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 - A Brief History of Tractors in Australia - by Bob Kavanagh. After Captain Cook's exploration of the east coast in 1770 the British Government decided to establish a penal colony in Australia. The first fleet arrived in 1788 and consisted mainly of convicts who were poorly equipped and new little of farming techniques. The colony remained far from self-supporting and it was not until the early 1800's that things started to improve. Free settlers started to arrive, they followed the explorers across the mountains and where land was suitable set up farms. T
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