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The CIS pistons had domes that squished the combustion chamber over so that when the piston was at TDC, the effective combustion chamber had the spark plug near the center. This allows higher compression (due to shorter flame path) and leaner mixtures (without misfire) for lowest emissions and best efficiency.
It's not really a performance gain, as you can't run radical cams with these pistons, due to inadequate valve to piston clearance.
You won't lose horsepower. You will lose some efficiency though.
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'69 911E coupe' RSR clone-in-progress (retired 911-Spec racer)
'72 911T Targa MFI 2.4E spec(Formerly "Scruffy")
2004 GT3
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