What's the impact of an aggressive port on engine compression and compression ratio? The more aggressive the port, the lower the compression?

 

In absolute compression terms yes because the port will close latter therefore less time to compress. But as your RPM increase you will get more air into the chamber in your intake stroke. Compression is measured at 250 RPM and the pressure will decrease as speed increases because there won't be as much time to draw in the air. The larger the porting the higher you can rev and still get a good charge of air. If you bridge port that RPM (horsepower peak could be in the 11,000 to 12,000 range. When we raced 12A's that were bridgeported they pulled easily to 12 but a stock engine poops out at about 8-9. Hope this makes some sense.