So this TTY vs ARP issue...
Fastener Pre-load: The amount of static force experienced by the fastener at the "tightness" that it is torqued too... for example (and COMPLETELY a random number example... not given based on realitY) say a stud or a bolt is torqued to 80 ft-lbs which results in a pre-load (sometimes... but not completely accurately... called clamping force) of 1000 lb's in this random example. Keep that in mind for discussion further down.
Fastener yield strength: Point where the fastener experiences plastic (non-recoverable, permanent, etc) deformation
UTY - ultimate yield strength: Maximum force level fastener withstands before "softening" toward breakage... metal exhibits a "hump" shaped curve... you get a linear load until yield.. then at yield you become non linear but force continues to rise until you reach the peak of the hill... at which you are at UTY and once you pass that point the force the fastener can withstand goes DOWN.. so if you continue to hold a load above UTY you'll get failure... hard to explain how the back side of the curve works in words.. but basically the fastener holds "less and less" strength as you have forces after UTY.
So now given those items... here is the deal with a head stud/bolt.
All fasteners get a certain torque which yields a pre-load of a given value... say the 1000 lb's from my first comment... until forces exerted on the head at that bolt exceeds 1000 lb's that fastener sees absolutely NO DIFFERENCE at all regardless of the internal cyldiner pressures etc.. so ARP or TTY bolt... neither cares or knows what is going on until that 1000lb's is exceeded.
Then the differences come in to play between the ARP's or the TTY's...
TTY:
with TTY's you have already started the yield process BY DEFINITION... you have NOT however reached UTY (otherwise they'd break rather quickly). So with the TTY you have yielded the bolt somewhat and put it well above the linear portion of the material's strength.. this is in now way bad... and in all actuality helps TTY fasteners be more even.. the metal will yield at about the same torque in every case and thus you are ensuring consistency to some extents by doing so. If you reach forces that exceed the pre-load of the fastener you will yield it further which will in most cases reduce the static pre-load by a small margin, but nothing of note. However, if you load a TTY fastener to a significant extent such that you pass UTY... then you will quickly lower the pre-load to levels insufficient to maintain proper clamping and you get blown headgaskets etc.
ARP/Studs:
Studs similarly are pre-loaded, but almost always as a general rule are NOT yielded by the forces involved. So it remains in the linear/spring portion of the material and does not deform the length of the bolt mechanically in any way. So this is fine and great for the pre-load and is easier to explain. However, what then needs to be considered is what happens when pre-load is exceeded. When pre-load is exceeded the fastener immediately sees a stress like the TTY. If that stress exceeds the pre-load, and then yields the fastener (which is very possible.. and all metal yields to some extent eventually when given a load of the proper size). Now the problem is that because this has yielded the fastener the pre-load will be SIGNIFICANTLY reduced because of the linear nature of the stud pre-load versus the flatter/curved pre-load of the TTY. If you exceed UTY of the stud, just as with the TTY, you quickly will move to failure of the fastener itself as well as blown headgaskets etc.
So all this mumbo-jumbo.. what does it mean.
That TTY or ARP/Stud... there is no mechanical difference in them until pre-load is exceeded. If the fasteners are not pre-loaded properly (easier to do with the ARP's because you use a straight torque value rather than a torque to yield as with the TTY's) then yes they will loosen up and you have issues regardless. But given equivalent pre-loads there is no difference at all until those pre-loads are exceeded.
Now if pre-load is exceeded, the TTY fastener will yield, but in most "minimal" circumstances will maintain a very good pre-clamp. A stud will typically take MORE pre-load exceed force before yielding... which is why people tend to like them for high performance applications. However, if you do exceed to the point of yield with a stud... then you will loosen up and have significant problems quickly. Similarly, with enough exceed for the TTY you will also get that result.
So there is no difference in these fasteners until the loads exceed the pre-load. Then the TTY will start to give up sooner.. but a stud that is over loaded will fail more catastophically... but it USUALLY requires more force to do so.
