Rage

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.

Rage

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.

Rage

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.

Rage

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.

SB

iTrader: (1)

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 ?

Keith Atlanta

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.

Rage

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.

SB

iTrader: (1)

Another quick question , what aluminum alloy are these pistons ?

donzi matt

iTrader: (1)

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.

donzi matt

iTrader: (1)

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.