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.

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.

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/

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

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.

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.

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

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

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.

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%

With the Whipple Stage 2 and 120 T stat mine runs 140-145* at speed and high 140's at idle. Even though my rev limiter is 5450, there is no reason to go past 5200 with stock internal/induction. No power to be made spinning higher. There is a very noticeable midrange power gain vs stock. While the power gain at WOT is not as pronounced as thru the midrange, it's noticeable and the engine revs much cleaner. I ran it pretty hard at times this summer with not a hiccup. Most time was 93 octane but there were times over the weekend I used marina 89 octane. No knock present when engine scanned.

Thought this was interesting quote from Raylar.... Even though I've always had great success with Mahle's 4032 pistons in previous builds when tuned right. IMHO, the piston and ring setup was fine for your power level. Your previous 300ish hours of runtime proved that. Is the ring gap on the tight side? Probably. With a critical loss of FP like you had, it's all speculation as to what could or would have held up when it detonated.

Bob,

Thanks a ton for the invitation! I surely will take you up on that offer. I imagine your Cobra build will be complete by the time the engine is ready to run. I am having shoulder rotator cuff repair surgery Dec 4.and decided not to try to accomplish the rebuild before then since I am pretty much a one armed bandit already. With a six week immobilization then a couple months rehab the plan is to start the rebuild in March. I am pissed that it has to wait till then but resigned to that fact. Looking forward to the visit. Thanks again!

^^^Awesome.

So, no one 'liked' BOBL's post #39 ?

Give him a big fat kiss and a 'Like.' LOL.

That is crazy good !

I know I'm late to the thread but I just went through the same piston issue with my blown bbc (#8 cyl) and hurt a few others. Fuel pressure dropped to 3.5 lbs on wot . (had no fuel pressure gauge in dash/ there is one now). Also I found out my plugs were 2 heat ranges to warm. Had ngk6 and now run ngk9 just to be safe. Back to running like a champ. Maybe you increased cyl pressure with your new cam set up enough to cause an issue with the plugs? Just a thought....

I'll throw another thought into the mix. I've always felt 32+ degrees of timing was too much for those fast burn heads. Maybe your engine changes, lost fuel pressure, etc. was enough to push it over the edge. I would love to see some dyno runs with the timing pulled back a couple of degrees. I bet it wouldn't lose any power.

FYI, have done some tuning on Volvo 8.1's with the mefi 4. I can tell you for a fact the stock HO does not like advancing the timing beyond about 28, degrees. You can read the knock retard on the mefi 4 and any timing you added it would pull it right back out.

Bob

Thanks Bob. The 32* originally came from the GM HP3 Gen1 engine which I initially copied along with the 12.8 AFR WOT on 87 octane that produced max HP per Innovation Marine. Of course that engine was 515hp at 5400 rpm and way different than what I am running now so yeah maybe time to back off the timing at WOT. The different Volvo experience could be related to it's cam maybe having more dynamic compression than the HP3 Gen1 cam. I know my stock 425hp 496HO had 175 psi compression and the HP3 Gen 1 was around 160 psi I think.

Could very well be. That is one of the big advantages of tuning an engine on the dyno. You can find the best timing for a particular combination I'll generally chase the HP until it shows no gains, then back up from there. Of course with the PCM you don't really have that option since you have to ship it off for tuning. My personal opinion(for whatever that's worth) is that combustion chamber design impacts timing requirements more than any other element in a build. Compression will affect the octane requirements but timing required for peak HP would not change significantly with changes in compression or camshaft.

http://www.raylarengineering.com/img/540-afr.jpg

AFR map from our tuner on a stage 1 540 build. If possible, you should be somewhere in the 12.2-12.7 range at WOT. In street vehicles, we've found that a lean cruise is fine at 14.0-14.5, but as soon as you dip into the throttle, power enrichment or the tuning should add more fuel to prevent a lean condition.

Here's a dyno from one of our old 600s which should be a pretty similar setup to yours:
http://www.raylarengineering.com/img...-dyno-info.jpg

Raylar,thats what I've told him about afr's,not sure he has the tuning capabilities of the 555 computer.also told him to look at his knock sensors to see if he has too much timing.
Raylar,what timing (max) are you running in your 540's?

Larry,

Don't forget that with a 13.5 and above 'lean cruz' we burn one up cause "lean doesn't care how fast you're going". There's a big pull on boat engines from getting on plane and above. You might be able to skate by once or so with really good gas and lucky day but anything over 12.8 is pushin' it unless you're just at fast idle and below.

We've spent tons of time and money on the 555. Spend the time and hang a Holley or something else you like on there. Unless you are rich enough to have one of the experienced tuners ride in your boat etc, it's just not worth it anymore to expect too much from that box.

Keith, yes changing the fuel pressure is a fairly decent band-aid up to a point but at Rage's level or something like that it's not even worth thinking about and there are a lot of issues going on at same time.

We just finished a 588 with raylar heads and 105mm. Going up 3 injector sizes (62LB) the duty cycle is 70%. Haven't had time to fully digest all the recent info but i went pretty far on the stock merc injectors (42LB) and prob could have upgraded a long time ago. This is a very dynamic conversation and you gotta take it all piece by piece with the changes and the equipment being used etc. There's a difference in AFR sensors, equip etc. IOE, if you are using a bosch type sensor with the Innovate hand-held then i'd recomend setting it almost one point richer compared to say an NTK in a nice, dry envirionment.

