Cracked piston club

[blksmok]

How many of you are like me, and a little concerned about the number of people who have had this happen.

What's your take on the cause? To me, it seems like EGT's have more to do with it than anything. It seems guys who have joined the cracked piston club have had instances of EXTREME EGT's or a very high number of heat cycles. How much does cylinder pressure have to do with it? Is the pressure in the cylinder alone enough to crack a piston or do you have to add high EGT's or metal fatigue due to heat cycling?

I'd love to hear thoughts on this one...

[saratoga]

I don't have an opinion on what exactly causes it, but that's one of the reasons I'm too paranoid to run a programmer after seeing guys with $300 programmers have this problem.

[Fingers]

I've been measuring and logging the pressures in my truck and recently have made up rigs for other to do the same. Though I max out as ~2800 PSI, I have now seen well in excess of 3000 PSI on other trucks.

That compares to the ~1600 PSI that we see with stock tunes/engines.

I am sure EGTs play a role, but I am leaning towards it being more the affects that a hot chamber has on the effective timing.

[blksmok]

I'm assuming you are meaning that you are seeing the spike in cylinder pressure at or before TDC, when the engine is running hotter?

[Fingers]

Max cylinder pressure is after TDC. Here is an example plot from a Dyno run at Danville.


http://dmaxcentral.com/forum/attachment.php?attachmentid=20&stc=1&d=1211180655

[blksmok]

That chart doesn't look bad to me. If I'm reading this correctly, the first green line is pilot injection, then the wide green is main injection. So main shuts off 5° ATDC which I think I read a post of yours earlier stating that is good. Then peak cylinder pressure happens 25° ATDC. This isn't good? Or are you saying that even with the same timing, the peak cylinder pressure moves closer to TDC as heatsoak increases?

[Fingers]

Correct on the read of the chart. The peak is late. It should be closer to 15° ATDC to be optimal. The trick is that the closer the peak gets to TDC the more sensitive to timing changes it becomes. Change the timing 5° when the peak is 25° ATDC and it will probably only move 5°. However, if the peak was say at 15° ATDC, a 5° move could shift the peak 10°. That is, it gets twitchy the closer to TDC you get.

Heat soak is another problem. A hot chamber has the same affect as increasing the timing. Our ECMs do no EGT monitoring to account for the transition from cold to hot chamber.

Did I mention boost? Yea, that affects the peak timing also. More boost moves the peak to the left. Less to the right. So while spinning the rollers, a laggy turbo will lull you into a lot of timing, but in the real world when the tubo is totally spun up, the peak pressure will have crossed into the ludicrous zone because of the effective timing change.

Fun stuff.

[blksmok]

Good info. Timing is the one thing I'm trying to get a good handle on with my tunes. I had JoshH do some special mods to his timing calculator for me, to incorporate some of your findings. (having the main pulse finish 5° ATDC, but it gave me some pretty scary timing numbers... I'm still working with it.

Thanks for sharing here. :thumb:

[Fingers]

The pulse end is much less important than the beginning. The 5° thought was a brain fart on my part that didn't pan out. Duramaxtuner has posted a couple charts elsewhere to that affect. Unfortunately, he had all the functional prototypes recently while I waited for the next shipment of components to come in. (they are here now!!) So he has gotten to play with this a bit more.

Need to get Nick (Duramaxtuner ) to stop in here. ;)

[Nick]

I'm here, I like it... .

I think I'll stay :)


It seems like when the peak hits around ~10*atdc the truck sounds good, is making good power, and keeping EGT's in check. I can be more general if you'd like ;)

These graphs should give you a better idea of which timing calculator to chose for your max torque area. It's my opinion that one size (percentage) timing calculator does not fit all areas of the map. Boost and differences in SOI (start of injection) to TDC (in microseconds) with respect to RPM play in significantly.

60% calculator at Max torque
http://i105.photobucket.com/albums/m201/vortecfcar/CPT_60_2575RPM.jpg

40% calculator at Max torque
http://i105.photobucket.com/albums/m201/vortecfcar/CPT40_2560RPM.jpg

20% calculator at Max torque
http://i105.photobucket.com/albums/m201/vortecfcar/CPT_20_2535RPM.jpg

Your max torque RPM may vary, depending on your tuning style. Mine is for comparison purposes only. :)

Notice the 'torque factor'number is pretty similar in all of these graphs despite the wide variety of timing. I think it's pretty safe to say that two tunes making similar power can have very different impacts on the life of your internals. I plan to back up this torque factor with Superflow numbers later next week.

