How are you coming up with the over torque value? By measuring the torque to loosen it?

 

So you set your Torque wrench to the specified torque setting for your spark plugs either in FT LBS or NM, you now know your Torque wrench will apply the correct amount of tightening Torque to your indicated setting for the install of your spark plugs.

The tightening Torque friction is created in this instance, by the sealing gasket (washer) the threads are merely guides and biting anchor points, as you start to crush the sealing gasket (washer) you get resistance friction, when your Torque wrench feels enough resistance friction it indicates this with whatever mechanism it has, and so you have reached your specific tightening Torque value +/- your torque wrench inaccuracy.

So when you apply Anti seize slipping compound the resistance friction is greatly reduced (EG: Braking in the dry VS ice!) and although you have only applied it to the threads on your spark plug body, as you wind your spark plug in, there will be displacement of your Anti seize slipping compound, pushed outward and upwards onto the sealing gasket (washer) mating face, this will now offer less resistance friction when in contact with the cylinder head mating surface.

As you continue to wind in your spark plug, installed with your chosen Anti seize slipping compound, the sealing gasket (washer) needs more tightening Torque application to reduce (squeeze out) the Anti seize slipping compound from between the 2 mating surfaces, to start the resistance friction and the crushing of the sealing gasket (washer)

The sealing gasket (washer) is now being subject to greater tightening Torque values from a reduction in resistance friction, which is not correctly transferred to your Torque wrench!

Deformation crush of the sealing gasket (washer) is now greater than if installed dry to get to the same resistance friction reading from your Torque wrench, which will give you a higher tightening Torque value, but not indicated and regulated by your pre set Torque value of your torque wrench.

As for loosening Torque values, this will not be the same as your tightening Torque value, the sealing gasket (washer) on the spark plugs is a crush washer and retains it's deformation crush, and when you have Torqued it tight, you have felt resistance friction during the deformation crush, all the way to your specific Tightening torque value as set/regulated by you.

On loosening you will encounter very little resistance friction, (unless partial seizure has taken place from cold welding over time) as the sealing gasket (washer) does not offer any spring resistance, it simply retains it's pre determined crush deformation, as regulated by your tightening Torque applied value.

In this case you will never measure an accurate Tightened Torque value to be the same from loosening.