Detonation Issues - "Why is the Gen 7 496 such a bad platform to build"
 

This post seemed to have all the knowledge 496 HO people involved. I hope my inquiry is relevant. If not I will repost separately (which is this new post).

I had been running a 2004 Mercruiser 496HO Gen 7 engine with Innovation Marine HP3 Gen II engine setup plus Raylar 517ci stroker crank and Mahle 4032 alloy forged pistons (030 over bore) with Rayar Cool Cap intake manifold, HP3 heads with 2,25 intake/1.82 exhaust valves and Jim Valako porting which dyno'd at ~625cshp on 87 octane since 2008. Refresh required in 2014. During refresh also increased TB from RV Morris bored / Jim Valako ported 77m to 92mm TB and increased cam lift from HP3 Gen II .596/.596 to .666 intake/.629 exhaust maintaining HP3 Gen II cam duration and LSA. to obtain 74.8% total exhaust/intake flow ratio versus 83.1% from previous setup. The engine recently detonated with the new engine configuration destroying two of the forged 4032 alloy Mahle pistons during AFR data logging sessions with 91 octane. Cause is believed to be fuel pressure regulator going lean off set point.

Question. What is the leanest naturally aspirated engine AFR 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.

What RPM were you at when the motor burned down? What was the AFR at that point? Do you have timing numbers for the point where you lost the motor and was there any adjustment to your timing table since the changes were made to the motor? In my experience increasing stroke on an engine generally requires less ignition timing, but you changed a lot of items at once so you can't necessarily go on a generality like that. It sucks you burned down a fresh motor like that but it opens the door for quite a bit of learning about tuning a setup like yours. One last question, are you still on the 555 ECM or have you converted to a MEFI setup?

The motor would still run but knew something was wrong. Leak down test confirmed it and pulled heads to find the broken pistons. Still running the 555 ECM. Maybe you misunderstood. The motor was bored,stroked, Raylar intake, HP3 Gen II cam, heads, back in 2008 and ran great for 350 hours. All I did this time was refresh, bigger throttle body and valve lift. Dustin retarded the timing a lot in the midrange rpm's this time versus previous build. When motor quit I lost all Diacom and Innovate data logger recordings of the run. But did watch the AFR during that run and the AFR was in the high 12's low 13's for cylinder #8 where the previous engine ran best. The fuel pressure was confirmed set at 48 psi at start but was 47.4 psi after run. However I suspect the initial damage likely occurred during the non instrumented run from the launch ramp to the boat slip the day before. After that run the fuel pressure was found to be at 42 psi instead of the 48 psi set point. Attached is AFR, MAP and timing data for current and preceding engine config.

I did misunderstand, I apologize. I thought you added the stroker crank this year. That's what I get for posting before my coffee.

To make sure I am understanding you correctly, were you just running from idle to 2200 from the ramp to your slip when the fuel pressure was low? If so I have a hard time believing at that low of a load that you would have melted a piston.

No, sorry for the poor writing skills. The fuel pressure was low (42 psi) during the run between the ramp and the slip but I did do a brief burst of higher rpm's during that run. Most of that runs rpm before and after the burst was ~2500 - 3500 rpm I guess. Also the pistons did not melt. A chunk broke off of two pistons at the same location, the intake valve relief. Pictures attached. The Idle to 2200 rpm inquiry has to do with the fact that is the only rpm range where the new engine shows leaner AFR than the previous engine when fuel pressure is at the set point that as arrived at from the fuel pressure vs AFR tests on the new engine. This is planning for the future when engine is repaired and I am wondering/asking if that could cause a detonation problem that could damage the engine then when it comes time to make more AFR runs to provide data for Dustin to recal the fuel tables.

Donzi Matt:

Looking at that damage, it certainly looks like detonation broke the top ring land. Do the Mahle pistons keep the relatively short ring land like the stock pistons or do they move the top ring down for strength? I would say with ~15% less fuel pressure than expected that would certainly damage the pistons.

As for your leaner condition at idle to 2200, personally I don't think you have much to worry about as far as damaging the motor, but I could see it having a lean sag around 1600 where it looks like AFR bumps up to mid 14's. Probably wouldn't hurt anything to have Dustin clean that up in the calibration a bit.

