I had the same issue, however I put a flush valve head on a 92 4d56, turns out, the problem is that the flush valve heads are for the 96 up intercooled 4d56 and the Hyundai d4bf, the pistons were redesigned in 96 to introduce more swirl and have a shallower relief in the top of the pistons, using a flush head on a protruding valve engine, regardless of wether it is a 4d55 or 4d56 drops the compression from 21:1 to around 18.5:1, which is great for performance if you throw enough boost at it, but cold drivability and efficiency will suffer.