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At upper rpms the 964 cams with their little bit higher valve lift and longer duration and later closing intake valve during the beginning of the compression stroke comes into it's own because of the inertia of the high speed intake airflow which has a ram effect that continues during the beginning of the compression stroke while the late closing intake valve is still open a little but closing.
More volume of air and fuel air goes into the cylinders at high rpms with longer duration and later timed cams.
Anotherwords more air and hopefully the right mixture of fuel is entering the cylinders at high rpms with 964 cams so the compression is a little higher than SC cams at high rpms (thats why they have a higher rpm powerband and make more horsepower) and can detonte because if that, while there is more valve overlap and less compression and lower efficiancy at low rpms as Keith said.
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