Cracked piston on high mileage stock bottom end
- Thread starter bison
- Start date
I wonder if it was cuased by excessive cly. pressure from running 30+ PSI and knowing you most likely alot of timing also
22 degree in 3rd is all it ever saw at over 27psi. It was more like 20 degrees advance at 33psi. No evidence of any detonation up top. The deck, head gasket and head have absolutely no carbon tracking from a compression leak. If this thing even rattled a little id expect it to throw the crank out the bottom immediately at that power level. It never had less than 110 octane and alky injection. 30+psi is nothing for that fuel combo. Im sure i could have gone to 40psi and not had any detonation. a/f was 10.6-10.8:1. Most likely was just tired from all the rpm and cylinder pressure and thats where it failed. Im not upset by any means. I never thought the block/caps would take that much abuse as long as it did. I could guess it had a combined time of at least 10 minutes of WOT at 600+hp. Most of which was in 3rd gear.
On the street. I run the same tune on the street as i do anywhere else. Never logged any detonation and never saw any evidence of it. If the piston cracked in the ring land area and there was carbon tracking all over the sealing surfaces id say detonation. But there is no evidence of that.
I'd love to see the underside of the piston to see what it looks like and see where the crack starts. Last one I saw had stress cracks where the wrist pin goes in the piston from what appeared to be over revving a stock unbalanced engine with heavy pistons. You my friend made our crack look mighty wimpy by comparison.
Im sure mine started out as a small crack underneath. It was fine one second then it dropped about 150hp instantly and had a lot of blowby. I got out of it immediately
Im referring to the RJC powerplate. I have heard but not confirmed that it may cause a lean condition on #1 with alky. I have not tested egt's on individual cylinders to be able to prove any of this though.
More pics
No evidence of detonation on the rod bearing in #1. No copper showing and the bearing insert fit snugly in the cap with wear tapering to nothing about .5inch from the parting line. Some wear to the outside edges of the bearing probably from the twisting of the crank which is very evident on #2 and #3 mains from the high polished look on them. The crank will take a polish from what ive seen so far. What is very interesting is the fact that there is very little metal transfer between the block and caps on #2 and #3. Its most prevalent on the left side of #2 but no where near what ive seen on engines in cars that were going 10 mph slower in the quarter. Break away torque was well over 110 ft/lbs on the mains so i knew the mains werent cracked. When they are it takes almost nothing to remove the bolt. Running the highest octane combined with meth with a good tune can be the only reasons. I was very conservative on the timing and a/f even with the good fuel.
No evidence of detonation on the rod bearing in #1. No copper showing and the bearing insert fit snugly in the cap with wear tapering to nothing about .5inch from the parting line. Some wear to the outside edges of the bearing probably from the twisting of the crank which is very evident on #2 and #3 mains from the high polished look on them. The crank will take a polish from what ive seen so far. What is very interesting is the fact that there is very little metal transfer between the block and caps on #2 and #3. Its most prevalent on the left side of #2 but no where near what ive seen on engines in cars that were going 10 mph slower in the quarter. Break away torque was well over 110 ft/lbs on the mains so i knew the mains werent cracked. When they are it takes almost nothing to remove the bolt. Running the highest octane combined with meth with a good tune can be the only reasons. I was very conservative on the timing and a/f even with the good fuel.
Attachments
It was only totally cracked through the side in the pic i posted in post #27. The other side it stopped before it passed through the oil rings. It could be hairline all the way through though.I'd love to see the underside of the piston to see what it looks like and see where the crack starts. Last one I saw had stress cracks where the wrist pin goes in the piston from what appeared to be over revving a stock unbalanced engine with heavy pistons. You my friend made our crack look mighty wimpy by comparison.
turbo nasty
Turbo Dojo / MNTR
- Joined
- Jul 19, 2001
These stock from Buick motors simply amaze me. As stated before you got your money out it for sure.
vacuum 6
Well-Known Member
- Joined
- Jan 6, 2006
There was no evidence of lean out here. No damage to the piston other than the crack. If it was lean it would melt the piston and there would have been aluminum splattered all over the combustion chamber. I ran 10.6-10.8 a/f and had no more than 3% correction at any time.
vacuum 6
Well-Known Member
- Joined
- Jan 6, 2006
Good to know.
I havent figured out how to manually focus on an object that close to be able to see it clearly. You can see it on the pics i posted but its faint and mostly on the left side of the engine.
Turbo1dr
9 Sec Wanna-Be
- Joined
- May 25, 2001
I've got one just like you! Car run mid to high 6's in the 8th mile at 28-30psi and it finally gave up after about 6 years of abuse. This also included street driving.
This is what happened after I got the forged pistons....stock rod...same engine running the same times. This engine had 580HP to the rear wheels.
This is what happened after I got the forged pistons....stock rod...same engine running the same times. This engine had 580HP to the rear wheels.
I've got one just like you! Car run mid to high 6's in the 8th mile at 28-30psi and it finally gave up after about 6 years of abuse. This also included street driving.
This is what happened after I got the forged pistons....stock rod...same engine running the same times. This engine had 580HP to the rear wheels.
Yours looks worse. lol. I went 108+ in the eighth at 3600+ running 28.5 psi. Had it at 32-33 for a few months and that happened. What cylinder was yours?
Looks like the pistons may actually be a limiting factor rather than the main caps/webbing when the engine hasnt been subjected to any detonation. Still probably a roll of the dice on what will fail at those power levels. Mine was over 700hp at the flywheel assuredly at 33psi. It was 670hp going nearly134 in the quarter at 28.5psi.
