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-   -   LBZ: Another broken piston. What's the cause ? (https://www.duramaxdiesels.com/forum/showthread.php?t=76059)

Dvierra86 04-18-2017 08:24 PM

Another broken piston. What's the cause ?
 
So my friend has a lbz dmax with 120k miles on engine and broke and cracked #5 while running 22psi on a 4094stg2 turbo. I took that same turbo and installed it on my lbz dmax with 225k miles and would see 36psi and broke my #3 piston within days. Both trucks are tuned by different people. What actually causes the pistons to break cylinder pressures ? And caused by too much boost or fuel timing ?

I want to replace my stock ones with the mahle performance cast pistons while still retaining the 4094stg2 turbo on my lbz but I don't want it to break a piston again. What's you guys/girls thoughts upon the mahle cast pistons. What kinda hp have you personally seen them handle without failure ?


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gmduramax 04-18-2017 09:09 PM

Use fingers pistons and it'll be fine.

WVRigrat05 04-18-2017 09:12 PM

Remember how fords used to catch on fire with the cruise set??

Well duramaxs especially LBZ/LMM's crack Pistons.

But yes, high timing, too much pulse width, boost and mainly heat that's caused from all those plays a big role in them lasting or not lasting.

Takes more timing and pulse width to work that 4094 with stock injectors so it's doesn't take long to pop a piston. Not to mention 4094's are known to have a little higher drive pressure.

c5fourj 04-18-2017 09:35 PM

Quote:

Originally Posted by WVRigrat05 (Post 1060444)
Remember how fords used to catch on fire with the cruise set??

Well duramaxs especially LBZ/LMM's crack Pistons.

But yes, high timing, too much pulse width, boost and mainly heat that's caused from all those plays a big role in them lasting or not lasting.

Takes more timing and pulse width to work that 4094 with stock injectors so it's doesn't take long to pop a piston. Not to mention 4094's are known to have a little higher drive pressure.

So, would you say lowering drive pressure would allow the pistons to last longer? Like with compounds?
Sorry to the op for hijacking...

WVRigrat05 04-18-2017 09:49 PM

I'm on the theory with many others that more air than fuel can help save it but nothing is a cure all. And tuning always plays just as big a role as to where it start making power.

Dvierra86 04-18-2017 10:16 PM

Quote:

Originally Posted by gmduramax (Post 1060443)
Use fingers pistons and it'll be fine.



That is out of my wallet. They seem fancy but I don't plan on breaking 700whp if even 600whp


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Dvierra86 04-18-2017 10:21 PM

Quote:

Originally Posted by WVRigrat05 (Post 1060444)
Remember how fords used to catch on fire with the cruise set??

Well duramaxs especially LBZ/LMM's crack Pistons.

But yes, high timing, too much pulse width, boost and mainly heat that's caused from all those plays a big role in them lasting or not lasting.

Takes more timing and pulse width to work that 4094 with stock injectors so it's doesn't take long to pop a piston. Not to mention 4094's are known to have a little higher drive pressure.



What causes the higher drive pressure ? Larger turbine ? I've got the Ppe manifolds and up pipes and port matched the ex housing of the 4094 to help eliminate any restriction possible and I was only in the throttle from stop to mid 3rd gear egts weren't even about 1300 st the time it went boom although I've seen1800 with my stock turbo month before. Bottom line I'm asking is the mahle perf cast pistons gonna pop by running the turbo and atp tuning ?



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ikeG 04-19-2017 03:19 AM

Mahle Motorsports cast Pistons will take serious abuse, let alone fingers' design. When people say they have a 'built' motor, a large majority of them are running one of these two pistons

Dvierra86 04-19-2017 06:00 PM

1 Attachment(s)
Attachment 45650

Just took a peek into my #3 injector bore and found this little guy, in hoping being just a hole and not cracked pistons the cylinder walls are good and I can slap some fingers pistons in and be done with it


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ikeG 04-19-2017 06:10 PM

The last one i fixed burnt a hole like that. Siezed the wrist pin in the rod bushing and spun it around. It also welded the wrist pin bushings in the piston to the pin itself. Cylinder required .040 overbore and obviously new pistons and 1 new rod.

