piston failure

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jet1
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piston failure

Post by jet1 » Mon Aug 12, 2013 2:49 pm

I have a piston that failed on a lsx454 crate engine. AFR was 12.-2-12..0 timing 28 degrees , fuel was 100LL. It is a endurance application and usually runs at 5700 rpm at WOT.
My thoughts are slight deto over a long period of time caused it but I amlooking for more thoughts on this. There is 4 other pistons that are starting to crack in the same manner also.
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Nick Campagna
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Re: piston failure

Post by Nick Campagna » Mon Aug 12, 2013 3:01 pm

Looks more like fracture from thin piston than burn through. IF mixture and timing are as stated, and using av gas 100 octane low lead, I can't see detonation, but I could be way off.
Is the defect in what I see, or what I'm seeing with ?

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Re: piston failure

Post by jet1 » Mon Aug 12, 2013 3:07 pm

The fuel was actually 103.2 octane. water temp 130 f oil temp 280 f. You can't see it in the pic but there looks to be some slight sand blasting texture going on at the edge of the piston also. I hope it's just to thin as it is the piston we have to use in this class.

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Re: piston failure

Post by BigBlocksOnTop2 » Mon Aug 12, 2013 4:16 pm

Pretty clean piston...lots of heat.

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Re: piston failure

Post by Ron Clevenger » Mon Aug 12, 2013 4:19 pm

Another area you might consider is piston pin flex.

What do the spark plugs tell you?

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Re: piston failure

Post by jet1 » Mon Aug 12, 2013 5:06 pm

We had to run it for a while after the failure to limp back but we had looked at them the night before and they did look a little lean. We richened up the carb but maybe it was to late already.
BigBlocksOnTop2 wrote:Pretty clean piston...lots of heat.
Heat as in lean or please explain.

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Re: piston failure

Post by wil8115 » Mon Aug 12, 2013 5:56 pm

what does the others look like and got a pic of the underside of the crown?

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Re: piston failure

Post by MadBill » Mon Aug 12, 2013 6:11 pm

The pebbling at the edge looks like a bit of detonation and that definitely does not look like a piston running at a true 12:1 AFR. I've seen lots of much darker tops on road race Corvette pistons that have run up to an hour at a time at 13-13.2:1..

Is that the relevant wrist pin to the left, appearing a little cooked?

How do the bearings look, especially the rod uppers?

X2 a bottom view of the holed piston as well as a couple of the cracked ones.

How thick are the piston tops?

Any pinched rings?

You'd have mentioned it if you were running boosted, yes? :)

Presumably there is no knock sensor system on the engine?

Any idea what is peak torque RPM on this engine? That would probably not be a good range to run for endurance.

I don't know what MBT spark might be for that engine, but 28° sounds a little on the high side. You might do steady-state dyno testing and develop a spark table for a couple of percent retard from MBT.

If you're stuck with that piston p.n., thermal barrier coatings would be a cheap precaution. If they aren't approved, Swain for one has an invisible "stealth coating" available.

Piston squirters are more work but if legal would help substantially if other avenues prove to be dead ends.
Last edited by MadBill on Mon Aug 12, 2013 6:22 pm, edited 2 times in total.
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Re: piston failure

Post by lorax » Mon Aug 12, 2013 6:17 pm

Whats the under side of that look like. Cooked oil? What about the others?

jet1
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Re: piston failure

Post by jet1 » Mon Aug 12, 2013 6:43 pm

It is NA.
The piston thickness is approx .250 where it broke through.
Yes that is the wrist pin beside the piston.
Yes and the plugs looked leaner that the AFR was showing.(a side note this same thing happened to another competetor 20 min later in the same leg of the race with the same setup and showing the same AFR)
Funny you should mention the knock sensor. I had a J&S electronics setup and and sold it before I decided tio run this engine. (whoops)
bearings look like new both mains and rods. No other rings were pinched other thatn the holed piston, no scoring on the cylinder either.
Peak tq is at 4900 and we run them at 5700.
Yup stuck with that piston and not allowed to do any mods to the engine.
I agree the 28 degrees seems high now and I think you absoulutly right that we did not get a good spark table figured out. Looking back the altitude density was 5500 ft when we dynoed and all of our races have been at 2500 ft air density. 28 degrees timing made best power 29 dropped power slightly (prob some deto) and 27 was less also but obviously safer) Should have know that it may be ok in a drag race but for endurance surely it would push it into some deto. All I can do is replace the pistons and change the tune. just want to make sure that that will do it.
Here is bottommpic of the piston sorry no pics of the others right now.
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Re: piston failure

Post by Strange Magic » Mon Aug 12, 2013 6:59 pm

oil temp 280 f.
That should be a good indicator that something is not right, especially with 130 water temp.
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Re: piston failure

Post by BrazilianZ28Camaro » Mon Aug 12, 2013 8:30 pm

What compression ratio, cam, spark plugs?

Are you sure the damaged area is 1/4" thick?

Looks thinner by the pics :-k
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Re: piston failure

Post by needforspeed66gt » Mon Aug 12, 2013 8:59 pm

I've seen plenty of pistons cracked through the pin bosses and almost through the crown where the aluminum had gotten just hot enough to sag in the middle but not quite burn through yet, each time it was traced back to a lean condition due to tuning, vacuum leak, or in several cases an odd spark scatter thing on 5.0L ford motors due to spark plug wire routing.
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Re: piston failure

Post by wyrmrider » Mon Aug 12, 2013 9:06 pm

quench distance?
deck clearance ?
gasket thickness?

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Re: piston failure

Post by Dan Timberlake » Mon Aug 12, 2013 9:28 pm

The valve notches look different (less severe, smoother transition) in this picture.
http://image.popularhotrodding.com/f/te ... piston.jpg

Did you have to deepen the valve reliefs?

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