item level requirements and ilvl

Hi, i'm a little confused about this one, and would love a clarification on the subject.

does an item's ilvl affect its level requirement when upgrading normal to rare?
so, i was looking for a 5l chest piece for my lvl 41 shadow, so i went with a Frontier Leather and got one from a player.
Frontier Leather's level requirement is 41, the ilvl of the one i got was 80, and once i used an alch on it, the lvl req changed to 57.
what are the mechanics behind this change? and was it always like this?

For non-unique items, the ILVL will only define which affixes can be generated, aswell as the maximum number of sockets & links.
In the scope of your question, it won't affect the RLVL of the item at all.

The final RLVL of an item is the highest between the base and affixes all together.
In your example, the alch you used created a series of affixes, for which the highest one has a level of 57, so it prevails over the base level (41).
But, let's say you rolled a different series of affixes, with the highest being level 30 for example, the chest would have stayed RLVL 41.
And yes, i believe it has always worked like that.

thanks for clearing that up for me xhul.

so, how would one ensure that the final RLVL would not exceed a certain number when crafting an item, so it is still usable by my targeted char?
and also, where in the wiki or other source is that information exist? couldnt find it in the item level wiki page.

Pleasure =]

I would recommend 2 different sites for such data :
http://poeaffix.net/
http://poedb.tw/us/mod.php

Now, i didnt go into details on my first post, but the RLVL of an affixe is actually 80% of its level (rounded to the nearest integer apparently).
So, if you want an amulet with 48% fire res for example, the RLVL for such affixe will be 67 (84*0.8).

those links are useful, i was peeking in there from time to time, but what i was actually searching for is the basic information about those mechanics.
now i see those would be in the affixes wiki page and the details in the links you shared, but still i cant figure out the magic formula for limiting an item requirement.
lets say im crafting a Frontier Leather(base RLVL 41), and im aiming for an RLVL not higher then 50 for example, what is the ilvl i should limit myself to when looking to purchase said item?
e.g , base RLVL 41 + ilvl ?? = max final RLVL 50.

You would then have to do the reverse operation.
In that case :
X*0.8=50
X=50/0.8=62.5 (63 actually)
That means for a lvl 50 character, that would allow affixes which levels don't exceed 63.
So, if you want to be sure to never exceed that RLVL when crafting the item, an ILVL 63 frontier is optimal.

oh, i see, sweet!
thanks a bunch xhul, you have been very helpful.