Is this normally what you see for an AFR line on this motor? Looks like it is all over the place, but I am not sure if you are measuring each cylinder individually or what. I am used to a much flatter AFR curve but I am also not used to measuring individual cylinders, just complete banks.

SB:

Rage, can you put just 8/28 and 9/2 runs together ?

On the 9/2 run, it looks like 1600rpm and especially 2400rpm (high AFR spikes) could be detonation causing those 'high spike' AFR readings. The 8/28 run appears possibily same thing, but I can't quite make them out. Too fuzzy on my computer screen.

Thanks.

MILD THUNDER:

I am not very knowledgeable on EFI mapping, but I personally think, these lean air fuel ratio numbers being talked about, are playing with fire while under load.

Back in the days of carbureted vehicles, spark knock, at low rpm, high load situations, was a real problem. My general opinion, is that while 2200RPM is a slow engine speed, it is not impervious to being rattled to death there, even getting on plane. Some boats can really load the engine down trying to get on plane. With a carb, its very possible that while planing, the loss of engine vacuum, will make the carb go into power enrichment.

When adding power, without adding displacement, or forced induction, you are simply raising cylinder pressure to make that power increase. Looks like you added cam lift, without adding duration. Generally, this will help build more power, esp at lower rpms.

Basically, what I'm saying, is, when the HP per ci goes up, the tune becomes more and more critical. What works at .85hp per ci, may not work at 1.3hp per ci.

I used to believe in textbook AFR numbers I read online. I also used to believe, lean makes big HP numbers, and rich kills power. While quite different than what we are discussing here engine wise, I have witnessed 900-1000hp roots blown carb'd engines, make peak power on the dyno, with an AFR of 11.5. Leaner than that, it lost power. My personal engines, run low 11's currently at WOT. I have had it in the low 12's briefly at wot, and the engines layed down, top speed suffered. I then decided, I'm gonna just start giving the engine what it wants in the fuel, and timing department, and kind of ignore, what the internet says.

COMPEDGEMARINE:

just a couple questions about those piston pics. I am no expert engine builder but looking at the pics it seems the top ring groove is pretty high up and there is not much material left in the valve notch. were the pistons cut for more valve clearance? the shape looks a little odd but it could be the picture. when the engine was refreshed is it possible the ring end gap is tighter than before and the ring butted and pushed on the top of the piston where it is the thinnest? just a couple thoughts and hopefully I can learn something.

Being on a better computer, photo #2 on post#3 (clicked on it to make it larger)really does look like what Compedge is saying, looks like the ring lifted. You can see tears at the beginning and end of where the piston top is missing.

I tried to look closer up, even, but I couldn't...was looking for the typical pits and such detonation typical causes on the piston top. Don't see any from here...but maybe my screen resolution ?

I am blown away that changing fuel pressure changes the AFR that much in different areas of the rpm range. I assumed it would be more consistant.

Donzi Matt:

Looking at that damage, it certainly looks like detonation broke the top ring land. Do the Mahle pistons keep the relatively short ring land like the stock pistons or do they move the top ring down for strength? I would say with ~15% less fuel pressure than expected that would certainly damage the pistons.

As for your leaner condition at idle to 2200, personally I don't think you have much to worry about as far as damaging the motor, but I could see it having a lean sag around 1600 where it looks like AFR bumps up to mid 14's. Probably wouldn't hurt anything to have Dustin clean that up in the calibration a bit.

Is this normally what you see for an AFR line on this motor? Looks like it is all over the place, but I am not sure if you are measuring each cylinder individually or what. I am used to a much flatter AFR curve but I am also not used to ensuring individual cylinders, just complete banks.


Sent the pics to Mahle Motor Sports for comment. They said it looked like detonation damage. I do not know about the ring position vs stock but the valve relief in the Mahle is deeper than stock.

