You squash a FIBROUS gasket between two surfaces to 81lbft, the run the engine, and have a leak.
At no time did you allow for the gasket to compress, due to it's fibrous nature it will.
You need to torque up then allow at LEAST 2-4 hours, then retorque, overnight is even better, you will then find when you recheck the torque it is loosened a bit, quiet a bit actually, I torque mine to 120lbft, let the gasket have a rest, then retorque to 120lbft