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View Full Version : Burnt piston in Volvo Penta 2002 5.7 GS



olli57
01-17-2011, 03:07 AM
I have bought a second hand offshore powerboat 34' with two Volvo P 5.7 GS (built 2002, only 100 hrs)and SX drives (built 84). Now I made the maiden voyage and after 20 mins one motor was blown.
Here is the story: when i buy the boat both engines have been completely serviced with new sparkplugs, oilchange, new cables and distributor as well v-belts. also motors have been rebulit with a cooling kit (we are having heavy corrosion due to high saltcontent here in Philippines) to avoid corrosion problems.
When I launched the boat, I started the engines and monitored all functions and they where running nice. I took a tour of 20 mins with max 3500 rpm and engines are fine. Next day we started our first trip and slowly I extended the rpm to 4000. Oilpressure was good, temperature in the lower part. Then after about 20 mins the one engine started to run bad and I immedeately stopped the boat. The engine gave some slight knocking noise and I decided the stop it and return with the other engine.
Back in the port i removed the sparkplugs and found the rear left damaged and covered with aluminium. After removal of the cylinderhead, the worst expectations came true. The piston was heavily damaged and burnt. aluminium is melted and the upper piston rings can be seen.
Now of course I have to repair - however what is my concern WHY this happened, maybe it will happen again after repair.
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RicardoMarine
01-17-2011, 10:43 AM
.......................
Here is the story: when i buy the boat both engines have been completely serviced with new sparkplugs, oilchange, new cables and distributor as well.............................
I took a tour of 20 mins with max 3500 rpm and engines are fine...............................
Back in the port i removed the sparkplugs and found the rear left damaged and covered with aluminium. After removal of the cylinderhead, the worst expectations came true. The piston was heavily damaged and burnt. aluminium is melted and the upper piston rings can be seen.
Now of course I have to repair - however what is my concern WHY this happened, maybe it will happen again after repair.

Your photos show what I would consider to be, typical Detonation damage.
The first clue for me would be the new distributor(s). Was any Ignition TA data checked or written down during Ignition set-up?

To make things worse, your engine(s) is fitted with the Full Dished pistons, that in my book, should NEVER be found in a SBC Marine engine.
Shame on GM for doing this! :mad:

The likely cause:
Incorrect fuel/air mixture complicated by an incorrect Ignition Timing Advance curve (i.e., TA [total advance] coming on too early).
The 3k to 3.5K RPM range on a Cruiser SBC, is the perfect recipe for Detonation....., and particularly with this piston! These are prone to being a contributor to Detonation!

You may have several other pistons that are shortly behind this one.
Have you pulled both cylinder heads?

Disclaimer: While this wreaks of the typical tell tale signs, I'm making an assumption that this is Detonation caused. So this is speculation only since none of us can have hands-on/eyes-on your engine!

If I were putting this engine back together, I'd get as far away from the Full Dished piston as I possibly could. :mad: There are many many Quench Effect style pistons (http://www.chevyhiperformance.com/tech/engines_drivetrain/shortblock/94138_piston_head_clearance_guide/index.html) that will work in this engine. You can create the same C/R, and use the same cylinder heads when the correct piston and head gasket choices are made.
The over-all cost is minimal compared to the advantages.

I'd also be checking your Ignition TA on the engine that survived. It too could shortly become damaged if the same Ignition curve has been used.
Again..... assuming that this is Detonation caused damage, and is Ignition related!


The article linked to above is primarily Automotive.... but there is a very nice cross-over when it comes to the Q/E and/or Squish Effect...... particulary with the Marine Cruiser SBC.
(the squish can be brought closer to the .038" range, since we don't turn high RPM.... the tighter, the more effective.)

Good luck, and I'm sorry to hear of your misfortune. What a bummer!

.

olli57
01-22-2011, 06:26 AM
Hi Rick, many thanks for your quick and good reply.
I have also checked based on this some more and it looks that I have to replace the pistons all to better type.
But have some comments/reply to yours also:
- the distributor was not replced , only the cap. so the original setting probably was not touched and as the workshop is Volvo dealer, suppose they have used / verified the official volvo settings(checking on this)
- the detonation is a logic explanation, only I wonder why I did not hear that. (I belive I have a sensitive ear for wrong sounds from motors)
also the other 3 pistons on this side are looking quite good. (I have not opened the other side due to lack of time)
- I have the other idea that maybe the uper part of the piston has cracked off, this also would explain why the sparkplug has mechanical damage. However the question is why this happened. Maybe this can be created by detonation as well?
- I probably have no choice but to overhaul also the other motor as I will not be confident on the conditon after this. Driving on open seas in Philippine waters (with no SAR or rescue existing) I have to rely on my engines. Thats why i bought this twin engine boat after I had lots of hustle with my old singe engine Mercruiser.

