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Tomas Wallin
10-26-2009, 05:44 PM
As some of you know I melted a piston in the beginning of summer, I never found any answer to why but today something struck me...

Would a loose spark plug might been the cause? Or was it an effect of what happened. Both from lack of heat transfer and also maybe from causing a lean mixture due to the bad seal.

When I removed the spark plugs after the incident I found that #6 was loose but I always thought that was caused by the detonation or the heat, but is that really that common?

One thing that always bugged me was that all the other cylinders looked so good and this melted, why would #6 get so lean and no other???


I have plenty more pictures of the pistons, plugs, valves etc...



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.

Chris
10-26-2009, 07:09 PM
What engine?

Detonation will definitely loosen a plug. Loose plugs don't cause ultra-lean conditions- and that's what melts plugs and pistons.

DAREDEVIL
10-26-2009, 07:37 PM
I would say ,,that looks like a timing problem,,,,since if it would be a lean problem ,,it would show on more then on 2 !!!!!!!!

TRUSST ME,,,I KNOW,,BUT PLEASE DON'T ASK WHY ....:rofl:

Ratickle
10-26-2009, 07:39 PM
Is it fuel injected? Or carb?

mj2562
10-26-2009, 08:16 PM
Mine went lean and got 2 # 2 & 8. How much run time on motor?

DAREDEVIL
10-26-2009, 08:20 PM
This looks like a little 4 and #6 thats why i think its timing !!

If they run lean on a carb its usually the outside ones that go first !!!!

Ted
10-26-2009, 10:51 PM
Recently a buddy of mine took out a cylinder on his boat with very similar circumstances. Loose plug for a bit (see exhaust on body) and lean in one cylinder. As best we could figure the plug allowed too much air past and/or detonated from being loose (couldn't shuck heat off because it didn't have good head contact). It was a very similar failure and he only did one run with the loose plug.

Ted
10-26-2009, 10:52 PM
BTW, this was a carb HP500, if your engine is injected look at the injector for poor flow, possibly while pulsing (lost one myself that way).

MOBILEMERCMAN
10-26-2009, 10:54 PM
I doubt a loose plug caused the damage..One could argue the loose plug wouldn't fire as well and would miss.

DAREDEVIL
10-26-2009, 11:45 PM
I doubt a loose plug caused the damage..One could argue the loose plug wouldn't fire as well and would miss.

AIR WILL NOT DO THIS AND A NON FIRING PLUG WILL NOT BURN ( LEAN OUT ) ANYTHING !!!!!!!!:USA:

No fuel ,,or way to much timing will !!!!!

WeaponX
10-26-2009, 11:46 PM
It's 5 and 8 that are the worst on big blower motors on detonation.

I've seen some just like that and alot worse.

WETTE VETTE
10-26-2009, 11:57 PM
I think a loose plug could cause this. Spark plugs transfer heat into the head at a particular rate, which is why there are different heat ranges. Hot plugs transfer heat more slowly than cold plugs. If the plug was not installed tight enough it could overheat because it doesn't trnasfer heat into the head the way it should. If it overheats it could glow causing pre-ignition which will eat a piston or worse. My opinion.

Ted
10-27-2009, 12:05 AM
The engine will not run more than a few seconds with pre-ignition. The only way to control pre-ignition is just keep any pre-ignition sources at bay. Spark plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.


http://www.enginelogics.com/detonation.html

MOBILEMERCMAN
10-27-2009, 12:09 AM
I can follow the loose plug overheats theory. I have trouble believing loose lets in air though.

If the plug was glowing wouldn't it have blued?

Thanks Ted, Great read.

Ted
10-27-2009, 12:13 AM
Looks kinda dark up near the insulator, it is a silver plug normally.

SFOcean
10-27-2009, 01:24 AM
I had the same problem last year on #7 BBC normally asperated iron head. Loose plug (yes, I know it was installed loose) caused extended pre-ignition. My plug looked similar to yours, no ground strap. Forged piston was good, just tuliped both valves and lost compression before piston failed.

Tomas Wallin
10-27-2009, 03:12 AM
Sorry guys,

It's a 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.

My thought was primarily the heat transfer subject and secondary the lean condition from sucking air through the threads.

If the plug was loose I guess it would get very hot with a bad heat transfer at wot, I run AC Delco MR41T so they're pretty cold.

MikeyFIN
10-27-2009, 06:49 AM
Iīd Tomas go with the loose plug theory going too hot but also Iīm interested about your head and intake gaskets sealing properly ?

And letīs just not forget a piston ring land failure.

I mean it detonated and only oil and too lean a combustion in a single cylinder would do this.

Ratickle
10-27-2009, 07:00 AM
Is it carb or injection?

