What caused my melted piston?

[Tomas Wallin]

Nope but like you said..it smoked already and you lost compression so where did it go if not a hole somewhere... because of detonation ..aka dieseling.

Yes, and most likely because of the glowing spark plug...

[MikeyFIN]

That was one of the opinions I got when I showed the piston to some folks, that the spark plug died. But when it dies it's warm enough to diesel (as Ted said) if it dies of heat of course. But I couldn't find any reason to why it should have died - until now...

Why did it start to detonate in the first place... if you lose a plug the one cylinder just dies and gives no energy to ignite anything.
So what raised the cylinder temp up to detonation?
A loose plug itself doesn´t do that IMO because it´s a blown engine..it doesn´t suck air as the intake is pressurized so therefore the A/F ratios didn´t go lean until the temp of the combustion chamber got uncontrollable.

[MikeyFIN]

Yes, and most likely because of the glowing spark plug...

And what got it glowing ?

You´re still at the starting point IMO.

[MikeyFIN]

MSD or Crane can jump a spark over a 1/2" gap.

in a Non compressed enviroment yes...

[Tomas Wallin]

Why did it start to detonate in the first place... if you lose a plug the one cylinder just dies and gives no energy to ignite anything.
So what raised the cylinder temp up to detonation?
A loose plug itself doesn´t do that IMO because it´s a blown engine..it doesn´t suck air as the intake is pressurized so therefore the A/F ratios didn´t go lean until the temp of the combustion chamber got uncontrollable.

Because of the plug not beeing able to transfer enough heat to the head and got too hot, same thing as running with too hot plug heat range.

[Chris]

in a Non compressed enviroment yes...

At 175 psi, an MSD won't jump a .100" plug gap. .070" will barely run.

[MikeyFIN]

Detonation can loosen the Spark plug.... by itself.
Believe me on this one the loose plug is not the issue here Id say it got loose because the detonation gave a hit on the threads. We´re talking of a 10 times pressure peak and force to the threads compared what they usually take.
Was it the first run after winter ?

[MikeyFIN]

At 175 psi, an MSD won't jump a .100" plug gap. .070" will barely run.

That´s what I meant...therefore I agree with you that the loose plug wasn´t the cause but the head had an air pocket...

[Tomas Wallin]

Detonation can loosen the Spark plug.... by itself.
Believe me on this one the loose plug is not the issue here Id say it got loose because the detonation gave a hit on the threads. We´re talking of a 10 times pressure peak and force to the threads compared what they usually take.
Was it the first run after winter ?

It was the first run a few days after a 15 hour dyno session.

[MikeyFIN]

It was the first run a few days after a 15 hour dyno session.

Aka the same thing as a first startup... unless you kept the water in the engine while installing it back in the boat...

[Tomas Wallin]

That´s what I meant...therefore I agree with you that the loose plug wasn´t the cause but the head had an air pocket...

What's the chance of an air pocket with free flowing water, dumps through two barely 1" dumps. Water temp was around 60F. Don't know the water pressure though more than it beeing more than 10PSI since that's where my warning light goes away.

[Tomas Wallin]

Aka the same thing as a first startup... unless you kept the water in the engine while installing it back in the boat...

Nope, didn't drain it. Anyways, Idled for 10-15minutes, warm up passes at 3000 to maybe 4500rpm for 10-15 more minutes before the wot runs.

[MikeyFIN]

Beats me honestly... sometimes they just happen and that would be the only thing IMO that could do damage like that...unless you lost also an exhaust valve before meltdown which you didn´t ?

[Tomas Wallin]

Beats me honestly... sometimes they just happen and that would be the only thing IMO that could do damage like that...unless you lost also an exhaust valve before meltdown which you didn´t ?

The exhaust valve seat was loose... But I had to pry it loose.

[MOBILEMERCMAN]

Two things not mentioned yet.. Water pressure less the 15 lbs is trouble. This should be closely monitored. Exhaust could have a pin hole in it. Not enough to collect and rust but, enough to change compression.

[Chris]

If it was cooling, the valve would have tulip'ed badly.

[MOBILEMERCMAN]

After 15 hours on the dyno what changed besides water pressure and exhaust ? Fuel?

[MikeyFIN]

Valves looks good so no blowby´s on the seats that´s what I was after.
I stick with the air pocket issue.

Not an ignition crossfire like Daredevil hinted as a timing problem ?
Those seems to be the only 2 reasons which could lead to detonation IMO as the plug hasn´t blued or anyother part to speak of.

[Tomas Wallin]

Two things not mentioned yet.. Water pressure less the 15 lbs is trouble. This should be closely monitored. Exhaust could have a pin hole in it. Not enough to collect and rust but, enough to change compression.

The 10PSi light goes off at around 20-30mph so I guess it should be higher than 15PSI at 80mph+

[MikeyFIN]

If it was cooling, the valve would have tulip'ed badly.

On a long term cooling issue yes.