What caused my melted piston?

[Geronimo36]

couple examples....

[Tomas Wallin]

I'm going with pre-ignition of some sort. How did the rod bearings look?

If it leaned out I would think it would have melted the piston in the 1-2 pm location near the intake valve relief since it's the thinnest area on the edge of the piston near the ringland..

As far as being able to catch it or stop it...don't think there's anything you could have done, it happens too quickly... Having an A/F meter would have helped before you went WOT but we dont' all have access to that!

My $.02.

The AFR was where it was supposed to be according to the dyno, I have a AEM wide band in the boat mounted in the pipe after the collector (SM Gen III) on the even side.

The rod bearings and crank looked like this:

[Tomas Wallin]

If it goes lean the exhaust valves will glow red...and detonate is there, that wasn´t the case but on the other hand no 6 is the leanest running cyl in a BBC.
Was there any carb tuning done at the dyno session ?
I don´t remember the dyno session ..you posted it somewhere didn´t you Tomas?

Yes we tuned the carbs and the timing on the dyno

93octane graph and an overlay of 91 and 93 octane (solid lines are 93):

[Ted]

The AFR was where it was supposed to be according to the dyno, I have a AEM wide band in the boat mounted in the pipe after the collector (SM Gen III) on the even side.

The rod bearings and crank looked like this:

That marking on the top bearing is consistent with pre-ignition/detonation. Oil actually gets squeezed out and the crank and brg touch.

[Tomas Wallin]

That marking on the top bearing is consistent with pre-ignition/detonation. Oil actually gets squeezed out and the crank and brg touch.

Ok, I hear you, that's good to know what it looks like!

[Geronimo36]

That marking on the top bearing is consistent with pre-ignition/detonation. Oil actually gets squeezed out and the crank and brg touch.

Was just about to type that!!!

[Tomas Wallin]

Was just about to type that!!!

But that we already knew, the question is why it pre-ignite/detonated.

[Geronimo36]

well I'm leaning away from lean per my previous posts,,,, so I'd be looking at timing, octane of your fuel, heat range of plugs, crossfire, cooling, etc. etc.

Might be a good idea to have your fuel tested... The guy at the gas station may have been hung over and hit the 89 button instead of 93.

[Chris]

...and we're back to why just one cylinder.

[Tomas Wallin]

well I'm ruling away from lean per my previous posts,,,, so I'd be looking at timing, octane of your fuel, heat range of plugs, crossfire, etc. etc.

Might be a good idea to have your fuel tested... The guy at the gas station may have been hung over and hit the 89 button instead of 93.

I filled it on a land gas station, and pushed the 93 button myself, so the fuel should be ok.

All this happened in the beginning of summer so I have refueled a few times since then...

Heat range of plugs brings it back to my original question about the loose plug. Is there any descent way to find out if it crossfires? Besides looking at the wires at night...

[Geronimo36]

I look at the wires very closely and see if you can see any white chalky marks. To be honest, I dont' think you're ever going to know exactly why it was only one piston....

Last time I detonated an engine I forgot to turn on a fuel pump and it toasted #7 bad and started getting #5 on one engine. On the other engine is blew the head gasket on #5.

A long time ago I detonated an engine and it only took out #7 and every other cylinder was good. So go figure!

Another boat I worked on (45 Sonic with HP500's) leaned out and detonated due to the chinsy fuel/water pump setup the late models had. The spark plug sorta welded itself into the GM style cylinder head.. I had to heat the head to loosen the plug.

[Tomas Wallin]

I look at the wires very closely and see if you can see any white chalky marks. To be honest, I dont' think you're ever going to know exactly why it was only one piston....

Last time I detonated an engine I forgot to turn on a fuel pump and it toasted #7 bad and started getting #5 on one engine. On the other engine is blew the head gasket on #5.

A long time ago I detonated an engine and it only took out #7 and every other cylinder was good. So go figure!

Another boat I worked on (45 Sonic with HP500's) leaned out and detonated due to the chinsy fuel/water pump setup the late models had. The spark plug sorta welded itself into the GM style cylinder head.. I had to heat the head to loosen the plug.

I won't get a definite answer but perhaps I will know more than I started out with. Just wan't to make my best to elimiate the risk of it happening again.

[Geronimo36]

I hear ya! What were your brake specific and A/F numbers?

Another example of lean-out....these pictures were from not turning on the fuel pumps....

[MikeyFIN]

You can ensure you have enough water pressure. Some add fittings to the rear of the intake too.

Correct!

[WeaponX]

I dunno, I don't want to dissagree but the valve relieve usually goes first on all the BBC's I've done it to.... The ring-land in the relief area is significantly thinner than everywhere else and it's also sharper..

I've blown a few up in my years from being lean and every single time it took out the piston in the relief area... 5/7 seem to be the worst, at least on the ones I blew up, by my own silly mistakes I might add..

I've lost 2 #5s and 1 #8 on two.

I think the blower size and intake style have alot to do with the moving air to the cylinders.

Always #5

[glh]

It's a carbed (2x Holley 800cfm) 509ci with Superchiller and 8-71 with 9.5PSI boost@6000rpm. Iron heads and 8.3:1 in compression, very little total run time on motor since rebuilt and 10minutes run time since dyno (warmup)... This happened at wot.

9.5 pounds of boost melted your piston I would not run anything above 6# with the crappy fuel they sell these days with humidity gathering ethanol.

If early in the year you might have had old fuel that would not help with that high a boost.

[WETTE VETTE]

Did you ever do plug checks when tuning? If the plug in that hole was loose the question becomes did the detonation cause the plug to loosen or was it never really tight? That plug may have just become a glow plug from being loose and caused severe preignition. I feel your heat range is good with the 41's. What condition are the plug wires in?

[Tomas Wallin]

9.5 pounds of boost melted your piston I would not run anything above 6# with the crappy fuel they sell these days with humidity gathering ethanol.

If early in the year you might have had old fuel that would not help with that high a boost.

But why did the other cylinders look perfect? Of course it has to start somewhere but still... The fuel tank was dry when I filled it up for this little trip - on a land gas station so the fuel should be in good condition.

Did you ever do plug checks when tuning? If the plug in that hole was loose the question becomes did the detonation cause the plug to loosen or was it never really tight? That plug may have just become a glow plug from being loose and caused severe preignition. I feel your heat range is good with the 41's. What condition are the plug wires in?

The dyno guy and his helper checked the plugs during the dyno so I can't guarantee anything...

My plug wires had 20 hours on them. http://www.keitheickert.com/detail.aspx?ID=7927

[MikeyFIN]

Tomas it doesn´t take much time to melt a piston. And the #4 IMO was already going too.. so if you´d stayed on it a little more there would´ve been two.

I´m still saying it´s cooling related. Either the Intercooler or head.

You Should switch to E85 or E95 as your engine seems to like to run like a Diesel
Or then the Dyno guy just tuned your engine too lean.

Regarding the loose plug issue...heat and detonation does it.

* No Voodoo Here...

[MikeyFIN]

I've lost 2 #5s and 1 #8 on two.

I think the blower size and intake style have alot to do with the moving air to the cylinders.

Always #5

5 and 7 are next to each other in the firing order so it´s not a bad idea to change the firing order. Also the no5 has the shortest runner so it´s always a problem.