I think the compression "ratio" is about 8.2 to 1 on a new 390. If you are asking about actual compression pressure, like measured with a compression gauge, then I do not know. With the compression release on the cam, its problematic to measure. You can back off the exhaust rocker just enough to measure I would guess. Do that and get a baseline. Then run it. It may actually increase if its truly new when the rings seat. Then you could measure it on occasion to see if it drops. I use a leakdown gauge to check but rarely an actual compression gauge. Some do.
