The question is are some carbs calibrated to run with a PCV and others designed to run without a PCV valve. So a race carb would be different than a street/strip carb. i did a some searching and learned GM did use different valves in HP motors. also a lot of questions and no definitive answers. Cheers!
When i was a kid i would plug it and put a leaner running carb off a 350 or 400 on the 455 BBB or 454 cheby.
Black magic. PCV valves are interesting in how they are internally tuned. They do not pass vacuum* at idle and they do not pass vacuum* (allow suction?) at WOT, but only in a middle range. Different HP engines typically had different idle and cruising vacuum ranges, thus differences in specified PCV valves. Unfortunately there were a bazillon different combinations that GM (and Ford, and Chrysler, etc) used and the aftermarket basically pigeon'd holed them into one part number, of which I have no idea what the operating range of them are or where to even look up those values. When I went to a lower engine vacuum cam, I got a V6 PCV valve with the assumption the V6 would be expected to be working harder and at lower vacuum ranges at part throttle (at least that was my observation when I ran a 231 V6). *apart from minor leakage from being an imperfect seal.
1. The difference a PCV valve makes is most noticeable at idle or off-idle. A PCV valve restricts the flow at idle, but does not prevent flow at idle. Therefore, a carb intended for use with a PCV valve will be ever-so-slightly rich at idle to compensate for the air/fumes coming through the PCV valve. 2. Yes, carbs are calibrated to run with (or without) a PCV valve. The tuning difference is fairly minor, and contained within the idle circuit. Any carb intended for street use will be set-up for a PCV valve's effect on idle mixture. Carbs intended for race use will not be calibrated for a PCV valve. Like all carbs, the calibration of EVERY circuit including the idle circuit is somewhat suspect when installed on a vehicle that doesn't match the factory or intended calibration points. 3. As stated, there's a zillion PCV valve calibrations, and Z-E-R-O source of info about them. The best you can hope for is to pick one intended for an engine of similar displacement and compression and cam timing. If you can't find one from a similar application, trial and error is the only real option. 4. There is (was?) a company making an adjustable PCV valve, but at about a hundred bucks a pop, I can't recommend it.
The GM shop manuals I've seen from the late 60's give the size range of PCV's in the various applications. The orifice size is particular to the combo. Can't speak for other manuals.
Very interesting thread and some really good information here. I would be interested to know what influenced GM's engineers to alter the leakage rates (if that's the term) Compression, Cam timing, Carb size (or throttle body if EFI) and /or other forms of crankcase breathing e.g sealed or breather caps on the rocker cover. If going from a sealed oil cap to a K&N style breather as I'm sure many of us have done would it be wise to change leakage rate" or delete the PCV valve?
The purple GM PCV valve on Ebay is going for 65 bucks! http://www.ebay.com/itm/Purple-NOS-...2-Z28-1968-1973-Corvette-Camaro-/321388869910 got some good advice at the track this weekend..research thru a FRAM catalog. AC Delco PCV Valve Genuine NOS New In Box AC PCV CV746C Part # 6484525 Chevrolet Small & Big Block SBC BBC Special High Performance Applications Genuine GM NOS CV 746C >>>>>>>>>>>>>>>> <<<<<<<<<<<<<<<< New Old Stock - New in Box - PCV Valve for crankcase exhaust emissions This is a genuine AC part still in its original package Hard to find PURPLE PCV Valve for the following performance vehicles PCV stamped with “AC SPARK PLUG TYPE CV-746C” 1973 Camaro Z/28 1972 Corvette 454ci 465hp 1972 Corvette LT1 330hp 1972 Camaro Z/28 1971 Corvette 454ci 465hp 1971 Corvette LT1 330hp 1971 Camaro Z/28 1970 Corvette 454ci 465hp LS5 1970 Corvette 350ci 350hp L46 1970 Camaro 350ci 1969 Corvette 427ci 430hp L88 1968 Camaro 302ci Z/28 1968 Corvette 427ci 430hp L88
just bought the same part number valve on Amazon for $6.00. oh, it's not purple, i'll live with my $59.00 Cheers!