How long did it take for that to happen total do you think? Sure made a mess......:ack2:

MikeyFIN
10-27-2009, 07:00 AM
Carb

Ratickle
10-27-2009, 07:05 AM
The engine will not run more than a few seconds with pre-ignition. The only way to control pre-ignition is just keep any pre-ignition sources at bay. Spark plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.


http://www.enginelogics.com/detonation.html

Great read Ted. Thanks. It would be nice to take that article and add pictures of all the items he described. Then have it here for the future.

Tomas Wallin
10-27-2009, 07:11 AM
Iīd Tomas go with the loose plug theory going too hot but also Iīm interested about your head and intake gaskets sealing properly ?

And letīs just not forget a piston ring land failure.

I mean it detonated and only oil and too lean a combustion in a single cylinder would do this.

The leak down was - cold, very cold... 2-6% on all 8 cylinders, so it was nice and tight. Total seal gapless rings, MLS head gaskets.

The intake gaskets looked just fine! No signs of any leaks.

Tomas Wallin
10-27-2009, 07:14 AM
Is it carb or injection?

How long did it take for that to happen total do you think? Sure made a mess......:ack2:

2x Holley 800cfm

After a few warmup passes I made one wot pass for maybe 2miles, turned back and made one more and after about half the distance it lost some power and I pulled off.

MikeyFIN
10-27-2009, 07:19 AM
The leak down was - cold, very cold... 2-6% on all 8 cylinders, so it was nice and tight. Total seal gapless rings, MLS head gaskets.


If the leakdown was at Cold 2-6 how do you think the Rings had any movement left while heated..

I mean on Audi WTCC racecars we had up to 20% leakdowns cold also the same with Turbocharged experimentals and Audi Sports head boss from Ingolstadt at the time told not to worry about that as they will seal up while getting warm especially charged engines and they did up to your numbers.

I never do a leakdown cold... just doesnīt make sense IMO.
As a machinist by profession I think I know something.

Tomas you still work for Scania..I think you could get some inhouse help?

But How was the Head gaskets?

Tomas Wallin
10-27-2009, 07:41 AM
If the leakdown was at Cold 2-6 how do you think the Rings had any movement left while heated..

I mean on Audi WTCC racecars we had up to 20% leakdowns cold also the same with Turbocharged experimentals and Audi Sports head boss from Ingolstadt at the time told not to worry about that as they will seal up while getting warm especially charged engines and they did up to your numbers.

I never do a leakdown cold... just doesnīt make sense IMO.
As a machinist by profession I think I know something.

Tomas you still work for Scania..I think you could get some inhouse help?

But How was the Head gaskets?

The low leak down is because of the gapless rings, the ring gaps are where they're supposed to be on the plus side. I ran one season with them and everything worked great, the leakdown was the same when new.

No, I quit there a year ago, couldn't find much inhouse help there anyway.

The head gasket looked like this:

MikeyFIN
10-27-2009, 07:51 AM
The low leak down is because of the gapless rings, the ring gaps are where they're supposed to be on the plus side. I ran one season with them and everything worked great, the leakdown was the same when new.

No, I quit there a year ago, couldn't find much inhouse help there anyway.

On blown engines I donīt like gapless rings (because of the heat variations running I like to have things a little on the loose side so I donīt need to worry ) or MLS gaskets but thatīs just me.

OK so you had no problems before, hmmmm ???

Howabout a water cooling problem on the head, the right side runs warmer and the middle ones seems to have gotten some coolant problems ?

Strange is what it is... but the more I think the more it seems like oil in the combustion or cooling problem which could lead to detonation problems. The cylinders are perfect so the water is at least up to there.

The 1-2 oīclock position looks a bit doubtful but detonation will do that quick, hard to see without a mag.glass..

Tomas Wallin
10-27-2009, 08:00 AM
On blown engines I donīt like gapless rings (because of the heat variations running I like to have things a little on the loose side so I donīt need to worry ) or MLS gaskets but thatīs just me.

OK so you had no problems before, hmmmm ???

Howabout a water cooling problem on the head, the right side runs warmer and the middle ones seems to have gotten some coolant problems ?

Strange is what it is... but the more I think the more it seems like oil in the combustion or cooling problem which could lead to detonation problems. The cylinders are perfect so the water is at least up to there.

The 1-2 oīclock position looks a bit doubtful but detonation will do that quick, hard to see without a mag.glass..
There's no rust what so ever in the heads nor the block.

Why do you think it seems like oil in the combustion? It definately started where the spark plug points if you look at this pic.

MikeyFIN
10-27-2009, 08:07 AM
Was there a ring land failure at the top piston ring?

If yes thats where the 7-8 oīclock meltdown started because of slight oil in the combustion.
Doesnīt need much to lower the octane of the combustion mix to a mess and when you have the spark there also voilá we have a dish served...

My theory is that the head for a while run dry and heated which lead to oil ( rings butting because of too much heat ) in the combustion and from thereone the spark functioned as a small torch for a millisecond.

What do you think ?

Tomas Wallin
10-27-2009, 08:14 AM
Was there a ring land failure at the top piston ring?

