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Discussion Starter · #1 ·
Hi folks. I have a 5.3l GM LM7 Vortec with a Magnuson MP112 supercharger. Do I need to have a PCV valve that is designed to handle the pressure range of the supercharger?
 

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RedLine Stage 4 ZL1 650rwhp
1971 BB Chevelle
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580 Posts
I dont know that particular set up enough to have an answer for you however..... I have A LOT of experience with that question re: 5.4L Supercharged Ford Lightnings, I've had 5 of them and at least 7 other Supercharged Fords & Chevys.

You have to remember that with a supercharger your PCV system will now have negative and positive pressure, (Vac & Boost). I can tell you with the Ford Lightnings the OEM PCV valve could not handle any more than the std boost so as the yrs went by and we all went to built motors and larger superchargers and more boost EVERYONE HAD PCV ISSUES, and i mean everyone.... Oil being sucked into the Throttle Body, Oil being sucked into the Supercharger, Intercooler, Intake, you name it, it sucked oil in....

Some shoved dimes in the line and drilled a small hole in it, others just closed it off and went to open breathers (personally I hate that because it always smells like oil). The masses went with oil separators which actually worked pretty well but you have to remember to empty them and it's nothing more than an Air/Water Separator you would see on a Air Compressor. What I found was by swapping the crappy OEM PCV valve and instead using a Ford Terminator (03/04 Cobra) PCV valve in moderate builds you were fine and had minimum oil sucking issues. However When running 22 to 25 lbs of Boost it still wasnt enough so someone designed a system that added a second reverse flow PCV valve on the other Valve Cover. Now you could Suck In or Blow Out without pushing oil up into everything....

Sorry for the long answer but you have to understand how it all works and it all depends on how much boost your running that will determine how well the std PCV will work, or if and how much oil you will suck up. Bottom line is pick up a new PCV valve and blow in it against the check and then suck in it from the other side of the valve. If it doesnt pass air in either direction you should have minimal issues, if it does pass air find a PCV valve that wont pass air in either direction. You can also get a little in line Check Valve to add to the line feeding the PCV that will only allow the pressure to go one way but remember with a Supercharger YOU WILL have negative and positive pressure so oil will tend to find a line and push up into it as the boost level increases...
 

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Discussion Starter · #3 ·
Thanks for your in-depth response, ProStreetRob. The boost on this model supercharger is only 6psi. The vacuum source on the supercharger is in the same position as the vacuum for the brake booster, which makes me wonder if this area of the supercharger never experiences positive pressure? The existing PCV valve is a stock AC Delco. It freely allows air flow from the crankcase, but marginally allows flow back. If that is supposed to be a one-way valve I conclude it has some leakage going back to the engine. So I am not sure what to do. I would not believe that the brake booster could tolerate positive pressure, and given that the boost is relatively low anyway, it seems like a stock PCV valve would work properly?
 

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RedLine Stage 4 ZL1 650rwhp
1971 BB Chevelle
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580 Posts
With 6 lbs you will be fine with a good working stock PCV Valve (y) I never had issues on any of my supercharged cars and trucks until i got into much higher boost levels. In my experience 12 to 15 lbs of boost is where oil sucking became a concern you had to deal with. Youll be fine, sounds like a Bad Ass set up you have CONGRATS 🍺
 

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Discussion Starter · #6 ·
With 6 lbs you will be fine with a good working stock PCV Valve (y) I never had issues on any of my supercharged cars and trucks until i got into much higher boost levels. In my experience 12 to 15 lbs of boost is where oil sucking became a concern you had to deal with. Youll be fine, sounds like a Bad Ass set up you have CONGRATS 🍺
That is good to hear. I believe the vacuum port on the back of the blower does not see boost which is where that brake boost comes from as well. I need to verify this. I can put a check valve in there as 64nailhead above suggests - great idea.
I am looking into putting an oil and water trap in the PCV system to keep the gunk out of the blower, and keep the water out of the crankcase as the car does some serious sitting over the winter. It is a replica 1936 Auburn Boattail Speedster built by Speedster Motorcars. It weights 3250lbs, is running about 410hp and has a limited-slip For 9" rearend. So it can scoot. But the tires are rather narrow so it looks more period-correct. It is a bit too much power for the tires. But it does an impressive burnout!
Thanks for your help!
 

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If you pull your dipstick, drill a hole, weld a 1/4 bung, reinstall the dipstick, then install a ball valve at the drain port of the catch can, then run a hose from the can to the dipstick, you can easily open the ball valve when your sight hose shows the catch can is full and lessen the mess. This can be done with 2 cans also if you have one for the boosted side. Just make sure to close the ball valve so you do not have a massive vacume leak.

If you want to be real cool you could have the thing open everytime the car is turned off using a 10 second "courtesy" relay that would open the thing using something like a power window motor then a spring that would keep it closed when the motor is not powered.
 
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