I would like to get some opinions of what might have broken the pistons on a LS7 motor which had around 40 hours total time. The engine was updated with 4032 forged pistons and a torquer cam. The engine was putting out around 610 horsepower at the flywheel. Although the car is tracked regularly the driver always shifts below redline, @ 6500 rpm. On a 45 degree day at Sebring, with 2 hours on the refreshed engine, smoke started coming out of the exhaust with oil pushing out of the vent system. Upon tear down, #5 piston was cracked in the valley of the valve relief area to the edge of the piston and down through the ring lands. Four other pistons had fractures on the under side of the wrist pin bores. If detonation caused this damage would there be signs on the tops of the pistons or spark plugs. The existing tune was 2.5 years old and considered fairly conservative. Should a good 4032 piston last more than 40 hours in a track environment? Could some kind of lean condition as a result of the unusually low outside temperature have some kind of effect on the engine? Top end and bottom end of engine are perfect. Any thoughts?

 

Maybe the tune was off when the temps were low. It is SD tune right? I had a problem w/ one of my tunes when it got cold. It turned out that not all of the tables were adjusted correctly.

 

That doesn't look like it was running lean. Did the pistons have to be cut for valve notches? Might of been a thin wall condition.

 

Per the look of the reliefs, they were already machined like that from the manuf.

Brand of piston?

 

You said 40 hrs total on the engine, and 2 hrs "refreshed". What was done to refresh? Maybe stuck rings from tight end gaps?

 

There has been a lot of chatter about the ability of the small skirted pistons to transfer the heat out of the piston at high horsepower engines with long run times.
With the small skirt area the piston cannot transfer enough heat and failure of the piston is the result.

Some people feel as though this is why you see squirters in many of the new high horsepower applications. The LSA and the LS9 both have piston oil squirters. With the deck height of 9.2 and a 4” stroke you can end up with skirts that are very small.
The less area is less ability to transfer heat from the piston. Also a tighter clearance should transfer heat more efficiently. With a forged piston and the additional clearance it may not transfer heat as well.
Since we don’t have access to the engineering we really don’t know where the limits are until this sort of stuff happens.

Just my thoughts, good luck!

Robin

 

That is what i thought when i saw it. Sometimes the design fails the piston.
They used to call the "slugs". Now they can just call them paper weights.

They did use the non 2618 material, so the PTB clearance should be less due to the better expansion rate of the piston.
The skirts are not coated though. I am curious to see who makes them. Sometimes there just isnt enough area under the lands to take the extra abuse i guess.

 

Once you step away from the original "design" all bets are off. Going from 505 (well really 530) at the crankshaft to 610 is going to intoduce a lot more heat to dissapate. While the block and the heads may be fine if the piston overheats it will lose it's metallurgical properties. At that point it will fail.

Robin

 

The owner went back to the tuner (LS2 EDIT) to review the tune and verified that everything is fine and that all knock sensors were turned on and low octane tables were active. No check engine lights came on and actually the engine was running well. In the middle photo, although not clear, you can see a small crack starting to appear next to the wrist pin bore inboard of the bottom of the skirt. four other pistons had these same fractures starting, but had no damage to the top surface. If a lean condition had existed, could this much damage have occurred without damage to the corresponding spark plug?

 

The engine was running fine, but after 40 hours of mostly track use, the owner thought it would be a good idea to refresh the engine. This consisted of rings, bearings and valve job. At this time, the pistons were Rockwell tested and determined that no replacement was necessary. None of the rings were broken or cylinder walls scored. I think this piston takes stock LS7 rings. I am in the process of getting piston manufacturer info. At the time of the refresh, the rebuilt engine was dynoed and was within a few horsepower of the original dyno results.

 

So if this was the problem would you expect to see piston failure almost immediately or is it more of a fatigue issue that may not surface until after 40 hours of road course use?

 

After many hours of running at extended RPM and Power levels.

With a street car or drag race engine they don't get the long cylcle times that a road race engine does.

The piston supplier is not the problem. It's heat, heat will kill the pistons over a long period. Figure out how to run the pistons cooler or transfer the heat better and you should see an improvement.

And it's only my opinon for what it's worth.

The road racing engine builders might have ideas also.

What does your builder say?

Did they Rockwell the pistons after they failed?

Robin

 

PM me if the block is salvageable and you want to sell.

 

Based on the damage, the builder is "leaning" towards detonation. He seems to think that this can occur without the more classic signs showing up on the tops of the pistons or spark plugs. I have even heard you can see signs of detonation on the bearings......Thats a great question about the Rockwell test I am going to look into that.