If yes thats where the 7-8 oīclock meltdown started because of slight oil in the combustion.
Doesnīt need much to lower the octane of the combustion mix to a mess and when you have the spark there also voilá we have a dish served...

My theory is that the head for a while run dry and heated which lead to oil ( rings butting because of too much heat ) in the combustion and from thereone the spark functioned as a small torch for a millisecond.

What do you think ?

The head ran dry of water or fuel?

How would I know if the ring land failed? The piston got pretty warm since it began to sink just between the piston center and 7-8 o'clock. But what happened first? If it sinks I bet the ring land will break pretty soon as well?

Then the oil started getting in and it detonated to hell

MikeyFIN
10-27-2009, 08:30 AM
Of water... as you have a Carb engine.
Offtopic
My team boss ignored the same thing on our formula ford rollout in the british series last year and lost 5 engines because of that.
I wanted to check the water was flowing immidately while the kid was coming back to the pit gate just to look into problems and leaks but Dad thought otherwise...

Kid told me next year dad ainīt coming along at all... or he doesnīt race.

In Karting I never did forget the water checks (couldnīt afford it)... on a boat with Raw water cooling I like to see a good flow too and Daredevil too...thats one of the reasons why he runs dry...
Right Scotty !!!?

Back on track.
Check the inside of the piston..do you see fractures along the top compression ring groove.
When things heat up it doesnīt necessarily break because it goes soft but youīll see fractures or something.
If the ring land is intact the compression rings just got too much heat and butted and gave a mix of air and oil to let the spark plug be a torch.

Thatīs my (long) shot theory.

Chris
10-27-2009, 08:41 AM
A loose plug would lose a portion of it's heat sinking capability, but I would think the ground would melt away quickly and the plug would stop firing.

MikeyFIN
10-27-2009, 08:54 AM
Chris I agree with you, a Loose plug would just die.

Ted
10-27-2009, 08:58 AM
A loose plug would lose a portion of it's heat sinking capability, but I would think the ground would melt away quickly and the plug would stop firing.

Note that in the article I posted it says "The engine will not run more than a few seconds with pre-ignition. " I think the plug got so hot that it became like a top fueler at the end of the track, the plugs are completely consumed and the engine is dieseling. Also with a good ignition system you would simply start scattering spark to the thread or head area, MSD or Crane can jump a spark over a 1/2" gap.

MikeyFIN
10-27-2009, 09:20 AM
But the MSD would not detonate and if it would diesel it wouldīve not shut down immidiately... it wouldīve kept dieseling until a hole in the piston

Tomas Wallin
10-27-2009, 09:33 AM
But the MSD would not detonate and if it would diesel it wouldīve not shut down immidiately... it wouldīve kept dieseling until a hole in the piston

The compression was really bad when I turned it off since it smoked alot through the breathers and also through the oil pan gasket, with that bad comp I guess the dieseling would have stopped and only took place at high revs?

Would it be able to diesel on only one cylinder?

Ted
10-27-2009, 09:48 AM
But the MSD would not detonate and if it would diesel it wouldīve not shut down immidiately... it wouldīve kept dieseling until a hole in the piston

He pulled out of it.


The compression was really bad when I turned it off since it smoked alot through the breathers and also through the oil pan gasket, with that bad comp I guess the dieseling would have stopped and only took place at high revs?

Would it be able to diesel on only one cylinder?


Yes, absolutely, all you need is a heat source hot enough to ignite the A/F mixture. Most blower motors are on the edge of dieseling anyway so even a small change is enough to start a hot spot. As the piston burned away and it started to lose compression and use oil it probably ended the event.

Tomas Wallin
10-27-2009, 09:50 AM
He pulled out of it.




Yes, absolutely, all you need is a heat source hot enough to ignite the A/F mixture. Most blower motors are on the edge of dieseling anyway so even a small change is enough to start a hot spot. As the piston burned away and it started to lose compression and use oil it probably ended the event.

I meant, would it be able to run on only one cylinder that's dieseling if the others are turned off. :)

Tomas Wallin
10-27-2009, 09:56 AM
A loose plug would lose a portion of it's heat sinking capability, but I would think the ground would melt away quickly and the plug would stop firing.

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...

Ted
10-27-2009, 10:00 AM
I meant, would it be able to run on only one cylinder that's dieseling if the others are turned off. :)

No, by that point it probably couldn't develop enough power to even turn the blower. You probably failed this thing in the last pull within second of when you lost power, blower motors leave no margin, especially under the load of a boat.

MikeyFIN
10-27-2009, 10:16 AM
I meant, would it be able to run on only one cylinder that's dieseling if the others are turned off. :)

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.

Tomas Wallin
10-27-2009, 10:19 AM
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
10-27-2009, 10:25 AM
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 doesnīt do that IMO because itīs a blown engine..it doesnīt suck air as the intake is pressurized so therefore the A/F ratios didnīt go lean until the temp of the combustion chamber got uncontrollable.