GM ALWAYS had a fresh-air inlet to any engine with a PCV valve. If the cap was sealed, there was another vent present. DO NOT delete the PCV on any engine driven primarily at part-throttle.
My info was out-of-date, or outright wrong. At a hundred and thirty bucks a pop, I can't recommend it.
I for one, will wait to here from someone who actually has a M/E Wagner Dual Flow Adjustable PCV Valve on their car before taking any recommendation either way. This is an interesting quote from Rebel Rodz Magazine "At $129.00 the M/E Performance valve is far more expensive than a generic pcv valve, but if you count all the trips to the parts store to try yet another valve, and all the beer consumed in pure frustration after finding out that the motor STILL didn't run quite right, it might actually save you a few bucks. Besides beer shouldn't be consumed in anger." I'm in the middle of another 455 build right now and if I decide to use a pcv at all it will be a M/E wagner, then I will be happy to post my results. Larry
Update! I was just looking at a thread posted on Steves Nova Site and they worked out a group purchase where the price dropped to $96.75 and free shipping if 6 (or more) of their members bought in. That was back in 2013 though. I will contact them tomorrow and inquire about it. If they are willing to negotiate a similar deal, then I will have to run it past Jim Weise for proper consideration. Even if it all falls through I am willing to be the first to try one. While I am busy building my Stage 2, I will try it out on one of my Corvettes. Larry
You're right. I was/am a parts guy my whole life. What I found interesting was the difference between the aftermarket and factory parts. When working for Ford the PCV would be listed for each vehicle, but also would have a different part number depending on the mileage of said vehicle. How about that? If your car has more than 60k miles it would take a different valve. Over 100k, different #. Difference in those same engines with more miles? Vacuum. Look up a PCV in the aftermarket. One fits all...more or less. Interesting thread.
I spent a half hour on the phone today with M/E Wagner Performance discussing the technical aspects of their adjustable pcv. Very knowledgeable and helpful. Though their pcv is normally not useful on a "Race Only" vehicle which would benefit from a vacuum pump for evacuation and ring seal, their adjustable unit has everything covered up to that level. They include an alternate spring for the more aggressive cammed engines with less vacuum and also have the ability to convert from dual stage to single stage adjustments depending on vacuum level. There was no hard sell at all and by the time I had finished talking with him I ended up ordering 2 of the adjustable units along with 1 remote mount. One unit to be used on a customers build I am finishing and one that I will be using on one of my cars to evaluate. I was also impressed with "If not satisfied within 30 days, full money back guaranty". They will be getting back to me with about the possiblility of a group discount.
Very interesting! Such a simple part that's not given much thought, at least I never did! Curious to see how it affects weekend drivers, especially with a decent cam with lower than stock idle vacuum.
I find it hard to believe that a PCV could make much difference in how a motor runs. It is basically just a small vacuum leak as far as the motor knows. Adjust the fuel mixture on the carb, if necessary. Once you change cam, etc fuel mix is out the window anyway......
Once you understand the "PCV Valve Basics" which is an easy read under the Technical heading on the www.mewagner.com website we can realize it's importance. The PCV Valve's main purpose is NOT to improve any carburetors fuel mixture in any manor. Its role is far more important than a "small vacuum leak". In case anyone doesn't have the time to go their website and do the reading, the main function of a PCV Valve is to "Remove Moisture and Harmful Acidic Vapors", PERIOD! Without some type of crankcase ventilation you end up with sludge in your oil pan, I can attest to that from past experience of which I will attach a photo I took proving it. At full throttle or near zero vacuum the PCV system has little to no effect on evacuation, and for track cars seeing mainly WOT, a vacuum pump/pan e-vac of some design over a PCV Valve is recommended. I should mention while talking to ME Wagner yesterday that we discussed the importance of incorporating an oil separator of some type into the system. Also, measuring the amount of oil collected in the oil separator canister can be a helpful tool in determining how well the PCV Baffle System is operating. In closing, does a PCV Valve produce a vacuum leak (The M/E Wagner being variable )? Of course it does, but as any experienced engine tuner can tell you, adjusting and or setting the air/fuel mixture at idle and thoughout the entire fuel curve should be done AFTER all other engine parameters and settings are established. Larry
