A cross-flow head has intake ports on one side, exhaust ports on the other.
The advantage is (supposed to be) that the charge exiting the combustion
chamber is moving in the same direction as the new intake charge, so it helps
pull the intake charge into the cylinder.
In the standard reverse-flow head, the exhaust flow is traveling towards the
intake charge, so during the overlap period of your cam, the two flow
directions are competing, thus reducing your flow efficiency.
This makes the use of a tuned exhaust (header) more desirable because it helps
scavenge the exhaust gases, reducing the contamination of your intake charge.
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