If you stay at Larry's 600hp level kit, then his instructions and stuff works really nice. If you use that same product and go further, it becomes a slide ruler on steriods. You really need an open EFI system that you can learn and tune. ...And it starts all over..

since I was the first to question the ring end gap and possible butting of the rings I would like to say one thing. thank you to everyone on this thread. most of these end up in some stupid pissing match about nothing but this thread has stayed on topic with lots of great info and helpful tips that everyone can and has learned from. this is what these threads are supposed to be like. I am not a master engine builder but I do build some and any time I can help and learn benefits all of us. thanks to everyone that has been civil and informative.

LOL Thats because most conventional engine guys on OSO either hate or dont know anything about 496/8.1

So here we are, the redheaded stepchildren.

you think the 8.1 is a redheaded stepchild? I restore cars so there is no telling what I end up rebuilding. I have done all the chevy and ford stuff which is pretty basic but when you get to the rambler or flatheads or packard or worse, some with multiple carbs, well you want to talk about odd ball. that is why I like threads like this that stay on track. things I learn here I can apply to others. and yes I hate some of these odd engines and I am pretty sure the engines hate me some days but I still have to figure out how to fix them.

I prefer being referred to as the white elephants lol.

This 555 ECM is really a killer. It has some really nice capabilities, but it is either uncrackable or nobody is willing to do it. Nothing against Whipple, but it is pretty impractical to try to dyno tune a motor by shipping off an ECM for each adjustment.

I am just an idiot with a toolbox, but is there really no way to crack the programming on this thing so changes can be made by the end user? I would love to hear what you guys who have spent years massaging these motors have learned about this obstacle of an ECM.

Should re-title this thread "Red Headed Step-Children & White Elephants". lol It's all good. Probably all engines can be made to perform but some are just easier than others. Take the big block Fords for example. Has anyone been able to get big power from a 460 Ford? Has anyone even tried?

Not exactly 460s

http://www.offshoreonly.com/forums/v...-upgrades.html

You can add "and the 555pcm blues too", there, now its very patriotic . lol
wouldn't it be easier and more cost effective to sell the stock unit and buy an aftermarket self learning unit?

It would sure make life easier if they could figure out how to make an O2 sensor work with wet exhaust and run a closed loop EFI like cars do.

Its often done. Lots of Jet/V drive guys use the big block ford.

Just need a good exhaust to run an 02 sensor. I've done it on quite a few boats. You need to get a good tune, then remove the sensor. The Holley system is really nice. Run the boat under as many conditions as you can, upload the learn table and remove the sensor once you're satisfied. If you have a significant altitude change you might need to reinstall the sensor and let it learn again. FWIW, many cars only run closed loop at idle and cruise (mainly for emissions purposes). They will go into open loop at wot and run 12.5 - 13 AFR.

I'm sure it's all about the $$$. Not enough return on investment in this small market. I know Whipple paid a large price to acquire the rights to it (Seems like I remember it being in the 6 figures). Mark Boos can program them. Not sure how he came up with the software. Also heard Raylar had somebody reverse engineer one several years ago. I'm not sure how that turned out. Maybe Larry can elaborate.

Seems there are lots of 525's and 496's out there that could benefit by an end user tuning solution, similar to MEFI burn.

As for ditching the 555, I have smartcraft gauges, I can't go to another ECM solution without changing all my gauges out, and I like the guardian safeguards as well.

I've done a couple where I piggybacked the ecu's to keep the pcm for the smartcraft function. Kind of a pain but it can be done.

I do not think it is just your opinion it is fact that combustion chamber burn rate is the primary if not the only factor that determines what the optimum timing is for an engine. That is my biggest unknown input parameter for modeling my engines in Performance Trends Engine Analyzer Pro engine dynamometer performance simulation software. 10 - 15 % difference in burn rate has a huge impact on best power timing required and detonation probability. I reverse engineered my burn rate using the Innovation Marine HP3 Gen 1 max timing spec but that is marginal because I do not have a dyno test on their engine to baseline model against nor detailed measurements of their intake manifold.

Tuning real time on a dyno is definitely the best but as you say the PCM555 fuel, volumetric efficiency and spark tables can not be accessed with out unlock key and Merc software to my knowledge. Also there apparently are a huge number of fuel tables if you could access them to tune.

Larry,

Always wanted to ask about that 600HO dyno test, isn't that AFR at the higher RPM's rather on the lean side or is that where that engine is the happiest?

One of the biggest drawbacks with the PCM555 is that there is no engine knock fault code or data output real time. It does attempt to control knock by retarding timing on the individual knocking cylinder similar to the J&S Safeguard system but that knock event timing retard is not available as data output either only the base timing is available.

How was your conclusion on the Innovate/Bosch versus the NTK AFR reading accuracy arrived at?

A lot of people, actually almost everyone I trust, are telling me to put a Holley EFI on the engine. Maybe I better listen.

OSO Thread Realignment
 

I would like to get back to the initial question that started this thread.

Question. What is the leanest naturally aspirated engine AFR [ with GM Gen VII 8.1L, Merc 496HO heads] that is safe from detonation damage from idle to 2200 rpm? Plane out is ~2200 RPM. Significant MAP/engine load increase begins at ~4000 rpm. Knowledgeable responses only please.

Yep, but the stock computer becomes the limiting factor with "tuning" these engines in boat applications. 50-51psi seems to work pretty well, but the best fuel delivery solution is an aftermarket computer like SDFever has on his boat.

I was under the impression that it pull timing and dump fuel on either 1357 bank or 2468 bank due to one knock sensor per side?