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 - When Push Comes to Shove - by Dave Patterson. When I was a “kid” (still am to a deree) about two I guess, my parents couldn’t find me one day. They were horrified (we lived by the railroad), my mother thought the worst: "He’s been run over by a train, he’s gone forever!" Where did they find me? Perched up on the seat of the tractor. I’d probably plowed about 3000 acres (in my head anyway) by the time they found me. This is where my love for tractors started and has only gotten worse in my tender 50 yrs on this “green planet”. I’m par
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