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correct me if I'm wrong, but wouldn't there be positive pressure during the exhaust stroke? If the valves are open allowing exhaust to escape, and the piston is rising up pushing the exhaust out, wouldn't there be more pressure on the exhaust stroke as rpm rises(less time for exhaust to leave the cylinder)? It seems the pressure would hit 0 for only a moment before turning into a vacuum as the piston rounds TDC and begins the intake stroke.
It is actually possible to have a below atmospheric pressure in the commbustion chamber with the exhaust valves open, in a naturally aspirated engine. This is caused by the reversion of the exhaust pulse, if it is timed correctly, drawing out more gases. The cooling of the exhaust gas can also assist in dropping the pressure below ambient.
If I understand the question correctly, yes there will be an increasing pressure exerted on the rising piston as the rpm increases, as the pressure difference between the cylinger and the exhaust port/system is what expels the exhaust gas.
As Steven pointed out, it is possible to have a 'partial vacuum' in the exhaust port, not only from the pressure waves in the exhaust, but from the velocity head of the exhaust gases. That's why, when designing an exhaust manifold, both the length and diameter are important.
