Sorry missed your post.
Yes it is 100LL av gas 103.2 octane
I would not say it is right below but is very close to it and is in the thinnest part of the piston.
leaning pre-ignition because of the av gas.
correct heat range of plugs for the heads?
maybe plug to hot helping the cause.
just wondering if you were getting steam pockets in the cyl head, thru cavitation with bad pump / ingress of air on the suction side / debris in the intake, this could give hot spots in the head that promote detonation.
also being tuned to 5500 ft ....then dropping to 2000 =/- will make the true comp rise and the mixture to run lean
MadBill wrote:As far as plug heat range (especially for endurance apps.), I'm a fan of running the coldest you can without major fouling issues.
PS: David, re "it is only a matter of time before a bullet hole (30.06) will develop in the crown of the piston just under the plug." The hole is actually far from the plug. Maybe in this case it's just the flow of combustion gases through the crack eroding a larger path.
The center of the piston is normally the hottest zone but this will vary with specific combustion chambers and burn path.
I guess I'm just used to multi-valve and hemi chambers.
Nonetheless, the emerging hot spot can be detected by piston crown hardness testing.
Sometimes, a simple coat of white paint can be very revealing of the burn pattern.
Awsome stuff guys . I think evryone is kinda hitting the nail on the head.
Ya my biggest thought was that we dynoed at 5500 ft air density and then ran at 2000+-. We adjusted jetting but not timing. GM has since told us that all there street strip testing was done at 27 degrees. Why they recomended 28 I have no Idea. Never the less should have known better.
I will figure out the heat range of the plug and maybe try some cooler ones also combined with 26 degrees timing and proper jetting verified by plugs not the AFR should do the trick.
We are not allowed to add piston oilers or do coatings of any sort.
Once again I am just amazed at the knowledge base here at Speed Talk. Thanks guys!!!!
Hey i'll give it a whirl.
The heat range of a spark plug is the operating temp that the tip achieves while under operation. The goal being to have enough heat in the plug to keep deposits from building up and fouling the plug but cool enough that it does not creat an alternate heat source to prematurely ignite the fual mixture.
A plug that is to cold may cause engine hesitasyion and of course missfires as the plug becomes fouled.
A plug that is to hot may cause hard hot stating and preignition or detonation.
Probably just the tip of the iceberg but I tried.lol
Jet1,
Aluminum cracks due to flex. Flex can be induced by heat related annealing of the piston due to temps aluminum temps beyond 650-700*s sometimes less. The posted piston shows lean ( imho) on the surface color ( assuming you haven't cleaned it for pictures.) and the shot of the underside discoloration. There should be some carbon build up when things are right A/F ratio wise. The pin appears to be very short 2.500 or less and unless the pin is less than .150 wall it should be fine. One other area that has become important on these "narrow pin boss designs" is the thickness of material under the wrist pin. If this area is too thin it will induce deck flex and fail the piston in a similar manner. I'm going for lean on the reason for this kill.
A racing jet boat is like an engine being on the dyno all the time it's running. This is a very punishing type of racing , component wise. Strange magic is right the recent oil temps are way too high for my comfort.
jet1 wrote:Thankyou points well taken. One of our issues was believing the AFR guage without really looking at the plugs. That will not happen again.
True, sometimes low tech beats high tech.
I also heard WOT boat racers should never use projected spark plugs, only non projected plugs, that no matter how "cold" a projected plug is, after so long under WOT it will start to glow and cause issues with pre-ignition
Spark plugs should not be read to determine AF ratio; that is the job of the lambda sensor. If the mixture is proper, a lambda sensor will remain accurate for hundreds of hours, even with leaded fuel.
Spark plug heat range ONLY refers to its ability to shed combustion heat through the center electrode in order to keep the electrode tip clean yet not overheated. Spark plug temperature is a function not only of heat but also of length of time to which it is exposed to high temperature. A hot plug can be used in an engine that only sees full power for short periods of time. Conversely, a very cold plug can be used in an engine that is run at full load for lengthy time periods.
Time and not just temperature will determine the proper heat range of a spark plug.
That said, we must recognize that heat ranges ONLY refer to center electrode or tip temperatures and do not relate in any way to the J strap, or ground electrode. The ground electrode can become very, very hot while the center electrode is still within the safety range. The correct procedure is to read a plug for temperature and not mixture.
If the ground strap shows evidence of excessive heat, then a plug with a shorter tip or even a recessed tip should be used.
For high performance engines (not just racing engines), a side electrode or surface gap plug should be used to prevent excessive tip temperature that can easily cause pre-ignition.
Ok but I can tell you the o2 sensor was reading 12.0-12.2 all the time and it is new. This engine and it's sister engine in another boat setup the same way did the same thing within 10 min. Both afr readings were in the 12.0- to 12.2 range. But yet the piston and plugs look lean. I can see an afr reading leaner from air leaks and such but not richer. That is why I was going to use the plugs to try and verify what the AFR was telling me.
jet1 wrote: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.
Just a wild thought. I believe with the valve guide issue in the LS7 there were running changes in the main and thrust bearings to try to perturb the amount of oil thrown up into the cylinders on initial start-up (to avoid the abrasive valve deposits during the super cooling phase of sodium valve operation leading to valve guide wear). However, in a race engine this decreases the amount of oil available to cool the underside of the pistons. All other things being held constant this could lead to the failures you are seeing.
GM appears to try to standardize bearing use (and lifters, for example). Warp Speed previously commented on the bearings seen in currently supplied LSX engines and they do not match photographic records of LSX builds in magazine articles from years prior.
It is my understanding that lead causes Lambda sensors to fail by plugging up the pores in the sensor's ceramic. First the sensor is slow to respond to changes in AFR and then it won't respond at all. The lead solidifies in the pores when the sensor is cold. Sensors last longer if they are removed during cold start and warm up.
