All things being equal (rider weight, power output, no wind, same course), how much faster is a disc than a say, 404, over a 40K TT? Minutes or seconds? Wasn’t there a paper floating around out there with some tests? I did some quick searches but to no avail.
Now, how about flat (575) vs. lenticular (HED, Zipp, Blackwell) discs, again assuming their weight is the same?
Depends on your position. If your CdA is very low, then the disc will make more of a difference, since the percentage change will be larger.
BUT, in your example, with no wind, a disc would only be very, very negligibly faster. A disc is much faster than other wheels on windy days, especially days with a lot of crosswind.
Lenticular vs. flat depends on the frame and which direction the wind is coming from, since most lenticular discs are only lenticular on one side. But the frame is probably much more of a factor, especially seatstay profile, and how close they come to a disc.
So the easy answer is that it depends… I’m sure that was helpful! 
“most lenticular discs are only lenticular on one side.”
$65. CH Aero covers are lenticular on both sides. From what I’ve read a lenticular disc is about 7-9 seconds faster over 40 kms than a flat disc.
Definitely seconds and very few at that…and thats with some wind.
I 2nd the no wind = no difference comment. Interesting how discs perform the best (relatively) with lots of wind yet thats when everyone is scared to ride them.
Differences between discs = close to zero in my opinion.
Dave
WAG = 20 seconds
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I haven’t noticed much if any difference between a disc and the 808 I now race. I waent to the 808 for 2 reasons. I use a power tap and can’t use a disc in Hawaii.
I’m pretty sure all zipp discs are flat.