Here's a mystery several very good mechanics who've looked at the car are unable to explain. I installed a new 390 long block in my '66 Galaxie over a year ago. Since then, it has never run quite right. What can't be explained is why it only has 80 lbs of pressure in all cylinders and only 5" of vacuum at the manifold. Those numbers are way below normal. A dry and wet compression test confirms that the rings are sealed properly and the valves are opening and closing in accordance with the timing (and the timing gears are aligned properly). With a lot of careful carburetor tuning and close attention to setting the timing, it manages to run reasonably well but it is clearly not right. There are none of the usual suspect vacuum leaks (intake manifold, carburetor mount, etc.) so it would appear to be an internal engine problem. But what? Any ideas much appreciated.