Attached is the individual cylinder AFR the previous engine build ran the most of its 350 hour life on 87 octane 625cshp with the spark advance shown as over lay. Pistons, valves, plugs et al showed zero evidence of any detonation on tear down. Fuel pressure set point was 48 psi. The previously provided AFR graph on the new engine includes the #8 cylinder data (bold Green Line) from the previous engine for comparison which indicates it ran way leaner than current engine. Dustin set this fuel table up on the previous engine and did not seem concerned about how lean it was at the lower engine loads. The new engine AFR numbers graph is cylinder #8 only (see title). Dustin will only tune with engine bank AFR to start then deal with fliers which are difficult. Also the data can be "Smoothed" more or less. My data is smoothed only 0.090. Max smoothing would produce a very much flatter curve.

I would want the big lean spikes brought down. What I am concerned about is when making the first runs to produce the AFR data to give to Dustin, that I not break the engine again. Once the initial AFR data is produced then I would ask that such AFR spikes be reduced to reasonable levels. This is the reason for my inquiry.

Another quick question , what aluminum alloy are these pistons ?

He stated above they are the Mahle 4032 forged.

Quoted from a Hot Rod article on Mahle pistons:

The part about high-silicon and less ductile is what makes me think he chipped a ring land from detonation, much like a hyper-u-crack-it.

Oh, and it is quite common on the LS motors with a shallow top ring land to break the top of the piston to the top ring when they go lean under boost. Especially with the Mahle slugs for some reason. Perhaps the alloy they are using, but that is just me speculating.

MILD THUNDER:

I am not very knowledgeable on EFI mapping, but I personally think, these lean air fuel ratio numbers being talked about, are playing with fire while under load.

Back in the days of carbureted vehicles, spark knock, at low rpm, high load situations, was a real problem. My general opinion, is that while 2200RPM is a slow engine speed, it is not impervious to being rattled to death there, even getting on plane. Some boats can really load the engine down trying to get on plane. With a carb, its very possible that while plaining, the loss of engine vacuum, will make the carb go into power enrichment.

When adding power, without adding displacement, or forced induction, you are simply raising cylinder pressure to make that power increase. Looks like you added cam lift, without adding duration. Generally, this will help build more power, esp at lower rpms.

Basically, what I'm saying, is, when the HP per ci goes up, the tune becomes more and more critical. What works at .85hp per ci, may not work at 1.3hp per ci.

I used to believe in textbook AFR numbers I read online. I also used to believe, lean makes big HP numbers, and rich kills power. While quite different than what we are discussing here engine wise, I have witnessed 900-1000hp roots blown carb'd engines, make peak power on the dyno, with an AFR of 11.5. Leaner than that, it lost power. My personal engines, run low 11's currently at WOT. I have had it in the low 12's briefly at wot, and the engines layed down, top speed suffered. I then decided, I'm gonna just start giving the engine what it wants in the fuel, and timing department, and kind of ignore, what the internet says.


Very applicable info. Thanks.

I am surprised by a couple of performance differences between the previous and current engine.
1.) MAP on previous engine is flat in midrange RPM vs new engine MAP has a bulge of increased MAP midrange. Fuel #/hr increases in concert.
2.) The unmodified fuel tables of previous engine with the new engine's higher midrange MAP [item 1.) above] shows a flat ( and lower) AFR midrange to WOT. The AFR of best run before engine failure did creep up from 12.5 @5600 rpm to 12.8 @5800rpm on rev limiter. This seemed about perfect to me at the time.

RE: "Basically, what I'm saying, is, when the HP per ci goes up, the tune becomes more and more critical. What works at .85hp per ci, may not work at 1.3hp per ci. "
Should I assume the additional air/rpm from the bigger TB and more valve lift as described earlier is significant enough to require a richer AFR for best power or is the change too insignificant for that to be the case? The richer AFR's in the new engine did not produce the 5800 rpm that the 12.8 AFR on last run did.

Looks just like mine when I hit some light detonation with insufficient ring gap. In my case, the top ring butted and lifted the land at the weakest (thinnest) point - looked just like yours, but a slightly larger area. No other obvious signs of detonation. When you will the rings, check for polished areas on the end of the top rings. It might be very light, but look for clean, slightly polished areas.

