I've been looking at a few of the more popular oil seperating systems for boosted applications. One common theme I see is a 3 port can with one line going to the inlet tube pre turbo. This line is suppose to see vacuum at the turbo inlet which in turn; pulls a vacuum through the catch can which is often connected to a valve cover. The theory is that the vacuum seen at the pre turbo inlet tube will act like a vacuum pump and draw crankcase pressure out of the valve cover. One popular manufacturer of oil seperator systems states that there is not any appreciable vacuum pre turbo to place a vacuum on the crankcase.
I'm hoping that the more technical guys here, step in and clarify this. Are the claims hogwash or credible?

Here's a post from one manufacturer along with pics. I know these pics are shown in the oil catch can sticky on this forum so some are probably familiar.

https://www.gm-trucks.com/forums/to...e-engineering/?do=findComment&comment=2074141

 

Not sure what problem/benefit you are trying to solve here? You want extra HP or just pull fumes out of the valve cover? It is clearly understood that a lower crankcase pressure ads horsepower. In fact, dry sump systems use a roots style lobe on the suction side to not only evacuate oil but to also lower crankcase pressure. Why would you want a lower pressure (vacuum) in front of your turbo intake? That is the same thing as running in a high altitude environment! If I am misreading your question my apologies.

 

It's not to fix any issue but more of a curiosity of mine. These companies claim that placing a fitting pre turbo and running to the valve cover or valley creates a vacuum on the crankcase. The idea is to mimic the function of a vacuum pump but to a lesser degree.
My question is how much vacuum can possibly be pulled from the pre turbo inlet pipe? Gimmick or credible?

 

The air coming out of the compressor has to come from somewhere. If there is a tube on the inlet side there could be a vacuum (low pressure). Did I hear that and air filter helped create this vacuum?

When my car was on the Dyno we hooked a shop vac to the oil fill on the valve cover and data longed the crankcase pressure though the dip stick (it also had a small air filter in the valley). It did pull a small vacuum, difference in power production on my car was negligible. They did say some cars pick up a lot.

 

There should be a vacuum depending on the air filter restriction if any. I'm just curious if there's enough vacuum to pull out any appreciable amount of crankcase pressure.

 

AFAIK, there should never be anything less than atmospheric pressure at the turbo intake. (Greater than atmospheric is even better). Remember, the turbo has a pressure ratio. Any loss in pressure at the intake is multiplied by the pressure ratio (PR) for an even greater loss in boost. If intake air is 2 psi lower than atmospheric then a 2x PR turbo will lose 4 psi in boost! I realize that plumbing or filter issues could lower ambient pressure but I do everything I can to avoid that. My plumbing goes to a high pressure area of the car. They are trying to introduce a very slight pressure reduction in the crankcase by lowering boost and fuel by some multiplier! The logic escapes me.

 

You lost me. Lowering boost and fuel by adding a fitting in the pre turbo piping?

 

You are adding a fitting there because your system is so poor it is below atmospheric pressure.

 

Doesn't air flowing past a venturi create a vacuum? The pre turbo fitting being the venturi.

 

Yes. Where is your venturi?

 

The pre turbo fitting threaded into the inlet tubing. Did you view the diagrams in the link I provided? The hose going from catch can to pre turbo piping is attached to a barbed brass fitting. The fitting is threaded part way into the pre turbo piping.

 

Ok, you got me to go over and look at the proposed system in your link. Darn, I didn't want to waste my time. I see where you have the venturi and yes that will create a low pressure source for whatever you want to use it for. But a venturi LOWERS pressure. So you spend $$thousands to increase pressure then you add a restriction in the middle of it?

 

It's not my system. The pictured system is used by 3 different catch can manufacturers. It peaked my interest.
I'm not sure how much of a restriction a 3/8" od brass tube sticking into the pre turbo piping maybe 1-2 inches is.

 

Maybe I missed the point of this whole thing. “Catch can” meaning a street driven system to keep your engine bay clean. LOL.

Great pains are taken to have a turbo system as free flowing as possible. This idea adds a restriction in that system. It has to be a restriction to get a lower pressure. I was not thinking in terms of street driving. I don’t want to give up boost and fuel. All the best

 

A catch can captures a percentage of the moisture, oil and contaminants escaping the pcv system. These would otherwise end up in the intake track, in intercoolers, piston tops and cause valve coking. It's a real issue especially with direct injection vehicles. Boost just seems to make things worse. I use a catch can to catch as much of this crap as possible. I don't like oil in my intake or in my combustion chamber. I spend thousands of dollars for performance only to have oil degrading the octane of fuel being introduced into the combustion chambers. Various auto manufacturers are now installing oil separators in new cars in an attempt to reduce the amount of crap. I went with a different system than in the link I provided.

 

I don't drive a turbo on the street except for my stock diesel truck. Dry sump on my competition car takes care of all the above.

 

Or a vacuum pump

 

Buick Grand Nationals have the PCV plumbed at the turbo inlet.

ks

 

Yes, a venturi in the turbo inlet will give you a vacuum to operate off of. Even a good filter will have some restriction to create a low pressure area.

You could avoid the whole coking/octane reduction/whatever by spraying water meth instead of doing the catch can.

It's clear you have been reading catch can literature. I personally don't believe the hype, there are an awful lot of very high performance OEM vehicles that get by fine without them, make it through their warranty periods fine, and well beyond. The pics you see of the coked direct injection vehicles are from an era when that tech was early in development in gasoline applications. The OEM's are using reversion during the overlap process in the intake stroke to keep DI systems clean. If you are talking about port injection or carb the fuel will keep things tidy.

Burning the engine blowby products is really not an issue, its probably better for the environment than landfilling that shit. Just looks bad if you open your engine up. In order to reduce your octane to where you would notice you would have to have a LOT of blowby, think of your oil usage vs fuel usage.

If you are going to pull vacuum off the inlet side of the compressor a catch can does make sense. Have you considered using an exhaust based off evac system?

 

Agree with BEN mostly, although I will add ALOT of industrial/ ag/ commercial engines have catch cans installed. John Deere has a seperate crankcase filter and sensor now. They all route back to the turbo inlet, like BEN said, there's a pressure drop through the air filter, so there's already some type of vacuum.
I've also seen guys run the hose to the airbox pre filter. Not as much "vacuum" there, but plenty of flow and speed to pull fumes, and the filter absorbs the vapor. Beware though that your filter will get really dirty alot quicker.