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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
Posts: 5,565
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 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.

Thank you for your time,
Old 04-28-2008, 11:39 AM
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