You can test the rings with a compression tester. You need a friend. Remove one plug place the tester over the hole and crank the engine over. Record for all 8 cylinders. They shouldn't be to far away from each other in numbers. If you have a low one, squirt some oil towards the top of the piston and wait a few minutes for it to get to ring, then test the low cylinder again. If the compression improves more than a few pounds you have found you're bad ring.
Oh though I walk through the valley of the rice, I fear no turbo, for torque art with me.
1978 Mustang Cobra II:2.3 Auto Black with Red and Orange Graphics
1976 Mustang Cobra II:302 4 Speed, Knocks like a Champ. Black with Gold Stripes. '70 302 in the works.