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Hi guys, my question is to do with a fully sealed engine - and in particular relates to an RB26/30.
If I have 3x scavenge stages drawing from the sump - some have said this will 'draw' oil down from the head. Others suggest one of the scavenge stages come from the back of the head drain specifically. A third idea is to T the head drain in the rear sump scavenge in order to split the duty there.
My question is a little difficult to articulate - but I am imagining the air is being evacuated by the pump at the sump, while air is 'entering' the engine below the pistons (due to blow by). This makes sense then that the air entering from below the pistons - drags the oil along with it and out the sump.
However in this sealed engine, no air is 'entering the rocker covers' and so the oil that has been pumped up there will not easily return to the sump since if it did it would need to be 'replaced by something'... ie. imagine taking oil out of an sealed coke bottle, that would 'create its own vacuum' in the rocker cover area... and that means air has to get up there from the crank case....
So! - do we need to have the rocker covers able to draw some small amount of air to promote the oil to migrate to the sump?
And what I peoples thoughts on scavenging from the back of the head vs just everything from the sump?
The dry sump engines I work with, generally have a breather to the drysump tank located on the valve cover. There is often a restriction in order to keep th vacuum up in the engine/crank case. Often the head is modified with an external drain to bring oil off the back of the head down to the sump pan.
When I was running wet sump (+ accusump) I had to have a large drain back at the back of the head or the head would fill up with oil and come out the breather. This was partly down to the 4age drain back not being compatible with predominately clockwise race circuits. This largely solved the issue.
When I went dry sump I had a scavenge stage attached to the head drain and it completely solved the issue.
At the first sign of any similar issue on my new engine I will be adding small scavenge stages to the heads.
As for the coke bottle analogy, this isn't totally relevant to the drain back scenario assuming that you don't have so much oil draining back that it is "blocking" ALL the drain back holes so air can't get up to prevent the "coke bottle vacuum" scenario (try the coke bottle trick with a half full bottle and swirling it so the fluid doesn't totally the block the hole thus allowing air to enter and you'll get the idea). If you are really getting that much oil in the head, you have a problem! Ideally you would fix the cause of the head filling in the first place however if that isn't possible then larger and/or better placed drainbacks are on option...or just scavenge it with the dry sump pump.
Oh, and don't "T" a scavenge stage...it won't work very well. Play with a Y connector and you'll get the idea, unless BOTH inputs are blocked, it just sucks from the route with least resistance.
I'll add a couple of things. Firstly it's important to understand that some engines have idiosyncrasies that need to be considered - The 4AGE and the RB both have issues with draining oil back from the head. If it was me I'd be inclined to retain the external drain from the back of the head but I'd personally run this into the sump and scavenge from here if you're only running 3 scavenge stages (imho you'll do more good this way but if you had 4 scavenge stages then I'd dedicate one stage to the drain back).
Secondly you're unlikely to want this engine to be fully sealed. Regardless of the dry sump system you're almost certainly going to still end up with positive crank case pressure under boost (admittedly this has a lot to do with how much boost you're running). This positive pressure should be evacuated via a breather (or pair of breathers) on the rocker cover back to your dry sump tank and then you have a catch can running off the tank. You can run one way valves in the breathers so that you can still pull a vacuum at low boost if you like - I did this in my old EVO drag engine.
I’m currently rebuilding a car which did run the dry sump tank in the passenger footwell
We breathe off the cam covers to the tank ( gc8 Impreza EJ20)
To free up some space I’ve considered moving the tank to the boot area, however I’m concerned about the breather hoses firstly being so long and secondly being lower than the engine in places where they would be routed through the cabin
We have run the current setup with 100% reliability for 6 years, as you may appreciate I’m concerned about moving the tank and introducing an issue.