PCV and A/F Ratio

[Wreckless Warren]

I was talking to Woods Allen about his 64 P-Code. It's now a 445 Stroker with an 8V induction system. Woods has O2 sensors installed in both banks and a meter in the car that he can switch from side to side. He has done allot of jetting work to achieve a good A/F ratio.

He currently runs a Road Draft Tube and is considering going to a PCV System.  I raised the question if the PCV Valve would alter his A/F ratio, is a PCV Valve a somewhat controlled vacuum leak?

Woods raised the question on the FE Forum and all they can seem to do is argue about what the PCV Valve does to the engine related to oil, completly ignoring the A/F Ratio question.

So forget about the benifits/downside of a PCV valve related to oil. What impact (if any) could it have on the A/F ration with his 8V induction system. Thanks. ww

[jayb]

Just guessing on this Warren, because I don't run PCV valves, but adding the PCV valve may have an effect on A/F at idle.  The carburetor is going to deliver fuel based on the vacuum drop above and below it.  Addition of a PCV valve will decrease the vacuum in the manifold, resulting in a smaller vacuum signal at the carb and less fuel being delivered.  So, it seems like the engine should run leaner.  However, at idle the mixture screws on the carb are controlling the mixture, so adjustments there should get the A/F back where it belongs.

At part throttle or WOT, I would expect the vacuum leak from the PCV valve to be a very small percentage of the total airflow through the engine.  So, it should have a negligible effect on A/F at cruise or WOT.

[cjshaker]

I think the same as Jay. If you suck through a PCV valve (preferably a new, clean one ), it's like sucking through a small straw. That can't amount to much as far as A/F goes at cruising or WOT. If anything, I would guess a two size step in jetting might be called for, for an easy driven street car. But an argument against even that is I don't think Ford had to make any changes to the carburetors when they added the PCV. But I also don't use PCVs. I won't get into the politics of why...lol. So this is all just theoretical hyperbole based on an absence of factual information

[Tom Gahman]

I think a correctly working pcv will be closed at wot.(backseated).
All crankcase pressure is relieved back through the breather(s).
(if I remember correctly)

[jayb]

So this is all just theoretical hyperbole based on an absence of factual information

Well, after all, this is an internet message board 

[cjshaker]

Well, after all, this is an internet message board 

In that case, it must be fact 

[CaptCobrajet]

I think Jay is right on his assessment.  The amount of airflow through a 3/8" hole is not significant after a high idle.  Your idle circuit can be tuned with the air screws and will not affect jetting.  It very well may run a few tenths of a point leaner, but I would try it.  Two points.....first, if you run PCV, finish the job by plumbing the other VC into the bottom of the air cleaner.  When the valve closes, it has to go someplace.  Running an open breather on the other side will do nothing but suck air in one cover and out the other.  A closed system is the berries when it works right.  It helps the rings.  If you really want it to work, it needs two valves and two breather hook-ups with check valves opposite from what the PCV does.  Then, you have really good flow and they will switch out when the vacuum in the breather overcomes the vacuum in the manifold.

Back on the A/F.  If the engine is sealed up really good, it will pull a vacuum on the crankcase fairly quickly.....about 30 seconds on most of mine, and then there is no significant new air being pulled in anyway.  I use brake booster check valves instead of regular PCV valves.  They are much more reactive and have a more positive check valve.

[Monzy]

Sounds like your buddy already knows the benefits of using a PVC valve and has the monitoring equipment to check for A/F ratio changes so why not just plug it in and see the difference?   The A/F ratio may change in just 1 or 2 cylinders depending on where he plumbs it.   It would only make sense that you would want to evenly distribute the these crankcase gases through the manifold so plumbing it into the carb base instead of the manifold would be best for sending these gases to every cylinder.   If he doesn't have baffles  in the valve covers that will be very bad at high RPM runs as sucking oil up and feeding it back into the manifold isn't good.   Use a canister/filter to eliminate this even with good baffles, unless the engine just idles around it wont matter as much to have the canister/filter.  As for the change in A/F ratio it will be depend on how well those piston rings are sealing and how much crankcase gases are generated.