Mine was not a marine build at the time and had .020 gap, so it didn't take much excess heat. What was the top ring gap?

I definitely broke not melted. The runs made were very brief with no engine warm up followed by cool down in dock slip to analyze the data. So likely not enough run time to start melting things, maybe. Mahle said it looked like detonation. I double checked all file cut ring gaps during assy and they were correct at 0.024" gap top ring.

Bingo ! Looking at that #2 pic really raised my eyebrow. Thus my speculation too, if that was the alloy used in thje Mahle. Which, it was.

So....

0.024" gap. I will look for what you describe.

0.024 is pretty small top gap for a marine HP engine with that bore I'd think. My piston maker recommended .028-.030; ring maker said .032-.034 both for same bore size, but less HP and compression.

Some pics of mine where they butted at this link http://tinyurl.com/ppe2kgj

Nothing to do with AFR, but how much does the cylinder grow from ambient to say 250_300 degrees? I've installed many couplings with interference fit, heat em up to about 275 and they slip right on. Takes a beefy puller to remove, like a 10 ton Porta power.

Maybe that was it. I discussed my engine with the Mahle Motor Sports guy and was given that gap as the upper end. Mahle ring sheet calc factor for my rings worked out to 0.021" for Blown gas and 0.029" for Blown Nitrous over 200 hp so thought 0.024" was plenty conservative.

Who is your piston maker and ring maker and what was the ring diameter involved?

Just the narrow bright line across the OD end of the ring?

Yes, the narrow bright area. no carbon/ crud on it and slightly polished. That was the only one I found like it, so either that cylinder detonated when the others weren't or the gap slightly was tighter. Or something else nobody I talked to thought about yet.

Icon (KB) forged pistons and Total Seal rings. I called both and discussed the application, fairly mild power but run for long times at 2500 while under moderate load and longer times at WOT. Both said that required increased ring gap. I went to 0.034 on the top ring. It is the gapless design, so no loss of compression to worry about.

What was the cylinder bore diameter that these ring gaps were assigned to?

Total Seal gapless rings sounds like a goo option.

I looked up KB pistons. All I could find available from KB were 4032 alloy for other than motor cycles or ATV's which the latter could be had in 2618. Is 4032 what you used?

From the research I have done it looks to be rather difficult to find a 2618 alloy off the shelf piston for a 8.1 liter. Just about all of the pistons I have found are of the 4032 variety as they are a drop in replacement. Mark Rinda had CP custom make him a set of 2618 dish pistons for a Whipple motor he did.

What are the Diamond Piston alloy?

2618 is available from JE and Diamond supposedly. I have been talking to JE for about three days for a set of custom pistons for my stroker but so far I am not getting a guarantee that pistons will be made from 2618. I do not know what is going on with them.

The Diamond drop in is listed as a 4032 in their catalog. I'm sure just about any piston manufacturer will custom make you a 2618.

Back to your original worry of detonation during your initial tuning run, when I would street tune my 4 cyl turbo initially after making changes to the setup, I would mix 50/50 with 100 octane low lead from the local airport to give me a safety margin against knock until I got a relatively clean tune in it I liked. Maybe it is an option for you.

I have always been paranoid about ring gap so I did a lot of reading on it. the general thought from those that had done a lot of dyno work with different gaps was that being too big did not hurt the power noticeably but being too tight could cause butting and break something. after that I always run them bigger than recommended and just take the slight loss rather than risk damage.

Here is JE's 4032 ring gap spec's:
Note: How high/low the ring are placed on the piston obviouly effect things - I did not take the time to compare heights between JE and Mahle

[Edit in: oh schit, wrong table. I'll be back. LOL.

I can't copy and paste their info.

Here is JE's 4032 piston and ring recommendations:

Click on: http://www.jepistons.com/PDFs/TechCo...instrc4032.pdf

Here is Mahle's, however they don't note which aluminum alloy for some reason:
http://www.us.mahle.com/media/motors...tion-sheet.pdf

Shouldnt standard pistons be more than adaquate for this build? Isnt the root cause your AFR? You have a bad azz engine but it isnt off the charts bad azz (in a respectful way)

I think we need to re-direct the topic here... Are you sure this isnt a combustion chamber or possibly a quench issue?

