WTF are you using for a ratchet? A breaker bar?
Intake manifold bolts are very light torque, as you're threading into aluminum. I have no idea how you're not stripping the fuck out of the manifold, but breaking bolts instead.
Use a 1/4" drive ratchet next time. You can't really overtighten with a 1/4" drive, and if you are, you know it because you're trying.
Just snug it up, and give it a little pull. That's all you need...
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