I like Ben's definition, except for where he said the pressure in the exhaust is greater than that in the cylinder. If this were true, your exhaust wouldn't flow out.
I would say, as the exhaust gas flows into the exhaust chamber, and the chamber is unable to accomodate the full amount of gas without offering resistance (as the gas is being compressed in the exhaust) that is back pressure.
In other words, a 4 year old kid doesn't exert more force than Stone Cold Steve Austin... but he does exert some force on him.
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I <3 forced induction.
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