I do agree, but remember, it's a 625-650hp motor. May seem 'mild' to today's standard, but it's still 625-650hp. Any tune issues, and these pistons are less forgiving.

Anyhow, we want to make sure ring gap spec was chosen wisely, and to make note that there are a lot of Mahle 4032 pistons that have broke this way on other type motor, and many in the same spot. Yes, that relief is a pretty normal spot for any piston if detonation and/or too much fuel if.....but, again, just seams more Mahle 4032' have done this.

My IMHO, and repeating myuself, we still have to respect 625-650 as a lot of N/A horsepower. Because, quite frankly, it is.

I agree. At 1.3 HP per ci, thats a stout number for normal pump gas N/A. Not crazy, but still up there a bit. If this was a 572ci, we'd be looking at about 750hp. Healthy build.

To take a step back from the whole EFI tuning thing for a second, what are we looking at for cam specs on this build? Is there a cam card available? I know you mentioned you added lift. Also, what is the static compression, and cranking compression?

From Reher Morrison

One of the persistent problems we encounter is piston ring end gaps that are too small. Many racers are squeezing the end gaps in order to improve leakdown test results. Well, if you want to see zero leakage on your meter, just put O-rings on your pistons. But if you want to make power where it counts – on the race track – you must have enough clearance to prevent the ends of the piston rings from butting under actual operating conditions. Even if you don’t reach the point of damaging the cylinder walls, you risk impairing the ring seal if the end gaps are too small. In our Pro Stock engines, we run generous .028-inch gaps on the top rings. Believe me, if I thought there was power in tighter end gaps, I’d reduce the clearance in a heartbeat.

This thread actually makes me wonder if some of the stock 496 piston failures when hopped up aren't just random piston failures or detonation, but rings butting and lifting the ring land. The production spec from Merc is 0098-.0161", that is pretty tight. I could see a motor built on the tight side of the spec butting a ring when wicked up. I'll make sure to measure after one of mine blows up lol.

4.280 bore. 2618 alloy Icon 778 piston (not 8.1L) http://www.summitracing.com/parts/uem-ic778ktd-std

I like the gapless top ring. I get no blow by at all now. Even if I plug the PCV, I get only a small amount. Used to get a huge amount that you could smell after a run.

EDit : they are pricey, but as a % of what you are spending, the increase is small.

When I talked with Total Seal, they recommended higher gaps than what was listed for any application on the sheet that came in the box. The reason was the sustained nature of the WOT runs vs the intermittent nature of circle track and the short nature of drags. IIRC, for blown drags the sheet recommended almost 0.010 LESS than they told me to run on a marine engine.

I know of a couple of stock 8.1L engines that have had a ring land lift just like this, so not just hopped up.

Just remember what the 8.1L was built for, not boats. As you know in our stringent emission world, tight pistons (this means higher silicone pistons if forged) and higher placed rings with close as possible gaps are the norm. Okay, more than a norm, pretty much a damn requirement. Any place for fuel to hide and not be burned is a no, no...emissions speaking.

Think closed cooling is just for 'corrosion protection' on these motors ? I say no. Tight clearances with hot pistons + cold cylinder bores = what ? Yup. OOps !

Rage,
I propose you get your engine back together and come down here and spend a week or so before you put it back in the boat. We'll beat the snot out of it on the dyno, where it hopefully won't blow up if something's out of kilter. In our spare time you can help me building the Cobra, or maybe I'll have it running by then. Even better! Between your afr meter and mine we can look at 4 cylinders at a time. You're retired, so time is not an excuse.

Bob

Rage did you take all the hard edges off the top of the piston before install? I usually run a tootsie roll over all the hard edges on the piston and smooth out a little. It doesn't look like this was done. Hard edges that see combustion can and will turn into hot spots acting like a glow plug and can cause issues. Doesn't look like the valve reliefs were smoothed out at all. On a small bore motor you really need to get that top ring away from the top of the piston. Where the piston broke is the weakest part of the piston. A lot more material in that area on larger bore stuff.