Intervals too short for FTP work?

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I know that the most efficient way to increase FTP is generally 2 x 20min intervals and that total time at the top end of Z4 is a key factor but is there a work period that is too short? IOW, I have a convienient hilly route that requires me to spend about 5min at FTP per climb, will 40 min total time at FTP on these repeats give me the same benefit as 40 min total time using 2 x 20 min intervals? Thanks.

I know that the most efficient way to increase FTP is generally 2 x 20min intervals and that total time at the top end of Z4 is a key factor but is there a work period that is too short? IOW, I have a convienient hilly route that requires me to spend about 5min at FTP per climb, will 40 min total time at FTP on these repeats give me the same benefit as 40 min total time using 2 x 20 min intervals? Thanks.
I'll wait till someone can answer more authoritatively, but my inclination is that 5 minutes at FTP, spread out at different times, would not be as effective as 2X20.

Unless you can really reduce the rest periods between the 5 minute intervals, I think FTP development is really helped by long, sustained intervals at Z3/Z4 intensities.

The minimum interval length that most people seem to point to for Z4 intervals is 12-15 minutes, at a minimum. If the hill is only 5 minutes long, you're probably better off doing it at Z5 intensity, or finding a more suitable spot for Z4 training.

I know that the most efficient way to increase FTP is generally 2 x 20min intervals and that total time at the top end of Z4 is a key factor but is there a work period that is too short? IOW, I have a convienient hilly route that requires me to spend about 5min at FTP per climb, will 40 min total time at FTP on these repeats give me the same benefit as 40 min total time using 2 x 20 min intervals? Thanks.

Sure, if you keep the rest in between the 5 min chunks short (30 seconds or less).

My suggestion would be to ride the gradients at 110-120%FTP, and the rest of the ride at upper L3(tempo). That way, your normalised power for the entire session will be ~L3/4.
I would also say that from my experience, I have found better FTP gains through riding 1-2hr sessions sustaining average power between L3 and L4.