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I can't recall 'which is which', but there's no real difference in chamber volume between the two heads in 'as cast' condition.
The 'long plug' 1.6 head is identical in chamber shape to the 1.5 head. I prefer this one, as the 'plug area' of the chamber is tigher in shape, allowing for unshrouding yet retaining compression. The 'short plug' 1.6 head is more open near the plug area, so it is more shrouded at the valves. BUT, if you unshroud the 'short plug' head you will lose compression as compared to the 'long plug' head.
I have ported/flowed both types, and the variance in compression is the biggest difference after unshrouding. I personally also prefer the long-plug version as the plug electrode is closer to the center of the bore by 1/4". They tend to have decent combustion when used with a flat-top piston, at least for an archaic wedge-chamber design.....
Bob
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