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High Overlap Valve Timing & Reversion
I understand that at higher RPM the momentum of the exhaust gasses is used to start the air flow through the intake valve.
I also understand that at idle the intake manifold vacuum and perhaps a bit of exhaust pressure pulls/pushes exhaust gasses back into the intake, and we call it reversion flow. I had an intake gasket suck oil in, and there was an oil film all through the intake, out the throttle body, and onto the MAF sensor. So I know this is real, and have no dispute. Top notch tuning experts say that high overlap valve timing also causes O2 sensors to get a false lean reading, at idle. They say that un-burned fuel/air mixture goes out the exhaust valve, and the oxygen is sensed by the O2 sensor, indicating a lean condition. However this is not a correct reading, because the fuel/air mixture was never burned. What is burning in the cylinder can actually be rich and still report a lean condition. I'm a bit confused how exhaust gasses can be getting sucked back in through the exhaust valve at the same time some of the fresh air/fuel charge is exiting the exhaust valve. I suppose gasses can go one direction and then the other within the overlap period. Anyone ever hear of a theory on this? |
It's not a theory, it's a fact. If the valve is open on the piston down stroke, it's going to creat negative pressure in the exhaust tract, and suck hot exhaust gasses back into the cylinder.
Well designed headers will create a negative pressure are at the collector, and that helps control reversion. IIRC, Doug Thorley used to build anti-reversion headers. GM get's around this by using a very wide LSA on a stock engine - around 115*. The intake track gets contaminated on the upstroke, when the intake valve is still open, but the exhaust valve is fully closed. |
The existing exhaust gases pull fresh air/fuel mix across the chamber and exit the chamber with the exhaust gas.
Now the "combusted" gas is contaminated with air/fuel. You then get high O2 readings, hence lean. |
If your intake gasket is sucking oil into the intake manifold, or the cylinder, then I would suggest that the intake gasket needs to be replaced. Exhaust gases getting sucked in due to negative pressure is one thing, but oil should not get sucked in to the manifold. That is a gasket issue. Unless of course you are creating so much negative pressure that you are sucking oil past the piston rings and all the way up into the manifold :eek:, not likely.
I suppose one other potential reason would be that oil is being sucked up through the pcv system and then back down into the manifold (I am not very familiar with the all of the functions of a throttle body system), but again, the intake manifold gasket seems the most likely. |
One cylinder affecting another, especially with a single plane intake, or so I've heard.
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I had always assumed that the low intake vacuum and reversion flow at idle was happening during the valve overlap (both open).
I think Bob has set me straight. It is happening when the intake valve is open ABDC on the compression stroke, because fuel air charge is pushed back into the intake. This makes more since now. |
There are a couple of different aspects of reversion. Both are related to the momentum of the comparatively dense intake charge.
With a camshaft that is designed to extract peak power at high RPM, it leverages the momentum of the intake charge through the port to continue cylinder filling ABDC. At low speed (idle) there is insufficient momentum to prevent reverse flow at the intake valve. At higher RPM the intake charge has higher velocity and therefore more momentum. When the intake valve closes the column of fuel/air compresses against the closed valve. When the momentum is gone, the column pushes(springs) back upwards through the intake port. This phenomenon is subject to harmonics, so it will behave differently at different engine speeds. Intake manifold designers can use this pressure wave to increase power, or implement designs to minimize the effect. With individual runner systems there is often an engine speed at which you can hold your hand an inch above the air inlet and get wet with fuel. This is the most obvious and visible (with a timing light) example of reversion. |
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