Cranks Between Ring Lands

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SRS_Chris
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Cranks Between Ring Lands

Post by SRS_Chris »

I pulled the pistons out of a 3.25 stroke x 4.030 bore SBF today. One of the pistons had cracking between the top and second ring grooves. The top land looked ok. There also appears to be cracking between the second and oil ring grooves. They are JE 4032 material pistons. Application is a mildy boosted (5-8 psi) street car on pump fuel. Compression ratio is fairly stout for a boosted deal at 10.5:1. Would detonation cause this?

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Re: Cranks Between Ring Lands

Post by SRS_Chris »

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Re: Cranks Between Ring Lands

Post by user-23911 »

Of course, it happens all the time. Lost count of the number of broken pistons I've seen.

You need knock detection on boosted engines.
At 10.5 to 1 what do you expect?
It'll do it at 8 to 1 if the tune isn't right or if you just can't supply enough fuel to it.

When it's really bad you get them ALL broken between the 1st and 2nd rings AND the 2nd ring and oil ring. That's when the dipstick pops out when you stomp on it.




Oh and it's "cracks" not "cranks"
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Re: Cranks Between Ring Lands

Post by SRS_Chris »

joe 90 wrote: Mon Feb 26, 2018 11:33 pm At 10.5 to 1 what do you expect?
I didn't build the original engine, so I'm not sure what the thought process was. However, I suspect the blower was an afterthought.
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Re: Cranks Between Ring Lands

Post by Geoff2 »

Ring gap too tight.
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Re: Cranks Between Ring Lands

Post by kimosabi »

10.5 compression on a pump fuel 8psi boost is madness. That's 100 octane territory.
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Re: Cranks Between Ring Lands

Post by Tuner »

There should be other obvious signs of detonation, such as piston top erosion (sand blasted look) or erosion of the side of the top ring land, pepper on sparkplugs, etc. if that were the case. That looks to me like it was hydraulic locked. Stuck injector? Flooding carb? Water got in it somehow? Check that rod for being straight and square.
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Re: Cranks Between Ring Lands

Post by user-23911 »

Nothing at all to do with ring gap, I've heard that one so many times.

Stock JZ engines can make 1000HP
As can RBs, 6Gs and lots of other engines when tuned properly

If they don't detonate they don't break.

If they DO detonate you can save them by giving them silly big ring gaps that give lots of blowby and make lots of other problems like having to fit a catch can.
But why would you let it detonate?

So?

No detonation?
Or detonation with big ring gaps?

That's what knock sensors and electronics are for.
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Re: Cranks Between Ring Lands

Post by 4vpc »

You sure that's a forged piston?
There is no S on the end of RPM.
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Re: Cranks Between Ring Lands

Post by quickd100 »

Detonation, I ve got several sets that are worse than that. I underfused the circuit my methanol injection was tied into. The fuse went at around 18lbs of boost, the fuel was 87 octane.
[img]http://img.photobucket.com/albums/v514/quickd100/9ff3c690.jpg[/img]
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Re: Cranks Between Ring Lands

Post by 4vpc »

Have you guys seen forged pistons with cracks in like these? I've only ever seen it in cast.
Forged ones i've seen just seem to wilt and deform, but not crack.
There is no S on the end of RPM.
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Re: Cranks Between Ring Lands

Post by SRS_Chris »

4vpc wrote: Tue Feb 27, 2018 4:55 am You sure that's a forged piston?
Yes. JE #206066
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Re: Cranks Between Ring Lands

Post by SRS_Chris »

Tuner wrote: Tue Feb 27, 2018 4:27 am There should be other obvious signs of detonation, such as piston top erosion (sand blasted look) or erosion of the side of the top ring land, pepper on sparkplugs, etc. if that were the case. That looks to me like it was hydraulic locked. Stuck injector? Flooding carb? Water got in it somehow? Check that rod for being straight and square.
That's what made me scratch my head. The tops and chambers don't show any obvious signs of detonation. The tune looks extremely rich though in all holes. I didn't see the plugs as the engine was brought to me without any. I will take a closer look at the rod and bearing shells to see if I can see any obvious connecting rod folding.
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Re: Cranks Between Ring Lands

Post by piston guy »

SRP uses 4032 alloy on that piston and 4032 is more brittle than 2618. The higher cylinder pressures caused by the supercharger are simply too much for the 4032 material. The staining also looks like the rings lost seal so even a perfect tune up would detonate and crack a land. A switch to 3618 material should eliminate the problem. Fairly common.
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Re: Cranks Between Ring Lands

Post by stealth »

SRS_Chris wrote: Tue Feb 27, 2018 10:33 am
Tuner wrote: Tue Feb 27, 2018 4:27 am There should be other obvious signs of detonation, such as piston top erosion (sand blasted look) or erosion of the side of the top ring land, pepper on sparkplugs, etc. if that were the case. That looks to me like it was hydraulic locked. Stuck injector? Flooding carb? Water got in it somehow? Check that rod for being straight and square.
That's what made me scratch my head. The tops and chambers don't show any obvious signs of detonation. The tune looks extremely rich though in all holes. I didn't see the plugs as the engine was brought to me without any. I will take a closer look at the rod and bearing shells to see if I can see any obvious connecting rod folding.
This is something I've seen with nitrous engines when the tune up is way too rich. The fuel gets down into the ring groove and detonates down in the groove. I'd bet the rich condition and pump gas did this in. Did the second ring look like it was butting together?
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