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 - Tuning-Up Your Tractor: Secondary Ignition Wires and Distributor - by Curtis Von Fange. In our last episode we spent the entire day checking out the spark plugs. Is that efficency? Actually, it took more time to explain than it really did to do the inspection. At any rate we decided that a compression test was in order so we could verify the internal engine status. That way we wouldn't buy any tune up parts until we were sure that the engine was in reasonable mechanical condition. Our tests showed a fictional compression reading of 85, 80, 82 and 86 psi. That is well within a
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