Done an outdoor ride today which had some hill repeats on a 15 minute climb @ 6/7% and got my best 15 minute power numbers of 254 @ 59kg
I was wondering if I could use this number to guesstimate my 20 minute power subsequently at FTP number from it?
Any help would be awesome
A guesstimate on a guesstimate isn’t going to be great in terms of accuracy. Though as the 20 minute test is 95% and the 8 minute test 90% one could argue that perhaps 92.5% would make somewhat sense? Though that’s completely ignoring all physiological aspects. 
Did you do 5 minutes above threshold before the effort?
I get you 
No - No threshold before 
If you didn’t do the 5 min @110% before it’s definitely for the bin I would say. 
I’d be pretty confident your FTP is somewhere in the 230-240 range. How long you can hold it for (TTE) is another thing.
Are you saying you can estimate an FTP number from 8 minute power x 0.9?
Do the 20. Not 1 second less. Stop with all the shorting. You’re not doing yourself any favors. If you’re worried about a low number just remember it’s a number. True performance in events and metrics like speed and climbing ability are what matter. No one cares what your ftp is.
There is an 8 minute protocol. It’s 2x8 min and you take 90% of the two efforts average. It’s in the workout library.
FWIW I did a 16 minute test (395W) without the 5 minute effort, right before my last block. I ended up setting FTP at 365 (92.5%), and the workouts felt just about right (370 was too hard) throughout the whole block.
Yes, but with limitations. Firstly, once you are well trained and have completed a good number of tests you get a sense for your FTP from a range of efforts. Problem for a lot of people is that they don’t ever do long sustained efforts (approx 1hr) so they don’t really get a feel for it.
Secondly, there is a issue around completing the efforts on a hill. If the hill is steep and you are getting out of the saddle regularly your power output may be different to what it would be in the saddle. Also if the hill is not a smooth gradient, you may have bursts of power that skew your data.
The third issue is around repeating them as intervals. The rest between the intervals, the number and the consistency would skew the data. For example, there is a big difference between six 15 minute intervals with a 2 minute rest, than three intervals with a 10 minute break in between.
Everyone is different, so would respond differently to the protocol you have used relative to their FTP. You can only find out when you start undertaking training using the numbers.
Final point, if you don’t do a consistent repeatable protocol, it is hard for you to determine to what extent our training is having an impact.
So in summary, it would give you a number somewhere in ball park, but there are better options
Intervals.icu will give you a decent FTP estimate from any long enough max effort. 15 minutes is long enough.
Just use FFT Magic Power-Duration Curve (FTP) Predictor [Link: fft.tips/curve] - Google Sheets, that is exactly what this does. from 254w@15mins <=> 245.5w@20mins (depending on phenotype) <=> 208w-215w@60mins; or sync your data to icu as David suggested !
Based on that one data point, I am 95% certain that your FTP is somewhere between 218 and 256 watts.
The op asked specifically about 20 minutes, however, the same could be said of any test. I think pacing for 5 to 10 minutes is insanely more difficult than 20-40 etc…IMO, taking an estimate (algorithm of PDCs?) of an estimate (.95 of 20 min) used as a proxy for maximum lactate steady state, seems to me, like the whole point of testing is getting lost.
I guess I’m in the minority or a bit old school. I want a number to work zones to push the PDC up and right to compete. Getting familiar with riding longer maximum efforts are essential to this. Also, since my ftp seems to fluctuate 40w in a season doing the same test the same way seems critical.
If the OP’s FTP/60min power is 208w, then he managed 122% for 15 mins.
Why don’t you go ride your bike for 20 minutes instead of noodling about it on the Internet?
Okay dad i was just curious 
Sorry another plug for my s/w but Intervals.icu helps with pacing issues. If you go out to hard go as long as you can and if its only 18 mins you will still get an FTP number. If you get to 20m and have gas in the tank keep at it and you will get a higher number for every bit over 20m at that power.
David, I am very impressed with your software, been tinkering a little, unfortunately not had much chance recently though due to illness.
Probably only 5 or 10W difference at MOST. These two durations are so close together that an on or off day could make the difference between being able to handle a set power for either 15 or 20 minutes. Cognitive/mental fatigue, glycogen levels, motivation, warm up…all variables that could count. I’d go with 245W instead of 250 if you are looking to extrapolate to FTP. Otherwise 250W should probably be good enough to close the 5 minute gap.
