ATAC in general works much lower down the powerband than a powervalve does, and cannot be gradually reduced in its effect.
It works very well to reduce the effect of the pipes return pulse being way too early at lower rpm, but this effect kills the power badly as soon
as the pipe efficiency starts to rise - thus the valve connecting the Helmholtz chamber to the pipe must be switched instantly at a specific rpm.
In a 125 race engine the ATAC would be off at around 8000 rpm, where as the PV may not be closed fully to within 1000 rpm of peak power.
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