Pressure relief valve scavange side

neilca · 07-23-2011, 05:58 AM

I was talking with a Porsche racing "expert" yesterday and he was telling me that one of the pressue relief valves in the engine was to relieve the vacuum developed by the scavange side of the pump. I did not raise the BS flag since I was standing in his shop and frankly I wasn't sure.

So is there a relief valve for the scavange side?

Henry Schmidt · Henry Schmidt's Garage

Etiquette has it's value but I think I would have thrown a flag on that claim.
Here is the oiling diagram for an early 911 engine. You decide if either valve affects crank case vacuum.

neilca · 07-23-2011, 06:05 PM

I had studied that diagram in the past and thought I had it right.

But Henry, why the two pressure relief valves?

Walt Fricke · 07-24-2011, 06:15 PM

Neil

One of the reliefs sets the oil pressure. The other opens at a higher pressure, and is there as a sort of safety in case the main one gets stuck, or can't shed enough oil quickly enough. All to protect the oil cooler, I imagine.

The diagram shows a 911 engine without a remote (front) oil cooler. Where there is such a cooler, the thermostat which decides if the front cooler gets hot oil also has a pressure relief. But generally the scavange side has no need for a pressure relief, because the piping ends up being open to the atmosphere in the external oil tank, after it has passed through the oil filter.

On the 2.7s, the tank was vented via the filtered side of the air box, after the air filter. Later CIS cars vented through the rubber boot between the two sides of the CIS. I think the 3.2s vent in much the same way. The MFI and carbureted cars must have vented to filtered air somewhere on the air filter system.

At some point, the newer (like the 996/7? Or the GT3?) motors were set up to have negative pressure (a vacuum) in the crank case. To deal with this, it appears that crank seals were installed the other way around, so as to prevent gunk from being drawn into the case or something like that.