RicardoMarine
01-22-2011, 10:41 AM
Hey, you're welcome! Sorry to see and hear of this damage!

Yes.... you will certain need to replace all pistons.
But let me clarify something, if you will.

It may not have been the quality, per se',of these pistons that contributed to this Detonation damage. The quality was probably just fine. And what the actual cause was, may be debatable and may never be determined. But with my somewhat educated speculation....., this would appear to have been from Detonation!
(be sure to not confuse Detonation with Pre-Ignition)

But piston quality was not necessarily what I was referring to earlier. What I was referring to, was piston style ..... I.E., a piston that offers a Quench or Squish Effect.
(I believe that I linked you to an article)
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The SBC Marine Engine should NEVER use the Full Dished piston.
Shame on GM and any re-builder who is using them in this engine for Marine use! :mad:
SBC cylinder head chambers have become smaller due to the runners becoming larger. 64cc chambers are very common these days. The full dished piston brings C/R into specs with this chamber. GM does this as a cost savings, and with no favors to any of us!
With this configuration, often ignition TA is held back in order to combate detonation.
But there is more to this!
The SBC cylinder head chamber also offers a "wedge" area. The portion of the piston that comes up and under this "wedge" should "mirror" the wedge...... not be a "dish".... and certainly not under the "wedge".
Mirroring and bringing these into close proximity creates a Quench or Squish effect!
We want this! This makes good horse power and performance, and greatly reduces the rish of Detonation!
The dished area is OK if kept in/under the main chamber area, but it should NEVER be allowed to come up/under the "wedge" area of a cylinder head. It is a recipe for Detonation! :mad:
There are much better pistons availble, they will give us the C/R that we want, give us the Q/E that we need......, and the cost will not break the bank.
OK... off my horse now! :)

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Whether or not your original ignition timing was touched, something had to contribute to this. (Sounds like I misunderstood you on that earlier.)

Unfortunately, with the engine now torn down, there is no way to verify ignition timing via any forensics. That would have been first on my list, since a shop was responsible for setting timing in the first place......., correct?

Yes..... any source that may cause excessive cylinder temperatures can contribute to Detonation damage......... Incorrect Ignition timing, lean fuel/air mixture, heavy low rpm loads w/ too much TA, a combination of any, and so on..... spark plugs if they were holding heat.... but this may be more along the lines of Pre-Ignition.

Use caution if you search for information regarding "detonation" definitions, causes, and explanations. Some articles are great.... some have some technical un-clarity and/or errors in them.
But what you show in your photos, certainly appears to be Detonation related damage.


When all is said and done, and your engine is back together, be sure to use an advance curve/limit that is correct for this engine. (it can be slightly different if you get away from the dished pistons)

Then when you set it up, set/check your ignition BASE advance and TA (total advance) "dynamically" using a marked off harmonic balancer, and a good 'ole "Old School" timing light in Standard Mode. You will want to see your Ignition advance curve and TA limit in Real Time and in Real Degrees. (Not via an algorithm from a Chinese made circuit board as per the digitally advancing lights.)
The guys love their digitally advancing timing lights..... and they may very well work just fine in automotive and Marine EST w/ ECM/ECU etc.
Our ignition advance is very critical on our marine cruiser engines. It may take only a few degrees too much TA to cause detonation and damage.
Your call on that!

Docksidemarineservices
01-22-2011, 11:23 AM
Take a good look at the cyl head. If there is a crack in the combustion chamber it will cause a lean condition and do the same damage.

Everyone has a different Idea of what a rebuilt engine is. New Bearings and piston rings is not all that goes into a rebuild.

RicardoMarine
01-22-2011, 01:00 PM
Yes, good point.
As I mentioned earlier (post #2), a lean condition can also cause detonation.......... and what the actual cause was, may be debatable and may never be determined.
Re; your second point.... Amen.

I've been involved in/with/around the SBC since the mid 60's. I have NEVER built one using that silly dished piston.
It did not make sense then (althogh they were not around until the 70's) and it does not make sense now!

Docksidemarineservices
01-22-2011, 02:54 PM
The purpose of the dish piston was to lower the compression ratio to avoid detonation caused by the low octane fuels. Also to de-tune the SBC engine when they came out with the smog heads.