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Gentlemen:
Regarding the subject and the various subsequent discussions which have appeared on it, let me say first that I thought Georgy Braly did a very clear job of explaining why an increase in CR by raising the dome on a piston or by moving the pin a bit lower relative to the crown HAS ABSOLUTELY NO EFFECT ON DISPLACEMENT.
As I recall the original query, the person simply changed pistons from the 8.5 to the 9.0 version (which has a slightly larger "compression distance", the distance from the center of the wristpin to the edge of the piston crown.) He did not change the bore or the stroke.
Displacement is strictly a function of Cylinder Bore, Crankshaft Stroke, and number of cylinders. It has absolutely nothing to do with the shape of the piston crown or with the distance from the wristpin to the piston crown..
The Lyc 320 and 360 have the same bore (5.215). The 320 has a 3.875 stroke; the 360 has a 4.375 stroke. Displacement of an engine cylinder (also known as "swept volume") is the number of cubic inches of air the piston "displaces" when it moves from BDC to TDC (as in: both valves closed, one plug removed, very slow rotation).
Total Engine Displacement is calculated by the following simple arithmetic:
DISPLACEMENT = BORE * BORE * STROKE * # of Cylinders * 0.7854.
Try it.
(Incidentally, to avoid another tangential discussion, 0.7854 is PI/4)
COMPRESSION RATIO = (Swept Volume + Chamber Volume) / Chamber volume
which reduces to
CR = 1 + (Swept Volume / Chamber Volume)
(Chamber volume is the volume you would measure by the following procedure: Position the engine with one bank of cylinders facing up, remove one plug, rotate the crank until the piston is near BDC, shoot in a bit of oil, turn it over a few of times to seal the rings, and stop at exactly TDC, move the engine so the empty plug hole is facing up vertically, and using a burette, fill the chamber with kerosene and record the number of cc's of kero it took to fill the chamber.) (That''s just A way, not the ONLY way.)
So it is easy to see from the arithmetic that altering chamber volume alone (by any means) will alter CR without changing displacement. On the other hand, altering displacement without altering chamber volume will also change CR.
If the arithmetic is too much, the same conclusion can also be reached quite simply by thinking about what goes on in the engine. If you don't change the bore, then the piston crown has the same projected area, no matter what the shape. If you don't change the stroke, then the piston moves the same distance as before and hence displaces the same volume of air, regardless of the shape of the crown.
If you add a big dome to the top of a flat piston, then that dome projects further into the combustion chamber at TDC (thereby REDUCING the volume of the combustion chamber and increasing CR) but the dome has exactly the same amount of projection at BDC as at TDC then, doesn't it. The distance the piston moves hasn't changed, has it?
Regarding the "power gain" from the change, I'd be really surprised if you could effectively measure the difference. I've never wasted my time doing that particular mod, but I can tell you that in an angle-valve IO-360 (A, C or D) when we have changed from the stock 8.7 pistons to the 10:1 pistons (LW-11487-S), with no other mods (except fuel schedule) we barely saw 10 HP at 2700 RPM. I can assure you that it takes lots more than 9:1 pistons to get 180 (real) HP from a '320 (as long as we are comparing apples to apples and testing at 2700 RPM).
Hope that helps.
Jack Kane
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