When I fus ro dah someone's ass I'm often wearing a full face helmet shouldn't that blow the front of the helmet off ?. Then again as my neck is weaker than the metal of the helmet shouldn't my head come off ?. The conclusion I must therefore come to is that when fus ro dahing with a full face helmet on the front of the helm should blow out and my head pop of like a champagne cork.

interesting theory... in all honesty i wouldnt know as i am yet to see s1 using this shout with a full face helmet on in real life

On the same token, when you shout Fire Breath, you should burn yourself. I think the shout forms outside of your helmet.

its called magic for a reason not physics

You're thinking too much. Lol.

Thu'um doesn't need to use logic.

Shouts are essentially magical effects - a dragon can't shout if you drain its magicka, for example - so normal rules don't apply

Same as walking around with a destruction spell in your hand(s) and you don't get hurt.

Speaking of logical - how is it possible to put on\take off armor in seconds ?

Yer but when you shout it damages what ever it hits as it hits your helmet first its kind of like spitting while wearing a dust mask.

every time your dragonborn uses a shout, he also whispers another shout called "**** logic"
its the best conclusion i can come up with.

Not everything - try it on a tree or a wall....

Speaking of walls, shouldn't Whirlwind Sprinting into a solid surface mash you flat ?

It's likely that the magical effects of using a Thu'um start a few inches from your lips, more than enough to bypass full-face masks.

If you'll note, the shouts are in a cone in front of you. If the shout itself was the cause (i.e. the sound,) then they would work as an AOE type effect, similar to a Flame/Frost Cloak spell, but with a larger range. Since they aren't AOE-type effects, but Cone-type effects, this means that the effects of a Thu'um are NOT caused by sound. Therefore, we can't assume that full-face masks get in the way of the Thu'um.