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Hi Guys! What Is Primary/Crankcase Compression And How Is It Measured? What Effect Does It Have On Performance In A Outboard Engine. What Should It Be In Terms Of Ratio. Thanks.
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If I remember correctly, primary (theoretical?) vs. effective crankcase compression ratio. The theoretical crankcase compression ratio is equivalent to the volume of the crankase at top dead center (TDC) divided by the crankase volume at bottom dead center (BDC). Effective would be the same as above but instead of BDC you use the point at which the transfers open. Memory doesn't serve me well on what good numbers would be...I would be guessing.
There are many things that coincide with crankase compression ratio - effects of tuned length on carb locations, port timing, type of exhaust pipe used, etc.. A general statement could be made, the tighter the crankcase compression ratio, typically the better the engine will operate at higher RPM. Again, a generalization.
Hope that gets you started.
-Paul Fuchslin -
Very well stated, Paul.
The other consideration is net result. Decreasing crankcase volume does not always pay off. Sometimes the power increase provided by the improved cylinder scavenging is less than the power consumed by the increased crankcase pumping.
It is obvious that a huge crankcase volume will be inefficient, but smaller is not guaranteed to be better than small.Comment
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