LS7 Engine Failure
TECH Apprentice
Joined: Oct 2007
Posts: 301
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With tall the talk of heat, do the titanium rods transfer heat as well as steel? As I learned it, most of the heat is transmitted from the crown down thru the piston to the pin where it's path goes down the rod, to the crank, to the block, thru coolant to the radiator, then into the atmosphere. Of course, a lot of heat is shed thru the oil, but most of it makes it to the radiator.
Thread Starter
Teching In
Joined: Feb 2010
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briannutter,
Thanks for taking the time to write up such an in depth reply. Actually, I would like to thank everyone for their input. As a new member to this forum, I am amazed at how much really good information can come out of this site. I will try and get the information you requested, in order to further evaluate what was going on. My personal interest in this situation is an engine project that I am about to start. I would like to avoid this outcome if possible.
Thanks for taking the time to write up such an in depth reply. Actually, I would like to thank everyone for their input. As a new member to this forum, I am amazed at how much really good information can come out of this site. I will try and get the information you requested, in order to further evaluate what was going on. My personal interest in this situation is an engine project that I am about to start. I would like to avoid this outcome if possible.
On The Tree
Joined: Aug 2006
Posts: 107
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From: SW Virginia
I'll bet it condensed water in the oil. It's been real cold, and it takes forever to warm up. On a clear tubing oil circuit I can see that tan discolored emulsion-oil color before the water blows off. I've had to reset my fans to 240* and just let it idle to get rid of it. If you have ANY oil getting by your catch can, the condensation in it will hydrolock and crack anything, even under perfect tuning conditions.
I was reading up on this post to see where it has headed since I last posted and I was looking at the photos of the piston. I've got an 02 Mustang GT that I just pulled the 4.6 mod motor out of. It broke the #8 piston in half almost perfectly from an injector leaking thus causing hydrolock. Could it be possible that it could've gotten something liquid in that cylinder on start up when it was freshened up and started a crack?? I'm just throwing theories here.
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Teching In
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As I stated earlier, four other pistons had cracks on the underside of them. I am thinking that the piston that failed started out the same way. I am going to try and get pictures of those pistons. As far as hydrolock, the car was driven to two track days, a month apart from each other, never indicating any kind of compromise in the performance at the time. The problem was discovered on track when blue smoke started coming out of the rear of the car.
TECH Regular
Joined: Feb 2006
Posts: 437
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From: Cleveland
Sure thing GTII.
Another response was speaking of where the piston is able to dissapate it's heat. The majority leaves through the rings into the cylinderwall. The top land of an average forged piston is .050 smaller than the bore and the bottom of a piston is abot .004 smaller than the bore. The gradiant of heat loss in the pistons is how we determine taper of the skirt and the land diameters.
rsz288 brought up an excellent point that I wasn't going to get into. The slotted valve pockets are weaker even when we design a forging to accomodate it. Having the truss between the valve pockets (or at least a bit of a radius'd peak coming between the two pockets) provides addtional strength.
Another response was speaking of where the piston is able to dissapate it's heat. The majority leaves through the rings into the cylinderwall. The top land of an average forged piston is .050 smaller than the bore and the bottom of a piston is abot .004 smaller than the bore. The gradiant of heat loss in the pistons is how we determine taper of the skirt and the land diameters.
rsz288 brought up an excellent point that I wasn't going to get into. The slotted valve pockets are weaker even when we design a forging to accomodate it. Having the truss between the valve pockets (or at least a bit of a radius'd peak coming between the two pockets) provides addtional strength.
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Teching In
Joined: Feb 2010
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OK, here's the latest development. Upon honing the block to get it ready for a new set of pistons a small crack in one of the cylinder sleeves was noticed. To determine the extent of the damage, the sleeve was bored and then removed revealing about a 2 and 1/2 inch crack in the parent material behind the sleeve. The damaged sleeve was in a different cylinder than the badly cracked pictured piston although, again, several pistons showed signs of starting to crack.
The question that I would ask those that have been following this thread is, does this change anyone's opinion as to what the root cause of the problem might have been? In other words, is "heat fatigue" of the pistons still a viable cause of the failure and, if so, could this somehow translate into a cracked cylinder sleeve?
Thanks for all the great input.
The question that I would ask those that have been following this thread is, does this change anyone's opinion as to what the root cause of the problem might have been? In other words, is "heat fatigue" of the pistons still a viable cause of the failure and, if so, could this somehow translate into a cracked cylinder sleeve?
