[alpha1]

Originally Posted by IshaanIan

that causes a low pressure zone in the manifold

Why should there be any instance when the pressure at exhaust manifold is lower than atmospheric? From what I can reason out, the pressure at the exhaust manifold will be much higher during the exhaust stroke, however once the valve has closed down, it has to become equal to atmospheric.

[IshaanIan]

Originally Posted by alpha1

Why should there be any instance when the pressure at exhaust manifold is lower than atmospheric? From what I can reason out, the pressure at the exhaust manifold will be much higher during the exhaust stroke, however once the valve has closed down, it has to become equal to atmospheric.

Not sure of the specifics as I am more into the driving aspect than the engineering behind it though I would love to keep the discussion going on. However, why would it need to be lower than atmospheric pressure to pose harm? You do realize we are talking about an exhaust pipe that would be submerged in water right? Pressure under water is obviously much higher If you read my narration of my personal experience, you will note that I had to pilot my car through water was above the headlights with the bonnet nearly submerged at some parts.

PS: did some reading up, apparently there are 3 components to an exhaust pulse and the 3rd component is said to initially match atmospheric pressure but the momentum of the high and medium pressure components reduces pressure to lower than atmospheric level

[alpha1]

Originally Posted by IshaanIan

However, why would it need to be lower than atmospheric pressure to pose harm?

PS: did some reading up, apparently there are 3 components to an exhaust pulse and the 3rd component is said to initially match atmospheric pressure but the momentum of the high and medium pressure components reduces pressure to lower than atmospheric level

Actually I agree with your reasoning, but my question was different. Let me be more specific with my question

I agree that IF the pressure is lower than atmospheric, the water will be sucked in the exhaust pipe.
I also agree that IF the water gravitation head is greater than the pressure at exhaust, even then the water will make its way in the exhaust.

However, my question was different and perhaps answered by the second part of your post:
Why or rather how will the pressure at the exhaust manifold become less than atmospheric under normal engine operations?

Now you have mentioned about the exhaust pulses. Which in other words mean that there is a standing wave formation in the entire exhaust pipe and the node and anti-nodes of this standing wave refer to the points of low pressure and high pressure respectively. In this case the node, having a low pressure may encounter less than atmospheric pressure (incidentally, this is the reason why people fit expansion chambers in 2 strokes, since lower pressure at outlet is better at sucking the exhaust gases out).
But then, water will have to make its way through the anti-node too, where the pressure is much higher than atmospheric.
How will the water sneak in through this region?

In other words, have we actually encountered a case of water ingress via exhaust pipe in a running engine?