MikeyFIN
10-27-2009, 10:27 AM
Yes, and most likely because of the glowing spark plug...

And what got it glowing ?

Youīre still at the starting point IMO.

MikeyFIN
10-27-2009, 10:29 AM
MSD or Crane can jump a spark over a 1/2" gap.


in a Non compressed enviroment yes...

Tomas Wallin
10-27-2009, 10:32 AM
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 doesnīt do that IMO because itīs a blown engine..it doesnīt suck air as the intake is pressurized so therefore the A/F ratios didnīt 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
10-27-2009, 10:36 AM
in a Non compressed enviroment yes...

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

MikeyFIN
10-27-2009, 10:37 AM
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. Weīre 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
10-27-2009, 10:38 AM
At 175 psi, an MSD won't jump a .100" plug gap. .070" will barely run.

Thatīs what I meant...therefore I agree with you that the loose plug wasnīt the cause but the head had an air pocket...

Tomas Wallin
10-27-2009, 10:38 AM
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. Weīre 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
10-27-2009, 10:41 AM
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
10-27-2009, 10:43 AM
Thatīs what I meant...therefore I agree with you that the loose plug wasnīt 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
10-27-2009, 10:45 AM
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
10-27-2009, 10:49 AM
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 didnīt ?

Tomas Wallin
10-27-2009, 10:52 AM
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 didnīt ?

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

MOBILEMERCMAN
10-27-2009, 11:07 AM
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
10-27-2009, 11:10 AM
If it was cooling, the valve would have tulip'ed badly.

MOBILEMERCMAN
10-27-2009, 11:10 AM
After 15 hours on the dyno what changed besides water pressure and exhaust ? Fuel?

MikeyFIN
10-27-2009, 11:11 AM
Valves looks good so no blowbyīs on the seats thatīs 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 hasnīt blued or anyother part to speak of.

Tomas Wallin
10-27-2009, 11:12 AM
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
10-27-2009, 11:14 AM
If it was cooling, the valve would have tulip'ed badly.

On a long term cooling issue yes.

MikeyFIN
10-27-2009, 11:16 AM
The 10PSi light goes off at around 20-30mph so I guess it should be higher than 15PSI at 80mph+

so you are in fact guessing Tomas....
Damn you, get a meter there for it ASAP...

The warning lights are aka IDIOT LIGTHS...

Tomas Wallin
10-27-2009, 11:16 AM
Valves looks good so no blowbyīs on the seats thatīs 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 hasnīt blued or anyother part to speak of.

New (since 20hours) Keith Eickert plug wires, new MSD crab cap distributor, MSD AL6 box and MSD coil.

Wouldn't it take a while to blue the plug? All this happened within a very short time period.

Chris
10-27-2009, 11:17 AM
Detonation almost always fractures the tops of the pistons around the ring lands. How about a cam lobe?

Tomas Wallin
10-27-2009, 11:18 AM
so you are in fact guessing Tomas....
Damn you, get a meter there for it ASAP...

The warning lights are aka IDIOT LIGTHS...

I have a gauge for it but's that's on the IC. I rather have lights than too many gauges. :)

Tomas Wallin
10-27-2009, 11:22 AM
Detonation almost always fractures the tops of the pistons around the ring lands. How about a cam lobe?
Wouldn't think so, engine runs since I put it together again. RMBuilder cam also new since 20hours.

Chris
10-27-2009, 11:22 AM
On a long term cooling issue yes.

I've seen them go in seconds. At max power and losing cooling water, you have no way to draw heat from the valve (exhausts valves sink heat through the seats).

Having said that, are you using the proper head gaskets for your block? There are passage and restriction differences between Mark IV and Gen 5/6 blocks.

Chris
10-27-2009, 11:24 AM
Wouldn't think so, engine runs since I put it together again. RMBuilder cam also new since 20hours.

If you had lost a cam lobe you'd have noticed it on reassembly. I'm assuming they're hydraulic roller cams.

Chris
10-27-2009, 11:27 AM
Wouldn't it take a while to blue the plug? All this happened within a very short time period.

Max power, max RPM- the damage takes a couple seconds.

At 6,000 RPM your engine is experiencing 50 combustions and power pulses per second. Imagine smacking your thumb once with a hammer. Now imagine smacking it 50 times in a second.

MikeyFIN
10-27-2009, 11:29 AM
How did you Route them wires I Think You do know this but repetition is the key ?

It would take a while to tulip a valve or blue anything which IMO isnīt the case here.

MikeyFIN
10-27-2009, 11:30 AM
detonation almost always fractures the tops of the pistons around the ring lands.

agreed.

