I have had P mags (114's) on 3 aircraft with parallel Lycoming O-360's. My current aircraft has a FI O-360 with 9:1 pistons, venturi cut intake seats, performance camshaft, counterweighted crank, and P-Mags timed to TDC. After a very informative talk with Brad at P-Mag, I retarded one mag 2.5 degrees, the other left at TDC. Running on each mag individually, I found the retarded mag lost only 1 Kn in cruise (9500', wide open, 23oo rpm, leaned to peak) but the CHT's ran 10 to 20 degrees cooler!!! I then timed both mags to 2.5 ATDC, and run about 1-2 knots les that prior, but have cooler cylinders, and lower fuel flow. Sooo, it seems that the pumped up parallel engines are near their max advance boundaries, perhaps that is why the angle valve IO-360 and the IO-390 have magneto timing at 20 degrees instead of 25!
DAR Gary
DAR Gary