I have some friends who are in the process of installing a Moroso 3 vane vacuum pump. They do not have a vacuum relief valve. I remember seeing some posts that said you could run the 3 vane without a valve. Any truth to that?

If not, I see there are some relief valves out there but I notice that they are not filtered. Any thoughts on that?

I know how my Star is plumbed and I have had zero problems with it. Trying to convince this friend to do his the same way.

Just depends how tight your engine is and how much vacuum you want to pull. Tell your friend to install the vacuum pump and see how much he is able to pull and go from there. If he finds he does need one then install it between the pump and valve cover.

He can install one of these if his vacuum level is higher than he wants. Installing it right next to the pump inlet on the valve cover is best if you don't mount it inline, or in the pump.
https://www.gzmotorsports.com/relief-valves.html

Hey Vern,

I've also heard of some guys running that pump without a vacuum control valve on it. It's my understanding it won't pull enough vacuum to need one depending on the engine, but I can't verify that. Moroso makes a really nice pump mounted valve that's filtered. I'd use that one with that pump if needed.....

Vern, private message sent.

I mounted mine on a tee fitting right at the pump inlet. Doesn't need a filter because it isn't creating a vaccum leak directly into the engine.

It's not really required. It's really just a safety device to prevent you from pulling too much vaccum. If it's set to 12 inches of vaccum and the pump only pulls 10, then it will never open to create a vaccum leak and prevent the pump from pulling the vaccum lower.

I guess the best way to know if you need one or not is to install a vaccum gauge or sensor somewhere to measure crankcase pressure/vaccum during a run. If the pump isn't pulling an excessive amount of vaccum then you probably won't need a relief valve. All depends on how well the crankcase is sealed.

I ran a 3 vane for years on alky without a relief. Dyno only showed 8-10” of vac wide open so i dont think it was needed. I run the 4 vane now with a relief that Reher-Morrison sells that goes right on the pump itself. Nice piece and has a fine screen on it to keep debris out. It’s adjusted to pull 12” with the 4 vane.