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15 psi on a stock compression 6.5 = piston crack

4K views 34 replies 16 participants last post by  quantum mechanic 
#1 · (Edited)
This is my second crack, wish I knew which brand, as these went in 200,000 miles ago, it could be any, but suspect they were purchased from GM.

I have seen as high as 21 psi at the manifold, but never higher than 15 psi since I cracked the first piston 35,000 miles ago and installed the IC. I'm now of the opinion that anything higher than 12 psi on a stock 21.3:1 compression engine is trouble.
 

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#4 · (Edited)
Nothing happened to the block, I'll find one piston of the same bore and hone it for new rings.

Long story short I'm not putting my 599 std. bore back in, I've already bolted in a .030" overbore 599 with the heads modified for lower compression.
 
#13 ·
Long story short I'm not putting my 599 std. bore back in, I've already bolted in a .030" overbore 599 with the heads modified for lower compression.
What did you do to the heads to lower compression?
 
#7 ·
#6 ·
highest EGT ever is 950*F. Bearings look good for 200K.
 
#8 ·
Great. That makes me feel good.:D I hauled about #4500 to Phoenix last night and brought about #8500 pounds back with me. On the flats I was seeing 10psi with about 700-750Deg. But in the Superior mountains, which has a long unforgiving steep grade, which lasts about 25 miles, I was runnung 15-18psi and would back off the throttle just a hair when I hit 1250deg. She was most happy at 15psi and 1100deg.....but all she could do was only 45-50MPH. However, that speed is acceptable for 8500# plus.

QM, are you running a IC? I would think that one would be running on the line with these #'s I posted and maybe, just maybe, the IC would be the deciding factor? If you are running a IC, I may be asking for trouble pushing my truck to the limit while I pull. The temps were in the 60's....so there would be no way I would pull this kinda load in the summer without an IC. Good post QM....I was just contemplating these things on my trip last night.

Or, am I missing the whole point here? Doesnt matter if its a cooler 15psi...its just straight 15psi is what these pistons cant handle?
 
#9 ·
My bad...you are running a IC. I missed that at first.
 
#10 ·
:eek: :eek: :eek: Censored

I guess I will be doing some adjusting to the turbo master after work. That really sucks.
 
#11 ·
:eek: :eek: :eek: Censored

I guess I will be doing some adjusting to the turbo master after work. That really sucks.
Thats the thing though....If I adjust my TM any for less boost, my egts Hit the roof.

I guess time will tell for me.
 
#14 · (Edited)
I removed material from the precup and combustion chamber toward the valves. Nothing too serious, maybe a point of compression lower.
 
#15 ·
This is my second crack, wish I knew which brand, as these went in 200,000 miles ago, it could be any, but suspect they were purchased from GM.

I have seen as high as 21 psi at the manifold, but never higher than 15 psi since I cracked the first piston 35,000 miles ago and installed the IC. I'm now of the opinion that anything higher than 12 psi on a stock 21.3:1 compression engine is trouble.
I assume this came as no real surprise, did it? Were you experimenting with boost or simply seeing what a stocker can handle?
 
#16 ·
Not a big surprise, but I didn't suspect a piston crack 'till I saw it.
 
#18 ·
Last time I pulled a trailer with a tractor on it and implements, I was seeing what EGT's would do holding a sustained 15psi. Never went over 950*F. This might have been the beginning of the crack. Last piston crack was obious, this one smaller and subtle.
 
#20 ·
Pre or Post?

Guys, do we have a convention here on this list? EGT probe location is PRE unless otherwise stated? Just curious, several folks mentioning the famous EGT's on this string and I cannot tell who's measuring where. Thanks!
 
#21 ·
Guys, do we have a convention here on this list? EGT probe location is PRE unless otherwise stated? Just curious, several folks mentioning the famous EGT's on this string and I cannot tell who's measuring where. Thanks!
My pyro is post turbo. My boost probe is 3" from the end of the inlet on the upper intake.
 
#22 ·
Ditto, post turbo for me. Next time will be in the manifold though.
 
#23 ·
Makes me worried, I am pushing 25 psi all day long, when i am speeding at 75 mph, i am at 11-12 psi of boost for miles.

But, that is low compression and specially coated pistons I guess.
 
#30 ·
Why so much boost running empty, I'm making same 75 mph with 5 psi boost running empty, I only go to high boost when towing (I self limit to 12 psi max there as I don't need to drag my trailer above 70 mph) and 10 psi boost cruises me to 100 mph running empty 12psi boost for 115 but then I run out of fuel from IP delivery, or if I don't watch carefully run the fuel tank dry; I've never run the numbers I'm guessing about 6-7 mpg at that speed, it does go quick
 
#24 ·
Quantum,

If you look very close at your other pistons, you will probably find some cracks in them too. Cracks seem to start at the upper side of the wrist pin and then work their way up. When I cracked my first piston back in 96, I inspected all the others and found 5 cracked in all, but the others hadn't cracked through the crown yet, just above the wrist pin. I must have missed one however, because it let go 50,000 miles later and scored the block.

