This question has come up before, including an extended answer (which I am not satisfied with) in stackexchange
by Dr. Chung. But nonetheless I will ask it here in a different way for advice.
I live in a very hilly area and rarely train on flats. My power profile and CP curve shows that my FTP is more or less fine at 4,90W/Kg (I'm not aiming to be a pro, just your average amateur Joe) as is my VO2Max. My anaerobic capacity and sprinting sucks on the other hand, badly, even for the average amateur. This has been computed by several weekly 20' climbs and interval sessions always on hills (I take about 7-8% of Peak 20' power to consider for FTP). In particular, my Z5 sessions are typically 6x3' @ 117% with 3' rest + 4x2' @117% with 2' rest.
My Z6 sessions are 8x2'@130% with 3' rest + 8x1'@150% with 3' rest.
Looking at my annotations and RPE from these sessions I find the Z5 sessions waay easier, while the Z6 sessions have me crawling back home to cry alone in the shower until I recover.
Now I am visiting the Chicago area for a few months and this seems to be the opposite in every way to my town (I've heard the longest climb is a highway overpass) and have just started to ride. I tried a typical profile test including a few 20" sprints, 1' all out and 20' TT and my numbers came out more or less in line with the numbers I use to prepare my workouts so I was particularly happy to be able to attain more or less the same output for 20' than in the hills (a slight drop, but not much).
So next I went up for a Z5 session, decided to start with an easier session than my typical: just wanted to do 5x3'@117% with 3' rest plus one 6' TT. I could barely keep the power output at 115% for the first 4 intervals and by the 5th I was fried! I didn't even try a Z6 session. These were supposed to be the "easy" sessions!
So my questions are: should I keep the same targets for these sessions or should I aim for less?
Could it be temperature? (there's > 50F difference between the average temperature now here and what I was training a week ago home). Could it be altitude? (home is a hilly area, but it's on the seaside).
I am puzzled since I didn't suffer much in the 20' TT at 105%, but any effort above 110% is killing me!