Thanks for all the great input.
I use my car at the track a lot and it has run quite hot before. You are making me wonder if I should change pistons periodically? I do not have an answer to your question. I was thinking of how thick the pistons are in diesel engines.
cracked sleeve and block If there was no mechanical force put on the cylinder wall as in a dropped valve, the sleeve and block wall broke due to detonation in that particular cylinder. I usually see this in cylinder 7 with 5 being the next to crack. I had one a couple of weeks ago (blower motor) with a cracked cylinder 2 which was rather unusual.
That block can be welded and put back into service. Don't junk it, it will be much better than new with Darton Seal Tight dry liners installed. I have been repairing two to three LS7 blocks a month on average. Another leaving here tomorrow.
Steve
That block can be welded and put back into service. Don't junk it, it will be much better than new with Darton Seal Tight dry liners installed. I have been repairing two to three LS7 blocks a month on average. Another leaving here tomorrow.
Steve
__________________
Steve Demirjian
Race Engine Development
Oceanside, Ca.
760-630-0450
web: www.raceenginedevelopment.com/
e-mail: race-engine-development@***.net
Steve Demirjian
Race Engine Development
Oceanside, Ca.
760-630-0450
web: www.raceenginedevelopment.com/
e-mail: race-engine-development@***.net
You cannot compare apples to apples with diesel and gas engines really. Esp a race style gasoline engine that is much smaller than a diesel. Gas engines do not use steel crown pistons to take the brunt of the abuse either.
Thread Starter
Teching In
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If there was no mechanical force put on the cylinder wall as in a dropped valve, the sleeve and block wall broke due to detonation in that particular cylinder. I usually see this in cylinder 7 with 5 being the next to crack. I had one a couple of weeks ago (blower motor) with a cracked cylinder 2 which was rather unusual.
That block can be welded and put back into service. Don't junk it, it will be much better than new with Darton Seal Tight dry liners installed. I have been repairing two to three LS7 blocks a month on average. Another leaving here tomorrow.
Steve
That block can be welded and put back into service. Don't junk it, it will be much better than new with Darton Seal Tight dry liners installed. I have been repairing two to three LS7 blocks a month on average. Another leaving here tomorrow.
Steve
you can most definately check the bearings for detonation. Ti rods will shock the bearing(and the piston by the pin) more than a steel rod by it being stiffer. N20 users sometimes utilize Al rods to help soften the blow to the bearings and main webbing as they give more cushion between the piston and journals
imo you would usually pound a bearing out before losing a piston in a N/A app
imo you would usually pound a bearing out before losing a piston in a N/A app
Thread Starter
Teching In
Joined: Feb 2010
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you can most definately check the bearings for detonation. Ti rods will shock the bearing(and the piston by the pin) more than a steel rod by it being stiffer. N20 users sometimes utilize Al rods to help soften the blow to the bearings and main webbing as they give more cushion between the piston and journals
imo you would usually pound a bearing out before losing a piston in a N/A app
imo you would usually pound a bearing out before losing a piston in a N/A app
i feel the knock sensors are a crap shoot as far as getting evidence of a problem but sometimes when your not in your engine its one of the few things you can go on. since you tore it down you can do a proper investigation as the bearings usually receive the brunt of the punishment of pre-detonation. everyone here saying that the Al speckels on your plugs means your doing damage to your pistons but it really means if your tearing apart your piston surface imagine what your doing to your bearings and possibly burn a hole in your piston if your waaaaay off and you must really not check your plugs at all.
http://www.streetrodstuff.com/Articl...ne/Detonation/
http://www.streetrodstuff.com/Articl...ion/Page_6.php
Food for thought...pre-ignition not the same thing as detonation.
http://www.streetrodstuff.com/Articl...ion/Page_6.php
Food for thought...pre-ignition not the same thing as detonation.
TECH Regular
Joined: Dec 2004
Posts: 440
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From: Alpharetta, Ga
GTII, can you post what the car runs at WOT for both A/F and timing through the rpm range? LS7's don't like or need a ton of advance up high even though they won't show knock. Many have taken out some pistons by running too much advance up high under the theory that if the knock sensor's aren't tripped, they are golden. That isn't always the case and the added heat is detremental to longevity and in many cases, power as well.