What are the symptoms of cracked heads?... Driving normally the car feels good, but at WOT the car starts stalling (missfiring).

What are the normal compression rate of each pistons (mine was about 175-180) but two are 150, cylinder 4 and 6 are 150.... others are 175 and 180...

Thanks

Cracked heads can have the symptoms you describe especially once the engine heats up. If you have a cracked head depending on where the crack is you will see different signs. Sometimes you will see coolant leaking from an area. Other situations you will have misfires caused by water leaking into the intake or combustion chambers. Compression will usually be normal since most times it is checked with the heads cold. If you check the compression with the engine hot be very careful not to strip out the plug holes. This may give you the solution to the problem. Also check for a vacuum leak that can cause similar problems especially at WOT. A final thought make sure you have a good fuel filter as you may be getting a little fuel starvation.

try a compression leak down test to determine if the head has a leak.