MikeyFIN
10-27-2009, 11:31 AM
I have a gauge for it but's that's on the IC. I rather have lights than too many gauges. :)

So that makes you an.. :sifone:

MikeyFIN
10-27-2009, 11:36 AM
I've seen them go in seconds. At max power and losing cooling water, you have no way to draw heat from the valve (exhausts valves sink heat through the seats).

Having said that, are you using the proper head gaskets for your block? There are passage and restriction differences between Mark IV and Gen 5/6 blocks.


Yes you could do that but thereīs problems arising IMO already for a while..you just notice it in seconds, Thatīs my take. And yes Iīve ground a FEW Valves to know how they work... but anyways repetition is the key here for everyone.
Itīs like detonation... a 50$ hearing aid modified is a lot better (read sooner) detecting detonation than any knock sensor..

MikeyFIN
10-27-2009, 11:38 AM
Max power, max RPM- the damage takes a couple seconds.

At 6,000 RPM your engine is experiencing 50 combustions and power pulses per second. Imagine smacking your thumb once with a hammer. Now imagine smacking it 50 times in a second.


Howabout the forehead instead :rofl:

MikeyFIN
10-27-2009, 11:39 AM
Still we havenīt come to an agreement of the cause...

everyone put their take and reasoning on it behind ?

Tomas Wallin
10-27-2009, 11:41 AM
I've seen them go in seconds. At max power and losing cooling water, you have no way to draw heat from the valve (exhausts valves sink heat through the seats).

Having said that, are you using the proper head gaskets for your block? There are passage and restriction differences between Mark IV and Gen 5/6 blocks.
The head gaskets are Fel-Pro MLS 1075-1, Merlin II block and GM 088 heads.

New impeller, sea strainer that was clean and water was flat.

MikeyFIN
10-27-2009, 03:29 PM
Tomas it doesnīt matter ..sometimes for example big diesels have to use a water softener that takes out the surface tension because otherwise they would knock a hole in the cylinders.. the Ford Powerstroke for example is notorious of that.
Air pockets just come sometimes and thereīs not much one can do about it.
Lack of water somewhere is the only reason I can think of that raises the combustion temps so that detonation would occur if everything else is fine.
The other reason is a misfire that spikes the temps and just ignites the detonation cycle.

Geronimo36
10-27-2009, 03:33 PM
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.

MikeyFIN
10-27-2009, 03:41 PM
If it goes lean the exhaust valves will glow red...and detonate is there, that wasnīt 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 donīt remember the dyno session ..you posted it somewhere didnīt you Tomas?

Geronimo36
10-27-2009, 03:48 PM
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.. :(

MOBILEMERCMAN
10-27-2009, 04:11 PM
Air pockets just come sometimes and thereīs not much one can do about it.


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

Geronimo36
10-27-2009, 04:26 PM
couple examples....

Tomas Wallin
10-27-2009, 04:29 PM
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
10-27-2009, 04:36 PM
If it goes lean the exhaust valves will glow red...and detonate is there, that wasnīt 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 donīt remember the dyno session ..you posted it somewhere didnīt 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
10-27-2009, 04:42 PM
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
10-27-2009, 04:46 PM
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
10-27-2009, 05:05 PM
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!!!:driving:

Tomas Wallin
10-27-2009, 05:07 PM
Was just about to type that!!!:driving:

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

Geronimo36
10-27-2009, 05:10 PM
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
10-27-2009, 05:13 PM
...and we're back to why just one cylinder.

Tomas Wallin
10-27-2009, 05:15 PM
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...:rofl:

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
10-27-2009, 05:25 PM
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
10-28-2009, 04:26 AM
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
10-28-2009, 05:29 PM
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.... :( :ack2:

MikeyFIN
10-28-2009, 05:47 PM
You can ensure you have enough water pressure. Some add fittings to the rear of the intake too.

Correct!

WeaponX
10-28-2009, 08:43 PM
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:sifone:

glh
10-28-2009, 09:18 PM
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
10-28-2009, 10:32 PM
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
10-29-2009, 05:05 AM
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
10-29-2009, 05:44 AM
Tomas it doesnīt take much time to melt a piston. And the #4 IMO was already going too.. so if youīd stayed on it a little more there wouldīve been two.

Iīm still saying itīs 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
10-29-2009, 05:47 AM
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:sifone:

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

Geronimo36
10-29-2009, 09:07 AM
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:sifone:

I'm with you dude!!!

I also think GLH is right; I would think 9 lbs of boost, intercooled, with GM heads is kinda on the verge with pump gas. I wouldn't go any higher than 7 lbs on mine and once in the boat I turned it down to 6 lbs to be safe and even jetted up 1 size on the secondaries to be help with heat-soak on long passes.

Every time I've ever had a problem it happened so quickly you can't prevent it...even with a knock sensor... Once you're into detonation it's almost too late. The moment you see the engine lay down RPM it's done and we're talking 1/2 to fractions of a second at 5-6k rpms.

