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Most people think a scatter cam has a slightly longer exhaust duration in cyls 2&3 to cope with the fact that both are using the same port. This is not true.
A scatter cam is a cam that has slightly shifted lobe centres on cyls 2 & 3. The Lobe Centre Angle (LCA - The angle between the In/Ex lobe centrelines) is the same, but the actual position of the inlet and exhaust is shifted by a few degrees. It's nothing to do with exhaust ports.
As the piston starts to rise on it's compression stroke
the inlet closes. The piston next to it has just started it's inlet stroke and
will (due to the shared inlet ports of the A series) suck some of the charge
from the one which is trying to compress it.
To counteract this effect you use a scatter cam which 'staggers' the timing slightly so that the first cylinder has closed it's inlet more when the second one starts sucking.
It's an effect that only really causes problems at high
revs and using a scatter cam only gains back a few bhp at the top end. That's
why they are race only, and not much use on the road.
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