Probably a bit late, but I'm asking similar questions for the 20 valve (in engines), so far without response so I've tried to compare your and my figures...
Assuming yours is the 150 HP version then it's maximum torque is 225 ft-lb. Mine is 285, so scaling:
54 * 285/225 = 68.4, which is pretty close to the 69 mm^3/i that I am recording. So they might both be right; or wrong by the same amount, which is a bit less likely. (But I am assuming you are quoting mes data: mm^3/i?)
Did you ever find a true figure for your engine?