Also, what was total advance? If too much you'll detonate, if too little you can smoke the exhaust valves. contrary to popular belief, blower engines like timing. I ran as much as 34* on the dyno but run 31/32* in the boat. I see some guys run 28* but every dyno I went to EGT's were very high at 28*. Of course this depends on the style heads you have.

I think if you really want to prevent it from happening again I'd look into getting some AFR's or turn the wick down! :USA:

Tomas Wallin
10-29-2009, 10:13 AM
I'm with you dude!!!

I also think GLH is right; I would think 9 lbs of boost, intercooled, with GM heads is kinda on the verge with pump gas. I wouldn't go any higher than 7 lbs on mine and once in the boat I turned it down to 6 lbs to be safe and even jetted up 1 size on the secondaries to be help with heat-soak on long passes.

Every time I've ever had a problem it happened so quickly you can't prevent it...even with a knock sensor... Once you're into detonation it's almost too late. The moment you see the engine lay down RPM it's done and we're talking 1/2 to fractions of a second at 5-6k rpms.

Also, what was total advance? If too much you'll detonate, if too little you can smoke the exhaust valves. contrary to popular belief, blower engines like timing. I ran as much as 34* on the dyno but run 31/32* in the boat. I see some guys run 28* but every dyno I went to EGT's were very high at 28*. Of course this depends on the style heads you have.

I think if you really want to prevent it from happening again I'd look into getting some AFR's or turn the wick down! :USA:
Actually I just installed a J&S Safeguard knock retard controller that controls the timing individually, I'll get back with report on how it's working later on... So far it works very good! I will play around with the timing a bit to see what will happen.

Total advance was locked at 28.5deg around 1400F EGT's@6k

I don't remember what my AFR gauge said at that moment.

I will detune the engine to 8PSI (that has worked very well earlier) until next season!

WeaponX
10-29-2009, 08:36 PM
I'm with you dude!!!

I also think GLH is right; I would think 9 lbs of boost, intercooled, with GM heads is kinda on the verge with pump gas. I wouldn't go any higher than 7 lbs on mine and once in the boat I turned it down to 6 lbs to be safe and even jetted up 1 size on the secondaries to be help with heat-soak on long passes.

Every time I've ever had a problem it happened so quickly you can't prevent it...even with a knock sensor... Once you're into detonation it's almost too late. The moment you see the engine lay down RPM it's done and we're talking 1/2 to fractions of a second at 5-6k rpms.

Also, what was total advance? If too much you'll detonate, if too little you can smoke the exhaust valves. contrary to popular belief, blower engines like timing. I ran as much as 34* on the dyno but run 31/32* in the boat. I see some guys run 28* but every dyno I went to EGT's were very high at 28*. Of course this depends on the style heads you have.

I think if you really want to prevent it from happening again I'd look into getting some AFR's or turn the wick down! :USA:

Aluminum heads is a must. And at 1400 degrees loss of water to the block even for a short period of time would finish the piston.

1250 degrees is a safe # on temps in my book.
It's crazy the differance heads can have on timing the last power I put together with Big Cheifs hated timing. I had heard it was that way but had to try it for myself made it to 31* and lost some power on the top end but the torque was better on the low side. Going to put a 4100 rpm retard on it for next summer to get the best of both.:sifone:

Geronimo36
10-29-2009, 09:00 PM
I've heard that about the Big Chief's, JC originally told me about it! How did the Chief's hold up long-term?

Sounds like a cool idea with the timing retard!!

Real Baja
10-29-2009, 10:30 PM
I do not know about 4 stroke engines, but on multiple (triple) cylinder 2 strokes I have seen a loose plug create an extreme lean condition and quickly burn a hole in the piston. The plug gets so hot it glows and even with a bad connection the engine runs smooth, until the piston gives way.

MikeyFIN
10-29-2009, 10:41 PM
I've heard that about the Big Chief's, JC originally told me about it! How did the Chief's hold up long-term?

Sounds like a cool idea with the timing retard!!

If the "Heads/engine" hatre timing it Works!!!
The more timing the worse the breathing.

MikeyFIN
10-29-2009, 10:42 PM
I do not know about 4 stroke engines, but on multiple (triple) cylinder 2 strokes I have seen a loose plug create an extreme lean condition and quickly burn a hole in the piston. The plug gets so hot it glows and even with a bad connection the engine runs smooth, until the piston gives way.

This one is a force fed engine so it doesnīt got lean or draw air from the loose plug. Otherwise I agree with you but I believe this as a cooling issue.
Tomas just went over the hill...

tango
10-30-2009, 12:44 AM
if it was not a blown (pressurized) intake charge, but strictly a naturally aspirated engine I might suggest a bad intake valve seal which would allow oil to be drawn into that "1" in particular cylinder and thus start the detonation process, just a secondary thought. also look @ the plug threads in the 1st post "oil" soaked ? is that a result of it being loose before or after the initial detonation process ?

