piston failure

[MadBill]

The failed piston has a very sharp point between the valve reliefs, whereas the PHR one has a large radius. The former can't be good from a pre-ignition or stress concentration standpoint. The crack runs right across the point. Also it looks like a triangular piece has flaked off the tip...

[lorax]

The failed piston has a very sharp point between the valve reliefs, whereas the PHR one has a large radius. The former can't be good from a pre-ignition or stress concentration standpoint. The crack runs right across the point. Also it looks like a triangular piece has flaked off the tip...

The PHR is a completely different piston. There is a step from the two different reliefs for the intake and exhaust in the 1st piston and the PHR piston. The PHR piston has a single relief angle and depth.

[Warp Speed]

I believe the failure is tune related. The proper tune for a steady state differs greatly from a proper tune at 500rpm/sec accel rate.
Many times we consider being on the rich side as "safe", when it can be just the opposite, especially with max timing.
I presume from your name and water temp being run this is a jet boat?
I would suggest doing all my tuning in a steady state at the rpms being ran at.
How is AFR being recorded? Is it individual cylinders or bank averages?
If it is just bank averages, a couple of cylinders could be allot richer than the 12.1 stated. Couple this with allot of timing, and you can get what you see here, both piston and pin, in short order.

Also, what can be best timing power wise, may not be something that you can run for extended periods, even with low coolant temps. When using combustion data, many times you can still be gaining power by adding timing, but are randomly burning fuel too early in many of the cycles. These random spikes can destroy stuff quick!

[Truckedup]

This is my guess from being not as experienced a many of you..........The piston crown developed a fine stress crack ,the hot combustion gases then eroded the crack into the hole you see.

[BigBlocksOnTop2]

We had to run it for a while after the failure to limp back but we had looked at them the night before and they did look a little lean. We richened up the carb but maybe it was to late already.

Pretty clean piston...lots of heat.

Heat as in lean or please explain.

Being you said an area of the piston looked sand blasted....detonation (caught in time). Does the spark plug show black specks on the insulator? I'll lean to the tune on this one.

[jet1]

Your right AFR was only being measured bank average I am sure some are riche4r/leaner than others.
I will measure the piston crown and post.
We looked at the plugs the night before and no black specs but there was something a miss about the plugs. Very faint shiny specs on a couple plugs. That is why we richened it up some. Normally the location that we were racing has a density altitude of 4500+-ft so we just weren't thinking timing or really thinking at all.

[Warp Speed]

One thing to keep in mind, you need fuel to create heat..............

[Truckedup]

One thing to keep in mind, you need fuel to create heat..............

So "lean burn" piston damage is not from the lean mixture but from detonation caused by the lean mixture?

[jet1]

So it really looks like both were happening because it looks lean to me but I don't think it was so lean that it was on the other side of the bell curve.
The piston thickness at the thinnest part of the failre is .284

[David Redszus]

Race engines almost never actually run in a "lean" condition. This finding is supported by actual lambda readings.
Most all race engines will operate on the rich side of stoich. As the AF approaches stoich, we announce that we are running "lean", but are actually just running "leaner than sloppy rich".

The most dangerous AF ratio to run is neither lean nor rich but just slightly (+4 to 5%) rich of stoich.

It is also where the combustion process is most efficient and most susceptible to errors in timing. Timing errors can be due to ignition or other heat sources (typically wrong spark plug heat range) that lead to pre-ignition. Once pre-ignition has begun, it is only a matter of time before a bullet hole (30.06) will develop in the crown of the piston just under the plug.

When a piston hole develops, it is ALWAYS the direct result of pre-ignition.

If there is enough crown thickness to delay burn through, then increased piston crown temperature will lead directly to detonation. When detonation persists, there is rarely enough piston remaining to diagnose the true initial cause.

Pre-ignition can be confirmed by changes in piston crown metal hardness and by stressed wrist pins. It will also impact rod bearings adversely.

Higher octane fuels will not prevent pre-ignition.

[Barbapapa]

oil temp 280 f.

That should be a good indicator that something is not right, especially with 130 water temp.

Good point. I was going to guess this is a boat and has 'unlimited' water cooling. Is it jet1?
It takes a hot day, and nearly non-functioning oil cooler to get oil that hot unless the pistons are radiating an unusually high amount.

[jet1]

So from you are saying then it was preignition that caused the hole and not detonation. This would also confirm that the timing had too much advance but I should also be looking at maybe running a colder plug because of the sustained WOT throttle that it sees in this situation? Of course making sure that the jettingis correct is paramont.

[jet1]

oil temp 280 f.

That should be a good indicator that something is not right, especially with 130 water temp.

Good point. I was going to guess this is a boat and has 'unlimited' water cooling. Is it jet1?
It takes a hot day, and nearly non-functioning oil cooler to get oil that hot unless the pistons are radiating an unusually high amount.

Yes it is a jet boat and we noticed the oil temp creaping up about 5 min into the leg, about 4 minutes before the failure. Normally the oil will run at 240 though.

[MadBill]

As far as plug heat range (especially for endurance apps.), I'm a fan of running the coldest you can without major fouling issues.

PS: David, re "it is only a matter of time before a bullet hole (30.06) will develop in the crown of the piston just under the plug." The hole is actually far from the plug. Maybe in this case it's just the flow of combustion gases through the crack eroding a larger path.

[MELWAY]

Great discussion guys....this is what Speedtalk is all about...