Oil Catch Cans with the twins;, Yes? No? 1? 2? what say you?

 

Why not exhaust based crankcase evac?

 

never thought about it and quite frankly I am unfamiliar with the workings. Wouldn't this require welding bungs into the headers?

 

Yes, you weld venturi tubes into the exhausts, after the turbo though so it isn't pressurized.

 

The venturi tubes would go into the downpipes.

 

Ok, simple enough, what is the benefit of this over catch cans, other than not needing to clean the cans?

 

Less to fool with and works great. Pulls a vacuum on the crankcase instead of just venting the pressure.

 

The only downside to them is the check valves at the exhaust tend to burn out if your cruising a lot, which I don't think your car will have a problem with. No cans to mess with cleaning, less visible plumbing, increases vacuum with RPM, all good stuff for race car.

 

Since I am running twins, this would only need to be mounted on one side, correct?

 

Double your pleasure, double your fun! Put one in each valve cover and feed to each downpipe.

 

I know this may be an ignorant question, but as I said I am totally unfamiliar with exhaust scavenging. But if you develop a vacuum in the crankcase via the heads (valve covers) where does the cr4ankcase get it's make up air from?

 

It doesn't, that's why its a vacuum. It doesn't need to have flow, stick your hand over your vacuum cleaner while its running, it will still suck even though there is no air movement.

There is gas expansion occurring in the crankcase, there is pressure and gas coming past the rings. You are trying to remove that pressure. Pulling a vacuum in the crankcase helps with ring and gasket seal, as more stuff is put into the crank it gets pulled out via the scavenging. The negative pressure will remain.

The method of pulling the vacuum doesn't matter so much, could be intake vacuum, a vacuum pump or via exhaust scavenging. What your doing with the exhaust venturi is creating a negative pressure area via flow velocity, seems like witchcraft but it works the exact same way your carburetor does.

It's a strange thing to think of, but a vacuum is just the absence of gasses. We have atmospheric pressure, when you remove the gasses that make up the atmospheric pressure everything wants to fill that void, because its being forced in by that atmospheric pressure. Your goal is to keep that void so everything try's to fill it and can then be removed by your mechanical vacuum.

 

Hi Ben, Vacuum I do understand, maybe that is my problem since I am used to systems pulling 27 - 29"hg. I also understand that extreme vacuum like most other things always have a tendency to expose a weak link, in this case seals, gaskets or tin valve covers or oil pans. I guess one thing I do not know is just how much vacuum does the crank case see in these setups? would a guess of 14-20"hg be out of line?

Now for another couple of questions,

1. my googling has revealed the use of "check valves" in these system but no explanation of what they do. Are they specifically to prevent back pressure pulses form pressurizing the crankcase?
2. Where is the best place to draw from, Valve covers?
3. Do the valve covers need to be baffled, or does it matter?

 

How much vacuum it pulls will depend on the combination. How much blow by, exhaust flow, fresh air entry, etc. I don't think most exhaust scavenging systems pull more than 4-5inhg. The ones I've used pull a little at idle and goes up with RPM, but never anything like 20", at least on mine. If you pull too much vacuum, you need to introduce fresh air or it could draw air in past the main seals, dry them out, and wreck em. I think Moroso sells a vacuum check valve that will let fresh air in at a set vacuum point so you don't pull too much, altough I think those are usually used on engines with mechanical vacuum pumps.

 

Most pull from the valve covers, and yes, you'll want some form of a baffle. The check valves do just what you said. All sorts of drag cars have used these for years, and I've never seen a vacuum regulator on any. You don't actually need a vacuum, just a lack of pressure, so there's really no harm in allowing fresh air in, unless you're specifically after the vacuum benefits. Some also run the exh scavenge after a catch can/oil separator, so there's that option as well if oil control becomes an issue.

 

https://www.summitracing.com/parts/aaf-all34145
https://www.gzmotorsports.com/VCV101A-Vacuum-Control-Valve.html