@famous bob
Bob;
if you have 9-15 inches in the intake; that sounds like a fairly tame but fun street engine sorta like mine. If this is a street engine, all I know, then;
IMO
You need to provide fresh air to the crankcase/PCV system for the following reasons;
1) to get rid of the moisture that forms in there during running, that combines with the oil vapors to form acids, that destroy metals. Things like cam bearings and such.
2) the PCV is designed to provide bypass idle air. In your case it is just evacuating the crankcase. When the vacuum peaks, air no longer, rather, the correct amount of air, no longer enters the intake, and your low-speed circuit has to be adjusted to compensate for this lack of air. Inevitably this leads to low-rpm driveability issues. Now I know that you as a seasoned veteran of carb tuning,can get around these issues. But by the same token, you can restore the PCV function and tune for that.
3) consider what and why you are doing this. I'm guessing you are looking to recover a few horsepower,lost to windage. I know that you know there are better ways. You only need that power at WOT, so pan E-vacs are a much better solution.
4) IDK what 6inches vacuum translates to in pressure differential, but I have had a cam plug go on vacation at 4psi crankcase pressure, during testing, so don't recommend 4psi anymore, lol. And I know it is physically impossible so "suck" a cam plug in. However, I find it interesting that you can even achieve 6 inches crankcase vacuum, ..... and it makes me wonder if your system has the potential to do more, but achieves "only" 6 because it is already "sucking in" unfiltered air somewhere. Just wondering if your system is limited by the seals ability to work in reverse.
5) As for exhaust-powered E-vacs;
I had heard that the E-vacs only work on open headers. But if you know me, you know I question almost everything I have ever learned/been taught was true.
In my case, with full-length dual 3inch pipes, they worked just fine. In fact, they worked too good, sucking the oil right out of the valve covers. I couldda solved that, but here's the thing; my car is a streeter and burns rubber right thru first gear, and with 3.55s hits 50 mph @7000, and 65 in second-over is also 7000 and also still spinning. Obviously then, I don't need any more power, so I took them off.
6) other;
I ran the E-vacs simultaneously with a PCV and didn't have any other problems.
To solve my windage problems, I run a 7 qt pan, with just 5 qts in it, in conjunction WITH a tray.
And I don't do autocross anymore, just the one time,lol.