 

doesnt look like detonation to me... looks like the piston failed.... but that is just my .02 cents

 

I agree with some of the points here, but I want to hit on a few details from a piston makers point of view. First, that piston isn't a Wiseco, but this is still an important lesson that can strike any piston manufacturer and any racer that wants a "light" piston. "Light" needs to be defined because there is a HUGE difference between a light forging with a thick crown and a clunker forging design with a thin crown. The mass needs to be where the mass needs to be and no where else.

When it comes to endurance racing (long oval, marine, roadrace, and land speed); the number of cycles, the tune, ceramic heat barriers, and crown thickness are the key factors. Mother nature and the nature of racing only let us juggle these factors just so far.

2618 would have been better. It's method of failure is to "go out" like taffy rather than like shrapnel. You got very lucky with this 4032 piston that you caught the failure quickly before the piston grenaded. In fact, I'd be pretty suprised if it is 4032 based on the number of cracks you found. Fracture toughness is a factor that I've brought up before. Bill Miller put it best (or maybe someone before him): With 4032, when a crack does start, it shoots from silicon particle to silicon particle like lightning and the piston grenades. 2618 (no silicon) acts as a fuse and engine damage is minimized because a crack will travel to a low stress area and stop. It also has a better annealing curve than 4032. It doesn't soften as fast under the same conditions.

As I mentioned earlier, Raw gram weight is not the best comparitor unless it's done from brand to brand and forging to forging. The crown thickness is a better way to look at it if you don't have multiple engine builds for comparison. It's easily measured with relatively simple tools. Where the manufacturer drills the initial pin holes into the raw forging determines this crown thickness and how the rest of the net piston is formed. Identical looking pistons can be 50 grams different because of this factor and you won't know it unless you measure. A piston that is .150 thick will anneal much faster than one that is .250 thick. Yes, that added material adds another 21 grams to crown, but the lifetime can go from 1 season to several in a typical road race or circletrack application with a "good tune". Solid Domes also count toward crown thickness.

On the subject of rockwell, our pistons leave here in the high 70's and low 80's. When a customer sends a piston back, it's the first thing we look at. We've seen VERY lean engines (that failed within a short time) drop only 20 points before failure from extreme thermal shock. We've seen well tuned (slightly lean) engines that have run many years drop 50 points before failure-and yes we've seen pistons that don't register on the rockwell B scale at all

We prefer a strutted forging as well because the pin bosses under the center minimize the amount of unsupported crown area and heat can be wicked away better.

Other areas to look at:

1.) Big turbo dishes with short compression heights may have thin spots in the center if humped dishes aren't used to maintain material thickness over the connecting rods.

2.) Look in valve pocket corner radii and under flat valve pocket surfaces as areas where a piston manufacturer (should) compensate with an overall thicker deck to avoid thin spots in those areas.

3.) Thin slices of material left outside the valve pocket (from big valves and small bores) behave the same way in a nitrous engine and they lift lands sooner. It is better to grind these areas down with a die grinder so they don't tun into glow plugs and flake off.

A few helpful factors:

Oil Squirters are great....night and day difference in how fast a piston will anneal and/or seize.

Ceramic heat barriers can work very well. Our internal dyno testing has revealed an identical engine with identical tune (intentionally run very lean) resulted in a coated piston loosing a few rockwell hardness points versus an identical one that lost over 20 points with the same number of cycles. It wont always save you, but it does allow the engine to soldier on longer

Proper skirt cam/taper has more surface area touching the cylinder wall and more heat can be dissapated into it. Don't think (even for a second) that most piston makers do this correctly for every engine every time no matter their name or reputation.

Cheap rings aren't round, flat, and the RA finish is crap. They won't pull the heat out of a piston as quickly as a good ring will...especially one riding a nice flat well finished ring groove.

For the education of the forum members, it would be nice to see the A/F ratios, crown thickness, and Rockwell B if you can get that from your manufacturer.

 

I agree with most of what Robin is saying as road race is much harder on pistons than any street or drag since the duty cycle is higher. You have higher rpm and you are "on it" at WOT for way longer sometimes almost continuously at times.

I don't know however that that crack is from heat though. I see stuff like that on cast pistons here and there and on some 4032s but not usually on race 2618 pistons?

 

the crack is in near perfect line. And on the exhaust side, this seems like a heat issue. In factory setups we see the number 7 & 8 cyl with piston failure due to heat. Just a idea?

 

Is that a Mahle piston?

 

Unusual looking crown. Looking closely appears it may have started life with two seperate valve reliefs and extra material has been milled out between I & E. If the base forging is a design where it has the two valve reliefs reflected in the forging on the underside of the crown, then the additional material milled out is going across a very thin area of the crown.......right where the largest part of the crack is and has actually opened up into a "crevice".

What does the underside of the crown look like?