speedmaster 2.02 valve heads on a 318

On my 1965 273, when I swapped the stock heads for some 2.02 340 cylinder heads (I used reliefs in the upper cylinder bore), it slowed my quarter mile times about three tenths. In my case, I later assumed it was due to a loss in compression ratio. I had been using the 10.5:1 commando pistons but failed to figure the change in compression before the swap.