I've been thinking about this and I wonder if it isn't just the opposite for
the simpler head designs.
IOW if you consider a pushrod engine with the valves lined up along the
longitudinal axis of the engine, and you have an intake port going to one side
and an exhaust port going to the other side. Visualize this as you are looking
down on the head. Now because the valves are on opposite sides of the
cylinder/combustion chamber, as the exhaust is being scavenged it is wanting to
swirl the gases in the cylinder, and the swirl is in a direction that would act
against the intake gases that are trying to come in. With the intake and
exhaust ports side by side then as the exhaust pulls out it acts to pull in the
Now if you have a DOHC engine with the valves canted toward their respective
ports, then there is a straight shot from intake through the combustion chamber
to the exhaust port.
> 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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