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?