What is your current FTP test?

I use Trainer Road test or Sufferfest FTP test. Its a warm up, 5 min clear out or 5x1, then 20 min test.

What percentage of your FTP are you shooting for in the 4 hr TT?

It's possible that the FTP test you're using may not fatigue you enough prior to the 20 test. I prefer 10 minute warm up, 5x1 hard, 5 min recovery, 5 min hard (above your expected FTP), 5 minute recovery, then the 20 minute test. But I'm old school about this. Old, too...

You should stop estimating your FTP and just do a real test.

Of course. That's a given for anything over 2 hours. You should go over the definition of FTP again.

Every test is an estimate!

Are you adjusting your average power correctly during the 4 hour TT?

For more insight, see...
http://www.trainingbible.com/...4/ironman-power.html

"There" isn't any protocol for a long TT to estimate FTP - it doesn't make any sense. Nobody can hold his/her true FTP for 4 hours. You would want to set training zones based on your current FTP, then perhaps holding only 75-80% of it for a long TT or IM.

I am pretty sure he's asking if a 4 hour test could give a more accurate estimation of FTP that he would use to drive his zones for an IM race. He is not saying holding FTP for 4 hours. For example, 20 minute test estimates 280 FTP but when he does 70% of that FTP for an IM he feels wasted. Conversely, if a protocol existed, a 4 hour test gives him an FTP of 260 and 70% of that FTP for an IM keeps him fresh for the run.

I think the OP is just asking if there is a more accurate way to determine IM power than taking a % of your 20 minute estimated FTP.

I get you can't an FTP from long TT but I'm wondering if you ride say 300w AP for 4hr you could extrapolate your FTP more accurately than say 20 min FTP test.

From what I understand after reading Coggan's book, every person has their own power curve and hence it's a very personal extrapolation. That said, I would think that a 4 hour time trial estimate would be more error prone than the 20 minute estimate because there are probably more variables that could affect your ride over the duration (e.g, fueling and hydration which is not really an issue on a 20 min TT)

Perhaps someone who has analyzed large sets of power curves has some idea of what an approximate multiplier would be for a max 4 hour effort, if the data exists.

I think a more practical approach would be a 1 hour TT, or, take a look at your 5 minute effort on the 20 min TR test as well, which is supposed to be an all-out effort. The Sufferfest test (Rubber Glove) doesn't have that effort and probably overestimates FTP.

You could also just recognize that your FTP is overestimated and dial it down a bit for the long training efforts (trial and error) , which is probably the easiest and most pragmatic approach.

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Strava

When you look at your mean maximal power chart, what do you see as your maximum power in the 50 to 60 minute range? Taking 95% of your 20 minute power to estimate FTP is predicated on the idea that it's quite typical for many athletes to be able to hold ~ 95% of their 20 minute power for 60 minutes. While this is true, some can only hold as little as 85% of their 20 minute power for 60 minutes. A genetically lucky few can hold as much as 97% of their 20 minute power for an hour.

YMMV,

Hugh

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Genetics load the gun, lifestyle pulls the trigger.