Tomas Wallin
10-30-2009, 07:08 AM
This one is a force fed engine so it doesnīt got lean or draw air from the loose plug. Otherwise I agree with you but I believe this as a cooling issue.
Tomas just went over the hill...


Yes, maybe...:seeya:

But a loose plug can still cause the cylinder to pre-ignite because of the insufficient heat transfer between the plug and the head that caused the plug to glow.

It was loose enough to easily remove by hand. But if it was loose before or if it came loose during the "accident" I will most likely never know. :)

JupiterSunsation
10-30-2009, 08:15 AM
What caused my melted piston?


Capt. Obvious says "HEAT" caused it.

Tomas Wallin
10-30-2009, 08:20 AM
What caused my melted piston?


Capt. Obvious says "HEAT" caused it.

Capt. Stubborn says: What caused the heat.

JupiterSunsation
10-30-2009, 08:33 AM
Capt. Stubborn says: What caused the heat.

Just saw you were from out of the country, Mr Obvious is a radio skit (I turned it into Capt. since it was boat related).
http://www.youtube.com/watch?v=5XZMC1M9o04

Geronimo36
10-30-2009, 08:33 AM
Some pretty good suggestions have already been given. Depends what you choose to do with them! ;)

I remember a while back suggestions were given from a reputable builder about your engine as well...

Tomas Wallin
10-30-2009, 08:49 AM
Just saw you were from out of the country, Mr Obvious is a radio skit (I turned it into Capt. since it was boat related).
http://www.youtube.com/watch?v=5XZMC1M9o04

Yes I know that :USA:

Tomas Wallin
10-30-2009, 08:50 AM
Some pretty good suggestions have already been given. Depends what you choose to do with them! ;)

I remember a while back suggestions were given from a reputable builder about your engine as well...

Yup, and I'll do my best to use everyones opinion! Started this thread since I had some new info with the loose plug - and to gain some tech talk to the forum...

MERPerformance
10-30-2009, 07:09 PM
i'AM NOT GOING TO SAY THIS IS WHAT HAPPENED. I had a N/A 540 CID 12.5:1 with iron heads, tapered seat plugs, paint was on the seat area of the head for the plug, well after awhile of running the plug would miss- fire and break the center electrode out of the plug and ding the top of the piston. If the plug was loose I feel it would have done the same either N/A of forced induction.

WeaponX
10-30-2009, 07:14 PM
I've heard that about the Big Chief's, JC originally told me about it! How did the Chief's hold up long-term?

Sounds like a cool idea with the timing retard!!

Titanium intake valves are the trick to the Big Chiefs lasting. $1500. but it worth the cost.

We will see after another 42 hours.:26:

jeffswav
10-30-2009, 07:30 PM
Detonation almost always fractures the tops of the pistons around the ring lands. How about a cam lobe?I am only on page 4 of this huge post but that happend to me on a flat tappet cam. I am sure he must have a roller cam. I have since switched to a roller cam after blowing a hole in the piston from going lean. A cam lobe can definatly do some damage.

jeffswav
10-30-2009, 07:36 PM
That marking on the top bearing is consistent with pre-ignition/detonation. Oil actually gets squeezed out and the crank and brg touch.I did the same thing a few years back. I destroyed the tops of the rod berrings. Machine shop said the most likley problem was pre-detination, most likley bad fuel. It was the first trip out in the spring with E10 fuel using stabil. I have since never stored E10 fuel more than 1 month. Man you are bringing back some bad memories. :smash:

jeffswav
10-30-2009, 07:40 PM
...and we're back to why just one cylinder.Ok, I have blown mine up way to many times. When I had a cam lobe go down it only blew one piston. May not play a part in this one.

Tomas Wallin
10-31-2009, 01:23 PM
I am only on page 4 of this huge post but that happend to me on a flat tappet cam. I am sure he must have a roller cam. I have since switched to a roller cam after blowing a hole in the piston from going lean. A cam lobe can definatly do some damage.

Yup, a hyd roller and the cam is running perfectly since I put it together again.

WeaponX
10-31-2009, 06:31 PM
Pics of the last victims......#5 & 8:sifone:

14lbs and 7000 rpm on 604 cid. The wrong fuel mix. Easy to fix this time just two new pistons and rings.Got lucky:26:

Notice the eyebrows both missing the thin sides.

Coolerman
11-01-2009, 12:14 PM
Pics of the last victims......#5 & 8:sifone:

14lbs and 7000 rpm on 604 cid. The wrong fuel mix. Easy to fix this time just two new pistons and rings.Got lucky:26:

Notice the eyebrows both missing the thin sides.

maybe it's the light, but in that 2nd pic, it looks like the block got torched. Also, I thought these motors were at 6lbs boost :26::26::26:

indywhsle
11-01-2009, 12:25 PM
I have not posted here much if at all but I just read every page and since I just had the same problem with my 502 with a m3 procharger putting out 730hp and 785fpt I figured that I would chime in. Mine had dyno time plus 4 or 5 hrs. and long story short it was the water pick up. I would recomend putting a low water pick up on the boat. Don't know if you already have one but that was never asked in any of the posts.

