Test engine was a fresh 302 with 14” of vacuum at idle which is reasonable for the E series cam and 9.0 compression. Carb is a new 750hp Holley with holes in the primary throttle blades. Vacuum advance source was manifold vacuum that I wanted to keep despite the several hundred increase in idle rpm.
Purpose of the tests was initially to chase down why the engine needed to have the throttle blades closed below the transfer slot to get the idle down.. Engine was first tested for vacuum leaks and an intake manifold gasket leak was found and repaired but made little difference. Both valve cover openings were temporarily blocked and no vacuum was measured at the dipstick. In fact there was slight pressure not vacuum as it should be.
After that the test was to block the breather on one valve cover while leaving the PCV valve in the other side and measuring vacuum at the dip stick. I found information from a college mechanics test that it should read 1-3” of vacuum. With the hose pinched off the rpm should drop 50-80 rpm.
With an unknown Fram PCV valve with the number 2286 on it, the vacuum at the dip stick measured 7.5” vacuum - too high despite being checked for vacuum leaks. My 460 measures 2.9” with the same PCV valve.
#2286 PCV valve hose was restricted to 3/32-.094 and dipstick vacuum reduced to 3.5”.
Next an AC Delco #CV774C PCV valve was tested and dip stick vacuum measured 5.9”.
PCV hose was restricted to 5/64-.078 and dip stick vacuum measured 2.2” but the PCV valve would not suck up to the carb side. I switched the restriction to the valve cover side and it still would not suck up.
Next up was an AC Delco #CV789C that measured at 1.9” dip stick vacuum.
While this helped a bit with the idle it was not major so I temporarily JB Welded the Holley throttle blade holes shut and was able to reduce the idle speed by 300-400 rpm. This allowed the carb to be set with the primary throttle blades uncovering the transfer slot instead of well below.
In looking at catalogs I found that the same PCV valve is listed for big blocks as well as small blocks and of course if you have a big cam you are out of luck to find a match. Because of this I have always just pulled one off the shelf that physically fit but it is obvious to me that the internal pintle valve makes all the difference in pan vacuum.
Just so we are clear at idle with vacuum the internal pintle should get sucked towards the carb side and closes (but not entirely) and at cruise (lower vacuum) it sits in the center and is more open.
I also discovered an adjustable PCV valve which would be perfect for setting the PCV to work at idle with a big cam but there is no adjustment for the cruise setting, they just say it is formulated to be correct by calculations based on the idle vacuum setting.
http://mewagner.com/?page_id=444So, what do you guys think? Am I over thinking, under thinking?
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