Where does the "air" in AOS come from?

[johnb]

The air-oil separator on a 944 has a connection at the top to the j-boot, which sees a slight vacuum (the vacuum increases with engine power). So it's pulling air in all the time, post airflow meter. Where does this air come from, and why doesn't it affect the AFR? Some explanations say that it's combustion gas blow-by, so maybe it doesn't contain any oxygen. Then it wouldn't affect the AFR - but it would take up space in the cylinder and reduce VE. So then why doesn't that happen?

[Tom]

The air-oil separator on a 944 has a connection at the top to the j-boot, which sees a slight vacuum (the vacuum increases with engine power). So it's pulling air in all the time, post airflow meter. Where does this air come from, and why doesn't it affect the AFR? Some explanations say that it's combustion gas blow-by, so maybe it doesn't contain any oxygen. Then it wouldn't affect the AFR - but it would take up space in the cylinder and reduce VE. So then why doesn't that happen?

Hmmm, just spit-balling here, but I'm not so sure it is pulling in air all the time. If you pull a vacuum on a wine bottle, you'll create a lower pressure in the bottle, but you'll only pull out whatever air was in there to begin with (or a small fraction thereof if the vacuum is weak). In theory, the crankcase should be the same, right? In practice, small crankcase leaks might introduce a scintilla of unmetered air, and blow-by under boost likely introduces some previously-metered air that acts as a kind of an unintentional EGR system, but the impact of both probably just falls into the overall AFR tuning effort. Probably one of the reasons the same chip can (and has) produce slightly different AFRs on two separate stock motors. (And why everyone should be tuning their own chip using our DIY tunerpro tools )

[Bergerac]

(the vacuum increases with engine power)

Vacuum decreases with power, you have to remember it will take the path of least resistance which is through the fully open throttle body. Vacuum in the crankcase helps reduce windage and stabilises the rings.

I can only assume the stock tune allows for the effect of a small amount of air and oil vapour at idle. You have to remember fuel vapour from the charcol canister is also sucked into the intake.

[johnb]

(the vacuum increases with engine power)

Vacuum decreases with power, you have to remember it will take the path of least resistance which is through the fully open throttle body. Vacuum in the crankcase helps reduce windage and stabilises the rings.

I can only assume the stock tune allows for the effect of a small amount of air and oil vapour at idle. You have to remember fuel vapour from the charcol canister is also sucked into the intake.

I'm pretty sure vacuum in the j-boot increases. The more air that flows through it, the more the AFM becomes a restriction.

I think the fuel vapor purge is only during transient conditions, either when opening or closing the throttle (I forget which), so it might mess with AFR but I wouldn't be surprised if it didn't matter. Whereas I'm assuming that blow-by is being produced all the time in the crankcase under power. Maybe it's just not enough to matter though.

[Tom]

(the vacuum increases with engine power)

Vacuum decreases with power, you have to remember it will take the path of least resistance which is through the fully open throttle body. Vacuum in the crankcase helps reduce windage and stabilises the rings.

I can only assume the stock tune allows for the effect of a small amount of air and oil vapour at idle. You have to remember fuel vapour from the charcol canister is also sucked into the intake.

I'm pretty sure vacuum in the j-boot increases. The more air that flows through it, the more the AFM becomes a restriction.

I think the fuel vapor purge is only during transient conditions, either when opening or closing the throttle (I forget which), so it might mess with AFR but I wouldn't be surprised if it didn't matter. Whereas I'm assuming that blow-by is being produced all the time in the crankcase under power. Maybe it's just not enough to matter though.

Agreed. In the little tube between the j-boot and the AOS, Bernoulli and Venturi wrote the rules. The faster the air rushes into the turbo, the lower the pressure in the tube. Bergerac is thinking about Inside the intake/motor...

As for the blow-by, it probably depends on the motor. I had to drill the top of my AOS to 1" to prevent the dipstick tube from popping up under pressure, back when I was running tons of boost. I'd guess that has 'some' impact on VE and AFRs...

[Evan]

The air-oil separator on a 944 has a connection at the top to the j-boot, which sees a slight vacuum (the vacuum increases with engine power). So it's pulling air in all the time, post airflow meter. Where does this air come from, and why doesn't it affect the AFR? Some explanations say that it's combustion gas blow-by, so maybe it doesn't contain any oxygen. Then it wouldn't affect the AFR - but it would take up space in the cylinder and reduce VE. So then why doesn't that happen?

The air comes from the crankcase, but it's not fresh air. It's mostly inert blow-by gases with little oxygen.
The Air Flow Meter has already measured the fresh air for the engine. Since the AOS feeds in after the meter, the engine's computer (DME) doesn't add extra fuel for it.
It doesn't hurt performance because the volume of these recycled gases is very small compared to the main intake charge.