LM1 - A/F ration Data interpretation

[beepbeep]

Knock and pre-ignition are sometimes hard to detect.

Also, combustion-chamber geometry plays a significant role. With other words, if you swap out your nicely dished smooth pistons and install sharply recessed aftermarket pistons, squish area might change and you might get local lean areas while "total" AFR of cylinder still looks rich.

I attached pics of problems that a gentleman with race-prepped opel c20xe engine encountered. He run C/R of 12.1:1 and had good (=rich) AFR's. We discussed damage on another forum and everyone is scratching their heads.
I personally believe that new fancy recessed pisons messed up geometry inside the chamber and he was getting lean areas around exhaust valves.

It's a tough call to mate hot cams and smooth-dished pisons w/o doing piston recesses.

I also believe that the reason turbocharged engines survive much higher C/R ratio (altough dynamic) is that their pistons have smooth domes, thus making good combustion chamber geometry possible. It's the turbo that is doing the C/R raising. When you try high C/R with N/A engine, there is just not enough meat inside to do this...so you end up using tall pistons and then start cutting into them so that valves don't smack on them. All this cutting introduces lot's of sharp enges ino combustion chamber and it goes boom.

[Buckeye19]

Originally Posted by RarlyL8

From dyno data you have done what is the difference in power when leaning from say 11.8 to 12.6 AFR? I have not done enough runs to see statistically significant results.

I have personally done back to back dyno comparisons and there is no measurable HP difference going from an AFR of 11.5 to a 12.0. This may not be true for every vehicle out there, but this was true on my particular setup (11.3:1 C/R 993 single turbo with 6 psi making 412rwhp), and on several other platforms that I have tuned.

If it were me, I would never run a leaner AFR then 12.0 at WOT on an air cooled CIS turbo motor, too much risk with near zero gain. You have no knock feedback control features on that engine, and it only has to lean out a bit before you start to knock and damage parts very easily. Running mid-high 11's AFR is not rich enough to wash down the cylinder walls or dilute the oil, and it is worth the extra margin of safety, especially on an air cooled engine where head and piston temps can be a problem that leads to failures.

[NathanUK]

Interesting perspectives from you both Goran & Buckeye.

Goran, why is it that the melting of the piston always occurs near the rings? I would have thought looking at that piston you would get a hot spot on that very thin edge on the left hand side between the two valves cut outs? Do you think that thin edge caused pre ignition of the fuel?

[WERK I]

There could be other factors at play here. A/F contamination, carbon build up could be at play. A/F contamination can come from oil getting into the mixture. This will lower the octane of the incoming mixture. Valve guides, PCV and turbo bearings are the most common causes.
Carbon build up comes from over rich conditions or oil contamination. Carbon will raise the compression ratio bringing on pre-ignition or detonation.
Geez, even an improperly adjusted valve can give you headaches. If the valve has little time against the seat or is prevented from seating on seat due to carbon buildup, that valve will start glowing like a glow-plug.

[DonE]

Not to mention deck height, after-market pistons with Mahle cylinders, type of plug, and on and on.

On hard accel, I have mine set to 11.7 and it settles at 12.2 within 15 cycles. At no load, it's at 14.2 - 14.4. At cruise (2800 rpm), it's at 13.6 - 13.8.

As for the Innovate LC-1 and XD-16, make sure you re-calibrate often. Mine wanders after about 4 - 6 months. Before I open the laptop to tweak the tuning, I re-calibrate and usually find that solves the problem.

[beepbeep]

Originally Posted by NathanUK

Goran, why is it that the melting of the piston always occurs near the rings? I would have thought looking at that piston you would get a hot spot on that very thin edge on the left hand side between the two valves cut outs? Do you think that thin edge caused pre ignition of the fuel?

I would guess that extensive amount of squish around edges and somewhat complicated tumble of fuel mass leads to lean conditions around edges of the piston wich promotes knock. Also, exchaust valves are hotter than intake so in 4v engines, you will mostly see erosion around piston edges near exhaust valves. While center of piston in this case is covered with ridges, it's also postioned directly under the plug (thus ignited first) and probably exposed to good air/fuel mixture as well.

Remember, flame front has finite speed and in this case plug is positioned in centre. The plug is ignited before TDC, and while flame front propagates, piston is moving upwards. The piston edge under exhaust valves is hottest place, in this case with lot's of squish and reached by flame front last (and thus compressed by piston). That's the probable reason of why it got damaged in that position.

If you look at 930 piston damage further up, I would guess that hole appeared furthest away from a plug as well.

[DonE]

Both of my burnt motors had opposite problems - the hot spot was the intake side of the piston. When the last problem occurred, it burned (or frosted) the piston between the top and the first ring. The top of the piston shows no problem.