So i know how a pcv (positive crankcase ventilation) valve works in theory:
limited flow at high vacuum and greater flow as the throttle butterflys more fully open.
I know that as the rings and bores wear blowby gas increases and the pcv may need to changed to a higher flow device. I know that Ford makes a higher flow pcv valve for high mileage v8s.
I have read that somehow the 1994-1995 iron 8L V10 got by with no PCV mechanism, just a fixed sized hole.
I have read that a "Dry Sump" is a kinda pcv replacement... the vacuum pump sucks all the blowby gases away at all rpms and the crank/rods/piston undersides all move with less air to push out of the way. This saves more horsepower at high rpm than the vacuum pump consumes.
I have not thought through how a "big cam" with more than stock overlap affects pcv selection.
Big cam overlaps make idle vacuum lower. Big cams create "internal EGR" at idle making the idle rough and lopey.
What adjustment does it have that makes the custom PCV worth $127 ?
I have had a MEWagner valve on my motor for about a month, which equates to just a few drives. I first heard of it here on Moparts and decided to give it a try. I would recommend reading their website, but here are some thoughts for consideration.
PCV system is to remove combustion by-products from the crankcase. These by-products can be deterimental to the oil and parts in the crankcase. By-products in the crankcase results from blowby (ie imperfect ring seal - it can never be perfect). Blowby is a function of both the quality of the ring seating but also the size of the ring area. Larger bore motors and higher compression will naturally have more blow-by volume, assuming equivalent ring seating to other motors. This is why ring technology gets so much focus.
In the old days, venting the crankcase was done by draft tubes, which used the positive pressure in the crankcase to force the vapors out. Obviously not real good for emissions and it left oily deposit on road surface. Positive crankcase pressure may increase likelihood of oil seepage at seals and gaskets and slightly increases the work load on the motor. The PCV system pulled those vapors back into the combustion process via a controlled vacuum leak (ie the PCV valve) which creates a negative pressure in the crankcase under some circumstances. Manufacturers tune the PCV to the motor characteristics.
Motors which have an overbore, higher compression, and performance cams can result in the factory PCV valve not being a good match. In my case I have a 400/470 stroker which has a 4.375" bore, the compression is a good 2 points higher and the extra displacement does not completely cover up the overlap presented by the cam. Vacuum at idle is much less than a factory motor (10" Hg in gear in my case before the swap). The PCV valve closes at low vacuum, and a modified motor may put the PCV valve in a reduced flow area at idle, particularly when transitioning from park into drive. Cam is also a bit lumpy which causes vacuum fluctuations. Put it all together and the "controlled vacuum leak" is now a "fluctuating vacuum leak", which can make the idle itself less stable and less smooth.
Big bore motors will also need more crankcase venting under normal highway driving, and extra PCV flow can be helpful.
The ME Wagner valve has two adjustments: one for when the low flow channel opens, and one for when the higher flow channel opens (total flow is sum of both channels). I have been able to move my low flow channel opening point to below typical in-gear idle conditions and have picked up a solid 1" or so of vacuum in gear after the initial tuning (better idle vacuum with a better controlled vacuum leak). I don't have power brakes but for some folks this could be a benefit. High flow adds in at cruising conditions. Like all PCV, valve will close at WOT; vacuum pump or evac systems have an advantage here.
Of course the benefit of this setup will vary on a case-by-case basis. I suspect there is more benefit to automatic cars and stroker motors. I will be curious to hear of other's results.