Not all that frequent so hopefully not what you're dealing with. I'd be sure and get the injectors tested while it's down.

WolfLLY 04-19-2017 06:25 PM

The pulse width of stock injectors is the real culprit imo and from who I've talked to and what I've read. The problem is: stock injectors are too small to spray fuel in the optimal window (time frame) so you end up chasing the piston with fuel shot (think blow torch) piston gets too hot and weak to handle the cylinder pressure. Crack starts and forms a hole or cracks completely across and thru, thus losing compression to the crankcase.

Wikid 04-19-2017 06:26 PM

My LBZ had 194k miles on it when i bought it. It had stock turbo and injectors and the previous owner beat the crap out of it drag racing. I put an SDP twin kit with just the S475 on it, 60% over new exergy injectors and a mishimoto intercooler. I ran that for about 40k miles making just under 700hp with water meth. I had issues with rail pressure since the CP3 couldnt keep up which makes sense since it was stock and worn out. When my #8 pistion cracked I was at WOT. When I opened it up the piston was cracked and had 2 holes at the thinnest point. Now did the holes happen and then it crack or did it crack and then the holes happened, hard to say. With the twins we could get 60 PSI of boost and would see up to 85 PSI of drive pressure. From what Scott has mentioned that was about normal. High drive pressure can be hard on your pistons but its also good at lifting heads and poping gaskets. The manifolds help but I think the compounds are a better way to go to keep your DP down. But a good tuner can make a big difference also.

My cracked piston did the same thing as your 04jimc, we had to bore the block .020 over but i built mine with fingers, carrilos and all new internals.

WolfLLY, I also agree, Scott @SDP has mentioned to me that is if you are trying to reach a high power numbers and dont have the right size injector that keeping it open too long and chasing the piston can cause a lot of issues.

Dvierra86 04-19-2017 08:30 PM

Quote:

Originally Posted by WolfLLY (Post 1060614)
The pulse width of stock injectors is the real culprit imo and from who I've talked to and what I've read. The problem is: stock injectors are too small to spray fuel in the optimal window (time frame) so you end up chasing the piston with fuel shot (think blow torch) piston gets too hot and weak to handle the cylinder pressure. Crack starts and forms a hole or cracks completely across and thru, thus losing compression to the crankcase.



hmm. Makes sense I had no blowby at 220k ran a built trans 230 Ppei tune until 225 and notice very minimal blow by then put a 4094stg2 turbo with atp 230 tune and 3 days later boom. Egts were prob only 1200-1400 when popped piston and only 3rd gear. It doesn't appear to be cracked just a hole. And I'm hoping to run fingers pistons and ballance assembly and call it a day. I don't plan on doing compounds anytime soon so I'm hoping stock rods will hold up to that turbo and a 230tune. So being injector pw as the concern would 30% nozzles be fine or more like 60% nozzles and a retune. I do still have stock trans so it won't be hot rodded too much


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malibu795 04-19-2017 09:01 PM

Bad design IMO pistons are heavier than the LB7/LLY yet weaker over the wrist pin..
I've split piston with less than 300hp, and seen blocks with stock power split pistons..
Biggest thing I'd say is oil temp, hot metal is weaker than cold metal.
As the lbz/LMM pistons last almost 3x longer at 400 HP with an aux oil cooler vs no aux oil and less than 300hp
Between my two tow pigs
Either mahle race cast round bowl or fingers ovals can't go wrong with either

Bore n balance deck and main will set you back ~4500.00 in parts roughly if you hose LML head bolts, new bearings and gaskets
Obviously labor will increase that's drastically

oscyjack 04-20-2017 05:33 AM

Quote:

Originally Posted by Dvierra86 (Post 1060448)
That is out of my wallet. They seem fancy but I don't plan on breaking 700whp if even 600whp


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cheaper to do it right from the get go. Take shortcuts now and any savings will be gone next time you have to pull the engine. Not that there aren't other options to fingers


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