Thats basically it. The left side drum valve in the PV is like a two position valve. At lower RPM, the exhaust sub port is open to the chamber formed by the cyl and PV cover. At higher RPM, the drum rotates and closes off this chamber as it opens directly to the main port/header.
The PV cover has more effect on the 200s and 250s than the 300s, and also more effect if you are running a mid to top end orientated pipe. I like how it works on a newer 250. With this, a head mod, and good jetting, it will pull from a crawl at idle to a screaming top end. An older 300 with an old Gnarly pipe will not benefit or need a high volume PV cover.
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