WeaponX
11-01-2009, 02:30 PM
maybe it's the light, but in that 2nd pic, it looks like the block got torched. Also, I thought these motors were at 6lbs boost :26::26::26:

Block is perfect. Burnt through the MLS gasket between 5&7 a little.

Lots of talk on these motors but the facts are 1063 hp at 6.8 psi on pump and 1186 hp at 12psi could get more out of it at both psi but I had just one day on the dyno only ran six pulls.

And it was the five days before the Shootout. :sifone:

Tomas Wallin
11-01-2009, 02:37 PM
I have not posted here much if at all but I just read every page and since I just had the same problem with my 502 with a m3 procharger putting out 730hp and 785fpt I figured that I would chime in. Mine had dyno time plus 4 or 5 hrs. and long story short it was the water pick up. I would recomend putting a low water pick up on the boat. Don't know if you already have one but that was never asked in any of the posts.
Actually I did put on a new dual design pickup (engine and IC) after this incident so the flow is better now but it should have been sufficient enough before as well. I know the flow through the intercooler is alot better with the new pickupdesign.

I use the drive water pickup for the drive oil cooler.

DAREDEVIL
11-01-2009, 02:40 PM
Actually I did put on a new dual design pickup (engine and IC) after this incident so the flow is better now but it should have been sufficient enough before as well. I know the flow through the intercooler is alot better with the new pickupdesign.

I use the drive water pickup for the drive oil cooler.

PICS of the pickup ??

2112
11-01-2009, 02:59 PM
Pics of the last victims......#5 & 8:sifone:

14lbs and 7000 rpm on 604 cid. The wrong fuel mix. Easy to fix this time just two new pistons and rings.Got lucky:26:

Notice the eyebrows both missing the thin sides.


How did you catch this? Just on tear down after a Dyno pull or was it in the boat already?
.

WeaponX
11-01-2009, 06:59 PM
How did you catch this? Just on tear down after a Dyno pull or was it in the boat already?
.

After it ran. Just a bad guess on fuel blends. I saw a drop in power and boost and pulled out of it on a run for top speed. Got lucky on this one I've seen alot worse.

Geronimo36
11-01-2009, 10:37 PM
After it ran. Just a bad guess on fuel blends. I saw a drop in power and boost and pulled out of it on a run for top speed. Got lucky on this one I've seen alot worse.

Yeah man, you got real lucky, that's cool!! :driving:

I kinda got lucky like that once... Had an engine pop the head gasket out in the 11 oclock position on #7. Blew the chunk of gasket into the lifter valley...

Luckilly the gasket was the weak link otherwise the piston would have been moments later! :26:

indywhsle
11-02-2009, 04:33 PM
Actually I did put on a new dual design pickup (engine and IC) after this incident so the flow is better now but it should have been sufficient enough before as well. I know the flow through the intercooler is alot better with the new pickupdesign.

I use the drive water pickup for the drive oil cooler.

I thought the same thing. $6000 later I'm not so sure.

Tomas Wallin
11-03-2009, 02:18 AM
I thought the same thing. $6000 later I'm not so sure.
Here you can see the water flow at high speed.
http://www.salta.se/pics/alla/2009/Shootout/Pantera/slides/DSC_3786.JPG

Geronimo36
11-03-2009, 09:55 AM
Here you can see the water flow at high speed.

Are you bleeding water off the rear corners of the cylinder heads?

If so, how big are the hoses?

Tomas Wallin
11-03-2009, 09:58 AM
Are you bleeding water off the rear corners of the cylinder heads?

If so, how big are the hoses?

No I'm not.

How should that be plumbed?

I assume the bleed holes would be in the intake at the rear water runners in the heads, but where should the water be routed? Back to the engine somewhere or separate dumps?

Geronimo36
11-03-2009, 01:33 PM
No I'm not.

How should that be plumbed?

I assume the bleed holes would be in the intake at the rear water runners in the heads, but where should the water be routed? Back to the engine somewhere or separate dumps?

You can do it a few different ways; 1. Route it to the front of the intake and "T" in with the hoses coming out of your intake (if you have plugs in the intake just use them), 2. Dump overboard, 3. Some guys have bungs welded on the tailpipes for the hoses to connect.

I'm using two #6 lines on either side of the intake which connect together via a "Y" connector that runs a single #6 overboard. If you have low water pressure you might want to tee in to the front of the intake or use smaller lines, possibly as small as 1/8". All you're doing is giving air pockets a place to escape.

I decided to dump overboard so I can peer over the side of the boat to make sure I'm pumping water at anytime.

I'd highly recommend doing it to relieve any air pockets in the heads (especially steel heads) to reduce the potential of detonation on the back cylinders.