Who wants to try the 70% calculator,

Nick

ps- raise your hand if you just went and backed your timing down...

[blksmok]

Not raising my hand yet because I already had the timing backed down quite a bit from what others were running... but I'll definately be double checking it again... :D Thanks for the great info!

[Fingers]

Gotta love them there fancy graphs.
:D

[Fingers]

I am testing a revision of the software today that should greatly improve the frame rate for the chamber pressure monitor. I am varying the sample rate based on RPM. The trick was getting it to be backward compatible with the existing logs. Nick should have it by this weekend if all goes well

[Nick]

sweet, I really want to catch this thing right at the shift if I can. I'm just not that quick on the space bar.

[McRat]

My engineers believe pistons crack because I have not personally blessed them.

So we are working on a program where we can send a hologram of me anywhere in the country to bless pistons on site.

[Nick]

hahhahha......

I needed that.

[RickDLance]

I know it's confusing, but the correct names have now been assigned to their posts.

[neversatisfied]

:Get_him: Sure is crazy

[Bulldogger]

I read through all the posts on piston cracking in other sites. From what I recall there was no consistency to what caused it. Some pistons survive high horsepower conditions and others died in stock trucks. This would cause me to think more along the lines of quality control, either impurity in the piston materials during casting or improper tolerences when assembling the piston, like forcing in the bushing and stressing the structure. Certain factors like EGT's cylinder pressure and timing definitely are factors, but there are guys that bent rods and their pistons didn't break. But hey I'm not smart enough to do EFI live myself so I had kennedy diesel tune my truck.

[Nick]

There was another LBZ piston failure recently. The engine had 33k on it and was beat to hell it's whole life with a GT4094 and stock CP3. Shortly before it left this world, a modded regulator and new tune were added. It died on the shift, as it was pulled down to ~2400RPM, boost spike, low 20's main timing, egts, and full fuel pressure.

This is not the first one to die in this 'perfect storm'. hmmm...

[hondarider552]

dave and fingers;
there is a guy on the "other site" who just cracked his piston in the lbz on june 1st. interesting read..its in "425 ppe and cp3 mod" thread

[SteveFord]

Maybe this is the wrong place to ask but does my ppehot+2e.t. run alot of timing? I know it dosent rattle at low rpm under a load since the upgrade to the e.t. version back in October. I'm about to install my tranny upgrades after next weeks vacation and now starting to wonder if I should go with efi for a better fine tune.

[Nick]

dave and fingers;
there is a guy on the "other site" who just cracked his piston in the lbz on june 1st. interesting read..its in "425 ppe and cp3 mod" thread

For what it's worth, I talked with Henry about his piston failure and he indicated to me that he was not running PPE tuning at the time of the failure. His tune was custom tune with the timing and fuel numbers I mentioned earlier in this thread (I did not write the tune).

PPE is pretty conservative on timing down low, but they do fuel aggressively. I'm not here to say which is better, just the facts.

Nick

[blksmok]

Nick, do you know at the time of failure, what was going on? Specifically was he doing something that was pushing the engine extrememly hard and what were the EGT's it was seeing at the time or has seen in the past? I'm still wondering if extreme EGT's play a role either prior to or at the time of the failure.

[RickDLance]

I don't think what you are doing exactly at the time of failure is always the cause. I think the history of the piston's abuse may come into play too.

[blksmok]

I don't think what you are doing exactly at the time of failure is always the cause. I think the history of the piston's abuse may come into play too.

Absolutely! My thoughts as well. I tend to think a history of high EGT's has to play a role here.

[Nick]

Post 20 in this thread is all the information I'm comfortable posting.:) Maybe we can get Henry to post more details.

Nick

[SteveFord]

Thanks for the info Nick.

[hondarider552]

i agree nick, id also like to hear what henry has to say.

[duratothemax]

It died on the shift, as it was pulled down to ~2400RPM, boost spike, low 20's main timing, egts, and full fuel pressure..

thats when mine let go. EGt's were barely 1400* IIRC (they were climbing though).

[subman]

Anything new to report here, I know of another one that went down this past week.:(

[blksmok]

Anything new to report here, I know of another one that went down this past week.:(

Any details?

[BoiseRob]

Subman,

His was far from stock except for the stock CP3...

Rob