An important object of this exercise, now well off track, was to analyze the differences between the cam grinders figures and Subaru's specifications. At long last here are the figures :-
Subaru's figures relating to the standard cam are shown as, SS
Grinders figures relating to the standard cam are shown as, SG
Grinders figures relating to the modified cam are shown as, MG
All figures should be read as degrees of angle.
Relating the figures SS and SG:-
Duration: SS 180 + 2 + 54 = 236. SG 180 - 27 + 33.8 = 186.8.
Overall Difference = 49.2. At each end of the cam, 24.6
Therefore 24.6 is the estimated rotation relative to 0.05" valve travel.
Inlet, SG 27 ATDC - 24.6 = 2.4 SS 2 BTDC and error = 4.4
Exhaust, SG 33.8 ABDC + 24.6 = 58.4 SS 54 and error = 4.4
Corrected 0.05 travel figure = 24.6 + 4.4 = 29
Comparison between SG and SS figures:-
Opening SG BTDC (-27) Correction 29 = 2, SS = 2 ( No change)
Closing SG ABDC 33.8 Correction 29 = 62.8, SS = 54 ( 8.8 increase)
N.B. The correction figure has been calculated on absolute values and it is probable that Subaru's figures provide for some slight valve/lobe clearance, in which case the figure of 29 could be accepted as say 27.5. This would bring the figure of 8.8 increase, down to 7.3 which is near the obscure figure mentioned several times in the thread.
Relating the figures SG and MG:-
Duration: SG 186.9, MG 180 - 23.4 + 36.2 = 192.8, Increase = 5.9 = 3.2 %
Relating the figures MG and SS:-
Duration: MG 192.8 + 29 + 29 = 250.8, SS 236, Increase = 14.8 = 6.27%
The end result of the cam grind, as anticipated, indicates only a very limited degree of change which appears hardly worth the effort and expense.
The increase in lift shown is .784 mm which amounts to 11% increase.
Edit:- The actual effective increase in breathing can be shown as less than 3%, due to the limited time when the valve is fully open. However there could be some additional very limited advantage, due to a slight increase in the terminal speed of the gas flow.
Going for the reasonable maximum and to make a real difference, I will stick my neck out and suggest:-
Based on Grinders Measurements:-
Duration: 202, Opening 24, ATDC, Closing 46 ABDC.
Edit. I think you could go as far as say:- 210, 20, 50.
Based on Usual Measurements:-
Duration; 260, Opening 5, BTDC, Closing 75 ABDC.
Edit :- 268, 9, 79
Are there any takers ?
