Crankcase pressure under boost conditions with a Weiand 177 blower.

[Larry R]

I have a Chevy 350 with a Weiand 177 blower on it, as well as a Holley HP EFI throttle body on top of the blower. Everything was either new or rebuilt, but I'm getting what appears to be oil leaks from the engine. So this led me to wonder about pressure in the crankcase. I have a PCV system on the car using the port on the back of the throttle body. This port sees vacuum all the time as it is above the blower. Max boost I expect is 8 to 10 PSI. Has anyone dealt with issue and, if so, what solution did you come up with? Thanks, Larry

[andrewb70]

Hey Larry! I hope your calling fan setup is working out well!
How is the PCV system routed? Is it possible that you're getting some blowby when under boost? Andrew

[Larry R]

Andrew: Yes, my cooling fans are working great. In fact, we took my '57 on a road trip to Nova Scotia to visit friends and it worked flawlessly. It rarely got over 180°F and in the cooler weather it did not even get there. So I'll change some of the parameters to get the temperature where it should be. But my laptop has decided not to talk to my ECU, so I need to take care of that!

My PCV system runs from a PCV valve in the RH valve cover to the port on the Holley throttle body, (sees vacuum all the time). On the other valve cover there's an oil fill cap that has openings in it to let air in with some fine wire mesh in it. I'm not getting oil around the top of the engine, just some dripping from underneath. The engine is quite likely getting some blowby under boost, but I really have not pushed it too hard as it only has 750 miles on it. Thanks, Larry

[andrewb70]

@Larry R: Personally I'd unhook the PCV for now and see what happens. It's also quite likely that the PCV is acting as a vacuum leak and throwing off your tune. Andrew

[81 TransAm]

I was using a convectional PVC system. This type of system does nothing to create vacuum in the crankcase. I put a MAP sensor on the crankcase and I was getting positive pressure in the crankcase with a conventional PCV system. And I was always getting oil leaks. No mater how many times I sealed it up, it'd always leak oil. I had to keep mats under it to soak up all the oil.

So, this is what I did to fix the problem. I got a fixed orifice PCV valve. I then put a one way check valve in the line between the PCV and the manifold so it could only pull vacuum in the direction of the manifold. I then put a restriction in the vent line going from the other valve cover to the breather vent in the TB. I then played with the sizes of the PVC valve and the restriction in the vent line until I got about 7" of vacuum in the crankcase at idle. Then I "T"ed off the vent line and connected an electric smog pump. And then I put an one way check valve in this pump line. As I'm driving around, the crankcase gets it's vacuum from manifold vacuum, and when crankcase vacuum drops below 3.5" or I go to WOT the ECU turns the smog pump on and keeps the crankcase vacuum around 5" of vacuum. Since doing this all my oil leaks have stopped and the mats are now gone. And as a bonus the vacuum in the crankcase helps ring seal.
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[Gaz64]

I was using a conventional PCV system. This type of system does nothing to create vacuum in the crankcase.

PCV is not designed to create vacuum, it's designed to pull fresh air in from a filtered source to mix with blowby to be burnt. The fresh air filtered source is a large hose to the air cleaner, the larger the better. A half decent system will have 1.5-2 inch hose to a breather air oil separator. Oil can drain via a small hose back to the crankcase. I'd add a 2 psi pressure switch to the crankcase as a warning light circuit, in case of smog pump failure.

[81 TransAm]

I have an LED warning light as well as a warning on my 6.86" Pro Dash that will let me know if crankcase vacuum drops. My LSx engine is not as sealed as my SBC was. I only get around 2-3" of vacuum with it. I'm guessing it's leaking at the knock sensors in the lifter valley.

[Larry R]

Thanks for the input. I'm going to setup two crankcase ventilation systems and see it this works. If not I'll go to the next level, probably with a vacuum pump. There is a port on my TB that's for power brakes, (venturi vacuum). I have a hydroboost power brake unit so I'll use that port for one of my inputs to the engine, (above the blower). There's also a port below the blower that I can use as well, (manifold vacuum). I'll setup the port in the intake manifold with A PCV valve and a check valve from the valve cover. I'll tee off the hose from the PCV valve before the check valve that'll go to the port on the TB. That hose will also have a check valve on it to prevent back flow. If I'm explaining this correctly, I'll have vacuum being applied to the crankcase both under vacuum conditions as well as under boost. The input to the crankcase will be a breather cap on the opposite valve cover. I could connect the input to the air cleaner, but since I have a ram-air system to the intake I wonder if that might cause problems. I forgot to add there will be a catch can on the line from the PCV valve. Then the line will have a tee with a check valve on each leg to their respective destinations.

[Gaz64]

I have an LED warning light as well as a warning on my 6.86" Pro Dash that will let me know if crankcase vacuum drops. My LSx engine is not as sealed as my SBC was. I only get around 2-3" of vacuum with it. I'm guessing it's leaking at the knock sensors in the lifter valley.

I'm glad you have a warning setup. Some engines can't create much vacuum due to the type of crank seals.

Thanks for the input. I'm going to setup two crankcase ventilation systems and see it this works. If not I'll go to the next level, probably with a vacuum pump. There is a port on my TB that's for power brakes, (venturi vacuum). I have a hydroboost power brake unit so I'll use that port for one of my inputs to the engine, (above the blower). There's also a port below the blower that I can use as well, (manifold vacuum). I'll setup the port in the intake manifold with A PCV valve and a check valve from the valve cover. I'll tee off the hose from the PCV valve before the check valve that'll go to the port on the TB. That hose will also have a check valve on it to prevent back flow. If I'm explaining this correctly, I'll have vacuum being applied to the crankcase both under vacuum conditions as well as under boost. The input to the crankcase will be a breather cap on the opposite valve cover. I could connect the input to the air cleaner, but since I have a ram-air system to the intake I wonder if that might cause problems. I forgot to add there will be a catch can on the line from the PCV valve. Then the line will have a tee with a check valve on each leg to their respective destinations.

Sounds like you're on your way. Gary

[Larry R]

Gary: Thanks for your input. As I was implementing my PCV system it occurred to me that one way valve on the line to the venturi vacuum port is not needed as it's always in vacuum mode so there would be no back flow. I do have a one way valve on the line to the intake manifold. Since the one way valves have a threshold before they will let something in, not having it seems like a small improvement. I haven't driven the car yet, so I cannot tell if it works, but I will soon. Again, thanks, Larry