Which PCV valve to use with low vaccum

[BrianG]

Does anyone have a part number or reference to which pcv valve to use with a small block which has a big enough cam that it produces low vacuum at idle. I have around 10 inches of vacuum at idle in park. According to several artilcles that I've read, the PCV valve should limit its flow at idle where the vacuum is usually high (somewhere near 20 inches or so stock correct???). So, if I have only 10 inches at idle, my pcv valve thinks I'm in a partial wot situation and flowing much more freely that it should. I'm not the first to ask this same question, but none of the previous threads that I could find ever really answered this question.

[BrianG]

Originally Posted by BrianG

Does anyone have a part number or reference to which pcv valve to use with a small block which has a big enough cam that it produces low vacuum at idle. I have around 10 inches of vacuum at idle in park. According to several artilcles that I've read, the PCV valve should limit its flow at idle where the vacuum is usually high (somewhere near 20 inches or so stock correct???). So, if I have only 10 inches at idle, my pcv valve thinks I'm in a partial wot situation and flowing much more freely that it should. I'm not the first to ask this same question, but none of the previous threads that I could find ever really answered this question.

Bump...Anyone?

[trac209]

To be honest I wouldn't worry about low vacuum to the pcv valve. It will still do enough of its job to warrant using it. I don't think you can remove too much crank case gas to cause issue

[BrianG]

Originally Posted by trac209

To be honest I wouldn't worry about low vacuum to the pcv valve. It will still do enough of its job to warrant using it. I don't think you can remove too much crank case gas to cause issue

Thanks for the reply. I understand what you are saying, but the reason that I am worrying about it, is because my engine produces much less vacuum at idle than a stock engine would. The PCV valve restricts but does not block flow at high vacuum condition. So, when I'm at idle and have much less vacuum than a stock motor, my PCV valve is not restricting flow. I can not say for certain that the proper PCV valve will really be of much help, but logic tells me that when I'm already low on vacuum (thus making it difficult to tune and dial in at idle) having a huge open vacuum line isn't going to help any. I agree, cleaning out as much crank case gas as possible, is not a bad thing. I'm more looking at being able to tune it properly at idle, maybe even gaining an inch or two of vacuum if possible. I think I am going to try to find an AC Delco CV-746 C PCV valve and try that (or an equivalent in Fram or some other brand). That is the PCV valve for a 69 Z-28 with a 302. I did more research last night and this topic comes up a lot more in the Camaro and Chevelle forums. I may not even notice a difference, but for a few bucks, its worth a try.

[geezer#99]

There's a test in the last post here to determine if your pcv is the right size.
http://www.hotrodders.com/forum/pcv-...on-434178.html

[im4darush]

Finding the right PCV is a chore. That is a fact. If you consider the extreme and run a straight hose from manifold vac to your breather (no valve) you will in effect introduce a vac leak because you are pulling full vacuum at idle from the crank case. It is obviously not the same as leaving the port wide open, but you will loose some vac. That is the negative. In terms of tuning your idle..if your motor is in good shape and runs well in general you should only need to go slightly richer on the idle screws to compensate. Mine is set up this way and idles rock solid at 600. I lost about 1hg on the vac gauge running this way, but I see no idle issues what so ever. Keep in mind though...my motor is stock and pulled 19hg at idle with the valve. So going to 18hg is no big deal. If you are dealing with a big cam and are trying to squeeze every little bit out of it to achieve a good idle, this may not be the way. I took this route because every valve I tried resulted in a leak from my front timing cover seal. As soon as I removed the valve, not a drop. I have one of those aftermarket breathers with nipple that open up and give you the ability to use different valves. I opened it and filled it with a scotch brite sanding pad cut to fit inside. So far I have not seen any oil at the carb. Using a valve cover with a baffle sure helps if you go this route.