I'm not sure I understand what you mean by "actual compression ratio." A typical auto engine - especially going back a few years has a volumetric efficiency of 60-70%. That applies to gas, and also to diesels with a wide-open intake. Most naturally aspirated diesels only run a 60% volumetric efficiency. That means the engines can naturally draw in around 60-70% of their cubic inch displacment without help from a supercharger of some type - abeit belt, gear, or exhaust driven. The mechanical compression ratio is simply the ratio for the cubic inch displacement at it's largest (bottom of stroke) to its smallest (top of stroke). Add a supercharger that forces air in - and the "effective" compression ratio comes into play - which can be quite a bit higher.
As far as I know (perhaps not far enough?) - a engine with an 8-1 mechanical compression ratio is just that - and NOT less regardless of intake restrictions.
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Today's Featured Article - Memories of an IH Super A When I was ? up to 10, I worked on my Papaw's farm in Greeneville, TN every summer. As I grew older (7), it was the thrill of my day to ride or drive on the tractor. My Papaw had a 1954 IH Super A that he bought to replace a Cub. My Papaw raised "baccer" (tobacco) and corn with the Super A, but the fondest memory was of the sawmill. He owned a small sawmill for sawing "baccer" sticks. The Super A was the powerplant. When I was old enough (7 or 8), I would get up early and be dressed to
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