Could it be THIS simple?

[PKRWUD]

Originally Posted by ~The Jester~

Don't forget that most people think of the power valve "rating" BACKWARDS!!! If you have a 6.5 PV, it's open BELOW 6.5 hg.

Above that, it's as tight as an ugly 4th grader!

Correct, which also means as you're getting into your top end, and the vacuum is rising again, a 6.5 PV will close up sooner. That's why I feel pretty confident about suggesting an 8.5 PV.