donzi matt

Taking your thought one step further, a Whipple stage 2 ECM upgrade requires going to a 120 degree thermostat. I do not know what that ends up running for actual water temperature, but I would assume it cools things down somewhat. So you drop water temperature, increase timing, increase horsepower, and increase max RPM. Doesn't sound like a great recipe for tight ring gaps. I have no idea how many stock engines with upgraded ECM's have lost a piston. All of this gives me plenty more to contemplate and lose sleep over. I'm not happy that Rage lost a brand new motor, but I think this thread has some really good information and discussion.

SB

Rambling on :

Without any basis of saying so, I would think a steady 120F is fine. What I would not do with any high silicone, or hyper cracking pistons with high rings is to get all over the throttle nor get up on high load before temps stabilize. Sounds like a no brainer, but with EFI, cold starts and cold higher throttle/load situtations the engines seem to run just fine, because they do run just fine. And that's what can get people into trouble.

How many of your car customers jump in, turn the key, and go ? All of them unless they have remote start. Cars are light load (unless I drive them - lol), but not boats.

Anyway, these 8.1's start off at decent horsepower with emissions designed parts....they where never designed for tugboats with one speed transmissions trying to plane on water and run at medium to high medium rpms under a ton of load. GM made these 'good enough' where Mercruiser/Volvo/Etc could use them in marine applications.

Remember, these aren't 260hp small blocks....they are (up to) 425hp (sounds like schit power but it's not - especially compared to other manufacturers engines and on top of that factory BBC's from eons ago that had better forged pistons including ring height...450hp factory motor meant you where 'the man !') and 496cids.

How many 502's broke ring lands without major tune up issues ? Almost none. How many people burnt a hole thru the 502's pistons and the rings and lands where okay , due to tuning and/or fuel issue like injector or such ? How many 600-800hp SC'd 502's are out there with factory shortblocks including pistons ? To put a guess on the # I'd say a schitload 1 lol.

donzi matt

Actually, this motor was designed with endurance and industrial use in mind:

If you are interested, this is a pretty good read on the history and developmental updates over the production run:

http://www.performancetrucks.net/for...1-07-a-523619/

donzi matt

Sorry for derailing your thread Rage, I have been geeking out on this motor since I bought my boat last fall.

SB

I've read it before , and I followed it long time ago. Marine on their list ? Sure....But further down on it. Yes, the industrial aspect was quite large. They could have used 'stronger' components, but they didn't exactly need to, for their uses and for their level of OE tuning and etc. Add to that, a stronger piston is usually softer, and wouldn't give the life expantancy in terms of cycles that a higher silicone piston will.

Anyhow - all good table talk. And I'm finally 'piston'd out' in this topic. Lolllll.

Keith Atlanta

It would be interesting to see his EGT's.

Darrin Morgan, Reher Morrison Racing Engines (about 496/8.1 heads)
The stock heads are JUNK with a capitol "J"! Don't even think about trying to step the stock heads up with port work. The castings are thin, the short side radius is way to high and the combustion chamber is a total joke. The stock version of these heads are the worst I have ever seen come out GM...

I know you did a significant amount of work on the ports but did you work the chambers? We are still trying to design a better piston but we are still dancing around the root cause. If the combustion chamber dynamics are significantly disturbed and the quench is all messed up you have a detonation issue somewhere that needs attention. These are truck heads that were designed for a completely different use. The combustion chamber is less than optimal. You can feed it fuel but the issue remains. Especially at lower RPM, if the pston failure was at a stress point beyond 5000RPM I can see getting something with a more advanced material. But at this RPM, cmon, it isnt a material issue its a detonation issue.

SB

HArd to read above, but I think everyone agrees on the detonation and/or ring butting.

Black Baja

No doubt it was detonation. Coupled with a poor piston design.

Rage

Gentlemen, many thanks to everyone for all the good info and insight shared. It now appears clear to me that the pistons broke because of the ring gap which was insufficient to handle the higher than normal combustion chamber temperatures from the detonation event which likely brought on by the drop in fuel pressure from set point and not the piston material. A 2618 alloy piston may not have broken but some form of damage to the piston I suspect would still have occurred. I also initially saw that sections of the cylinder cross hatch had been wiped smooth in cylinder #4 and not the others during a cursory check for damage. That would be consistent with the ring binding in the cylinder from excess thermal expansion. I plan to double check #2 for a similar condition.

Reviewing Mahle vs JE min ring gap recommendations for a 4.28 bore shows the following.

min ring gap
JE BB 4.2 -4.6 NA 0.030" - 0.035"
super charged, turbo, nitrous, endurance 0.031" - 0.038"

Mahle 4.28 HP street- NA 0.019"
circle track, drag - NA 0.021"
Nitrous to 200hp/ supercharged up to 15# 0.026"
Nitrous over 200hp/ supercharged over 15# 0.030"

I used the same 0.024 top ring gap on both the previous 625hp engine that ran well with no issues for 350 hrs on 87 octane and the current engine that broke the pistons with 91 octane. I obviously can not be sure but suspect that if the fuel pressure had not dropped below the set point the new engine would have not broken the pistons. Regardless, the new build will follow the JE ring gap recommendation and the rings will be the Total Seal gapless rings. I am also thinking that the 4032 alloy is no longer off the table for my engine but probably not from Mahle.

Hearing about all the history of the Mahle pistons being prone to the type of breakage I experienced and seeing the Mahle vs JE ring gap recommendations it makes one wonder if the Mahle ring gap recommendation is at the root of that history.

Rage

I would like to see the EGT's as well. Exhaust coolant temps are the same as previous engine that had no issues
In case there is something else lurking in the engine config that I am not aware of that might precipitate detonation the following are some of the engine particulars for comment
Static compression ratio 9.66 (9.57 previous engine)
Dynamic compression ratio 5.88 (6.21 previous engine)
Cranking compression @915' elevation 150 psi (previous engine 156 psi)
Quench 0.039"
Cylinders bore notched to unshroud valves
No combustion chamber work other than blending to optimize port cfm c/o Jim Valako
Coolant thermostat 120F, actual running coolant temp 140F
Spark advance vs rpm curve for normal running idle to WOT shown on previously posted graph, new engine timing mid range significantly less than previous engine, high rpm timing the same at 32*
New cam spec Intake/Exhaust: Dur @0.050=242/242, Dur @0.200=160/160, gross lift=0.666/0.629, lobe CL=120/112, LSA116, overlap @0.050=9*, total Int/Exh%=74.4%
Previous engine cam spec Intake/Exhaust: Dur @0.050=238/244, Dur @0.200=154/159, gross lift=0.597/.601, lobe CL=120/112, LSA116, overlap @0.050=9*, total Int/Exh%=83.1%