I think the pistons cracking is a combination of high egts, high boost and timing advance(too much)
 
#25 ·
I am pre turbo also. My boost is dead center in the intake.

Camilleri, I run a 1.94TDC with my timing as per Heaths recommendations. I also run 15-18psi @ 1000-1100Deg in short durations. On flat ground, I run 10psi at 800-850deg. In your opinion, do you think I have a recipe for disaster?

Also....If I adjust my TurboMaster back even 1/2 inch, It results in 1250+ in EGT's @ 10psi.
 
#26 ·
I was abusing mine when it let go, I was towing a 13,000 lb gooseneck trailer that I built with my Jeep and living quarters up a steep forest service road and it pulled great in granny, but I wanted more so I kept hitting 2nd gear and mashing the throttle. Too high of boost didn't do in my pistons, it was too high of egt's. I hit 1000 post turbo and the damage didn't show up until two days later with very little load on it.
So my only advice is to limit egt's to 1100 max pre turbo and remember that being the first one up the hill can be expensive:eek:
 
#27 ·
How long did you hold that kind of EGT temp? Or I guess what I am asking is, how long does it take in your opinion, to do irreversable damage due to high EGT's?
 
#28 ·
I didn't think I held the high temps that long so who knows. This is why I say, pyro pre turbo only to rule out the guess work, use only high quality pistons ( I had a set of Silvolites crack and grenade with only 50,000 miles) they were no where near the quality of oem.:(
 
#29 ·
My pyro tap is pre-turbine.
 
#31 ·
WOW TD what gear you got in that thing. I run 68 MPH at 2300 RPMs and about 8-9 LBs boost. My will run up to 16-18 LBs before it cuts back. I don't let it go there. I turn almost 2800 RPMs at 80MPH. and I think 90 is around 3000 or so. I don't run that fast in this truck if I can avoid it.

OH welcome back TD
 
#32 ·
3:73 just like in the signature, IC is MANDANTORY for boost above 10 psi IMO so far so good with mine on longevity front, even with the IC I self limit to no more then 12 psi boost sustained, unless I'm getting squirrely running empty.

Opinions vary about how higher boost hepls on EGT, it may, but there is more than EGT to be concerned with than just the coolness of the gasses in the chamber.

Prior life abuse for instance, I have high confidence mine was in nearly new condition when I bought it at 37K, truck had never been off road nor abused it was then in factory new condition other than miles, my 1st mod was gauges, and exhaust and I've lived to limits of 12 psi sustained boost level, and if post turbo egt gets around 900 I pull back and go a little slower, (it never goes over 850 running empty BTW even up to 115mph on the occasional journey up there).

I'm not sure of status of this particular vehicle but when QM 1st showed, he was running hard with no gauges, and really working the vehicle, he has made some changes since then, but if memory serves me he hasn't done a complete bottom up overhaul with new parts, I suspect there have been some of the "used but good parts" in the previous repairs which may have had some stresses in them. Even in this particular repair he is looking for a suitable single piston to effect a repair.

This is fine if on a budget, but one has to remember as D CAM points out as well; the rest of the train may have cracks, the ONLY way to KNOW balance of parts are safe for use are zero time replace or eddy current or some other form of detailed NDT (non destructive test) of components; visual or FPI may be good enough for a quasi rebuild for operation at stock performance levels but that would be my limit to pushing it.

I no way would I run "used but good" parts at performance levels QM has been running at, even a 12 psi boost I'd be wondering when it was fixing to grenade.

I'm in a quandry myself, I have 110K on the engine also a set of new 18:1 Mahles on the shelf hollering at me to be installed (these are Diesel Direct shave tops) I don't think they are "at risk" the way I drive (someday I'll install them just haven't made up my mind when), there may be a concern with the "shave tops" I guess some day I'll be the "lab rat" for those maybe I'll pull the working 21:1s shelve them and run the "shave top 18's".
 
#33 ·
Good points TD,

Something I've considered is boring this block for a set of 8 pistons I have that are .010" over and having the machinst do the critical work like boring, honing and rebushing rods. I have a tight budget but I intend to see the engine work right and start with a clean block and new bearings, freeze plugs, oil pump, ect.

I replaced the one piston 35,000 miles back because I didn't have the time to wait for the next level of work to be done. as of now, I have bolted a fresh block in and should have it runnig this weekend.
 
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