - I would expect this to be the reason for the land failure, especially if there was any slight injector issue.

I would dismiss the trapped fuel argument, because combustion requires oxygen, although it may have some affect on ring wear.

The richer mixture I would also dismiss as while it will affect oil dilution, ring seal, and potentially cause deposits/plug fouling.

Another factor may be if the top ring closed up from the additional heat - it may be enlightening to check the actual ring end gaps?

I agree with Gord's mention that it's possible the ringland on the piston cracked on the over timed tune, and then came apart later.

While we can't know what happened with the info we have, and inspection now isn't going to reveal the whole timeline, I think we're on the same page that there's a good chance damage occurred when the engine was over timed, and then it was a time bomb from then on.

Thanks for the insights guys. Found a little more detail out & to clarify a few points. L targets are 1 while cruising, idling, light load etc & MAP is generally around the 70-90 region. Once it gets to 100kpa the target is .85L then ramping down to .76L

To Mikes comment & questions - No, this ecu does not compensate for variations in fuel pressure, also when I said it got upset @ .74L, I mean it started to loose power pretty quickly. It didn't rich misfire, in fact it never did misfire at all, just didn't like that much fuel.

Yes, I did dial back the accel enrichment as I did see .7L at one point when he was really working the throttle on our test run. Completely agree that the fueling will never be perfect, we also turned the long term trims on so they could drive around a bit & dial it in a little more. In doing so they managed to hit the rev limiter repeatedly & after a few test drives is when it started smoking. The owner believes it did survive the dyno just fine, & our road test & happened because of the limiter & he still thinks there's too much fuel. We'll see what happens after they put some pistons in & bring it back

I'll just say when someone blames failure on something that wasn't the issue, and doesn't learn from it, they're likely to do it again.

You know running it lean or overtimed isn't correct, so you're likely going to end up tuning it similarly again, the way they think blew it up.

"I'll just say when someone blames failure on something that wasn't the issue, and doesn't learn from it, they're likely to do it again"

That's the entire reason for starting this thread

I keep finding more & more out. Unless I misunderstood, the car's original carb'ed engine wasn't all that healthy so they grabbed an LS1, swapped it in, installed the Holley Terminator & tuned it on the street. Target ARF's & a reasonable timing value. He breaks it, second junkyard engine this time with the blower goes into the car. Likely the same calibration. I'll just let that settle in.

The Holley is simple. It has a timing tab where the basic timing inputs are Idle, Cruising & WOT. They were 18, 35, 22 respectively when I opted for the "advanced" setup where by having two axis, load & RPM

I'll go back in the next few days to get a little more information. The owner did say if it was overfueling it will be evident in the second ring being pushed down slightly

Likely when it comes back for a retune the first thing he'll want to do is remove fuel back to his .85-.87L. Before doing that I'll likely take 8-10*out before creeping back up on it.

Cheers,

Brian

Limiter bashing isn't healthy for pistons either...

What sort of limiter is in place? Is it cutting fuel and/or ignition? I have seen on some ECU's that they will favour a certain cylinder with the cuts due to the way that their randomiser works.

What condition was the wrecker engine in before it was dropped into the vehicle? Where any checks done of it as it may have this damage before it was even installed into the current vehicle and dynoed.

While we often focus on the technical side of things, do you feel the second time around the outcome will be something you and the owner will both be happy with?

I ask because I'm not getting that feeling from the info so far. If you decide to proceed, what are you doing to try to improve the outcome?

You'll do your best with the tune again, but ultimately many other factors determine the outcome.

Include any engine testing you want performed.

Are you confident the engine management system they have will allow you the level of control and safety measures required to achieve reliability?

Will they drive and maintain the vehicle responsibly?

Will they alter the tune after leaving your shop?

Reads like the owner is a damn-fool idiot!

TBH, I'd suggest either washing your hands of the whole thing - telling him to take it elsewhere, or insist it is at least checked over, and preferably rebuilt, by someone who actually knows what they're doing.

Limiter bashing isn't healthy for pistons either...

What sort of limiter is in place? Is it cutting fuel and/or ignition? I have seen on some ECU's that they will favour a certain cylinder with the cuts due to the way that their randomiser works.

What condition was the wrecker engine in before it was dropped into the vehicle? Where any checks done of it as it may have this damage before it was even installed into the current vehicle and dynoed.

It's not, & I'll be sure to set it beyond the 6krpm redline, & I'll stop when I see a 6xxx, dyon is configured to stop sampling @ 5800 for that car

TBH, I can't remember the details on the limiter. It's a softcut that much I do know, they tend to be a mix of both I find

Don't know the original condition of the wrecker, we know now what's into it though. I as a rule always open engines. Understanding rotaries are a $110 O-ring kit assuming everything checks out.

"While we often focus on the technical side of things, do you feel the second time around the outcome will be something you and the owner will both be happy with?

I ask because I'm not getting that feeling from the info so far. If you decide to proceed, what are you doing to try to improve the outcome?

You'll do your best with the tune again, but ultimately many other factors determine the outcome.

Include any engine testing you want performed."

In speaking with him, he reached out to someone that he trusts that has done some tuning for him in the past. That gent confirmed that the target L's I was looking for were correlating to his experience, although he thought my timing was still too far advanced. The owner admitted to learning a bit that day as did I. Be louder. At the end of the last tuning session we met after hours to get one last glory pull in after the charger had cooled & it made it's best 614. I told him I wanted to pull 5% & land around 600, it did 597 I think & he was happy. He was involved in the tuning from the beginning, he thought .87L & 22* was what it should be & it's not, having someone he trusts confirm it made it all easier.

"Are you confident the engine management system they have will allow you the level of control and safety measures required to achieve reliability?

Will they drive and maintain the vehicle responsibly?

Will they alter the tune after leaving your shop?"

No, the Holley isn't the most advanced & there aren't any actual engine protection features that I saw. It's simple & dumb

Probably maintain it ok, but the owner.... I've watched him do burnouts in the country store parking lot with his other C3... so responsibly... not likely

Well,it seems you a lot more info on the vehicle and customer now to make your decision.

If you tune it again, tune it like he's going to drive it the way you've seen him drive. It sounds like it needs a good margin of safety baked in beyond where it makes max safe power on the dyno.

Solid advice there. I typically only take a few percent off the top for timing & a little extra fuel for good measure in both the fuel table & the L target table & call it good. I'll be doing more than the historical 5% in timing for this dude

Cheers,

Brian