adding a 2nd PCV valve

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Rapid Robert

Rapid Robert

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I just saw something about adding a 2nd PCV valve & richening up the mixture for more flow which would certainly help me in my restricted 2bbl rochester class. first I ever heard of this but if legit it would give me a tremendous advantage over the chebby boys in my hobby stock class. 360 2bbl 6K rev limiter. thank you for your time. RR
 
Rat Bastid said:
I know there are some Super Stock guys doing it. It has some benefit.
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I'm not knockin it.....just never tried it. Never know what might and might not work...
 
All it does is add bypass air. The same could be achieved by drilling the t/blades. Typical PCV orifice size at opening is equivalent to about a 7/64" hole. A 0.078" hole in each blade would equate to that size.
 
Rapid Robert said:
I just saw something about adding a 2nd PCV valve & richening up the mixture for more flow which would certainly help me in my restricted 2bbl rochester class. first I ever heard of this but if legit it would give me a tremendous advantage over the chebby boys in my hobby stock class. 360 2bbl 6K rev limiter. thank you for your time. RR
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I get you.
but I think it won't work.
Here's why;
Look at the size of your port. And hose. Now cut open your pcv and see how small that orifice is. It's pretty small. Even a second one will not max out the hose/port. Maybe six of them, just guessing.
In order for this to work you would want the area in the PCVs to at least equal the area of the port and if you did that, the engine would not idle. It seems like it would be a great idea, but you would need to engineer a WOT air bypass, so it will still idle.
And after you get that done, realize that you are sucking oil-laden blow-by out of your crankcase, together with whatever comes thru the breather, and finally, how much oxygen is ultimately gonna pass thru that little port at WOT, considering that the air flying by it , is at 90* to it! The turbulence might just kill the gains. or worse.
And double finally, that oil-laden air may not play nice with your ignition timing and fuel-octane. Many times, it leads to detonation.

Now about that brake booster.....
 
AJ/FormS said:
I get you.
but I think it won't work.
Here's why;
Look at the size of your port. And hose. Now cut open your pcv and see how small that orifice is. It's pretty small. Even a second one will not max out the hose/port. Maybe six of them, just guessing.
In order for this to work you would want the area in the PCVs to at least equal the area of the port and if you did that, the engine would not idle. It seems like it would be a great idea, but you would need to engineer a WOT air bypass, so it will still idle.
And after you get that done, realize that you are sucking oil-laden blow-by out of your crankcase, together with whatever comes thru the breather, and finally, how much oxygen is ultimately gonna pass thru that little port at WOT, considering that the air flying by it , is at 90* to it! The turbulence might just kill the gains. or worse.
And double finally, that oil-laden air may not play nice with your ignition timing and fuel-octane. Many times, it leads to detonation.

Now about that brake booster.....
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The guys doing this stuff in Super Stock and such don’t use a cheap assed OE type PCV valve.
 
I have done it in NHRA SuperStock, drilled the top of 2 breathers, and mounted them in each valve cover with a power brake check valve in each hole drilled. The AMX I race is limited to the stock Carter AFB 500 cfm carburetor and any more air I can get in to the engine, even if it is dirty blow by, helps .
 
The jetting was already rich, so I didn’t change. I went to my U-Pull It, and walked around and picked some up. Some of the GM vehicles have a 3/8 hose nipple on the parking brake check valve, so that is what I picked up. I got the valve and grommet. right from the power brake booster.
 
@Rapid Robert Adding a second PVC valve must still have a valve cover breather. Your attempting to vent both sides of the engine with the PVC valves but the engine still needs air to get into the crank case.’

If there is a plus to this, I really don’t see it. Or much of it. I get keeping vacuum on the crank case will add power, but I normally see this on a drag engine. The amount of power can be substantial on a drag engine with the vacuum pump. But a simple engine vacuum via a PVC? If there is power to be gained, I don’t see a lot. Though I wouldn’t personally know. I do see a cleaner engine inside though.


Rat Bastid said:
I know there are some Super Stock guys doing it. It has some benefit.
Click to expand...
Dual PVC? Do they run a vacuum pump or can they?
 
where I was going with this is since I am restricted to a 2bbl that additional air/fuel would be a gain & I heard something about doing this. it does sound (quite) a bit out there but I wanted to pick the brain trust. & it wasn't that I wanted to evacuate the crankcase more but to add air (& fuel) for more power. I am limited to 9-1 & race fuel is allowed so I think I'm good on detonation. RR
 
Rapid Robert said:
there is a u pull it closeby. do you plug the same end of it into the carb that goes into the booster?
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I would see if the motor will even run at a low enough idle speed with all that bypass air before I went to too much trouble. Just vent the vacuum port you plan on using and see if Idle is still doable. This could also cause fuel distribution problems at wot so be careful. Time might be better spent optimizing the rochester 2bbl. that you have or finding one that is better. We work with those 2bbl carbs on the dyno alot and sometimes find big differences in power between carbs or by fixing problems in the carb you already have. A mopar with a rochester 2bbl racing circle track is a rare breed.
 
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with this cam I will be idling over 1K I'm sure. I probably should get this thing going before going off on a tangent!
 
Same approach as the cup engines built for restrictor plate tracks having the intake manifold castings so porous that it was a built in vacuum leak. Allows more air into the engine, since the carb was restricted by the plate. Nobody cared what the engine does at idle, just what it does at WOT at places like dega and daytona. Jet accordingly.

If you can't find a way to get more air in through the normal path, through carb, get it from somewhere else.
 
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