thule said:o.k. for example:
today i did 2x20 efforts with 328W and 324W. The second was really hard !
Maybe next time i try to reach 335W in the first and see what happens in the second, right ?
I think that 328 and 324 for 2x20 is good consistency and that is exactly what I like to see out of myself at certain points in my development, consistency. That the second effort was "really hard" says that you were "reaching" but since you were able to complete it, that means it wasn't totally beyond your capability. Assuming that you are pushing yourself and not riding too easily, it is normal for the second effort to feel harder than the first.
Had you tried to reach 335w on the first effort today, you might have been able to complete it but then given that you said that the second effort was "hard", I think that you might not have been able to reach even 320w for the second. Sometimes it's good to over-reach like this (335 for the first then <320 for the second) to find your limits, however, you may find that you make better fitness gains with a more consistent intensity for the two efforts. I know that I do.
I like to be comfortable doing 2x20 at a certain level before I try to make them more intense. So, if it were me, I would keep doing the 2x20's at ~325 until the second effort isn't so hard. Then you know that you are ready to increase the effort level. Then repeat the cycle and increase again once you are comfortable.
That's not the only way to approach this. You could also decrease the power and try to do 3x20. IOW, instead of 2x20 at ~325, try 3x20 at ~290. (This is what DaveRyanWyoming was referring to by increasing duration.) This can also produce great benefits as well. Or, consider doing L5, as John suggested.
I don't know what will work for you - only you can find that out - but I can tell you that if you keep doing the same thing, you probably won't make much more progress.