Always consult the Owners Manuals
From Axe-Fx II Wiki
Delay: supported by which Fractal Audio products
- Axe-Fx III: 4x.
- Axe-Fx II: 2x.
- FX8: 2x.
- AX8: 2x.
Delay: X/Y switching or channels
- Axe-Fx III: 4 channels.
- Axe-Fx II: yes.
- FX8: yes.
- AX8: yes.
Delays in the Axe-Fx III
The number of Delay blocks has been increased to 4 in the Axe-Fx III.
The Axe-Fx III also contains: Multitap Delay, Ten-Tap Delay, Megatap Delay, Plex Delay.
Read this: Spillover.
Unity gain (Delay Mix Law)
Firmware 11 and later:
Release notes: "Changed mix law for Delay block. The dry signal now stays constant at unity until Mix reaches 50% then decreases linearly to zero. Conversely the wet signal starts at zero and then increases linearly to unity when Mix reaches 50%. This eliminates having to compensate for decreased dry signal when increasing the mix".
Tape and Ambient delay types require more CPU cycles than the other types.
Position of the Delay block
Just like in analog rigs, there's a difference between delay effects before the amp and after the amp, when using distorted amp tones. When the Delay block is placed before the Amp block, the delay trails will be distorted by the amp and made louder (because of compression). With the Delay behind the Amp block, the trails will be cleaner. It's up to you what you prefer. Most people place delay effects after the Amp block.
About the position of the Delay block relative to the Reverb block:
"If there isn't distortion or modulation in the delay/reverb then the order is irrelevant since they are then Linear Time Invariant (or shift invariant in digital parlance). If there is a small amount of distortion or modulation then the order is probably still irrelevant. If there is a lot of distortion or modulation then the order may make a difference. However, typically the biggest difference, as noted above, is series vs. parallel since h1(t)*h2(t) is not the same as h1(t)+h2(t). If LTI h1*h2 = h2*h1. It may seem counter-intuitive that the order doesn't matter but try it and you'll be surprised." source
More information in the Owner's Manual.
True Tape Echo
Axe-Fx II firmware 4.00 added a "true" Tape Echo mode to the Delay and Multi Delay blocks. It's mono only (the Stereo Tape is the same one as in previous firmware).
Release notes: "Added Tape Echo algorithm to Delay and Multidelay blocks. This algorithm simulates a tape echo where modulation occurs due to tape speed variation. In the Delay block the algorithm is implemented as a two head monophonic tape “deck”. The Time/Tempo parameters set the distance between the record and first playback head. The Ratio parameter sets the relative distance between the record and second playback head as a percentage of the first playback head. The Multidelay block is implemented as a monophonic deck with four independent heads. See the updated manual for full details."
"The algorithm is a single tape with one record head and two playback heads. The signal from each head can be fed back (Feedback) and panned and leveled. Head 1 Time sets the delay time of head #1 when the Motor Speed is 1.0. This is analogous to the distance between the record head and the playback head in a tape deck. As you increase the time you move the heads apart and vice-versa. Whatever the time is set to is the delay time when the Motor Speed is 1.0. As you increase the Motor Speed the tape moves faster so the delay time decreases and vice-versa. The relative position of the heads do not change. Ratio sets the delay time of head #2 as a percentage of head #1. If you set Ratio to 50% then the delay time of head #2 will be half that of head #1. This is analogous to positioning head #2 exactly half the distance as head #1 from the record head. In brief: Delay Time = distance between heads, Motor Speed = tape speed. The LFOs modulate the motor speed. Modulating the motor speed (which changes the tape speed) is a very different effect than modulating the delay time. You can hear the difference between by varying the Motor Speed manually and the Head 1 Time manually. In a typical tape deck the tape speed is not constant. In the days of tape recorders much effort was expended on trying to keep the tape speed as constant as possible. This included periodic maintenance. However, when applied as a delay effect, the variation in tape speed could create a desirable modulation. Tape Echos, in particular, were built to cost points and suffered from considerable tape speed fluctuation. The two primary components of speed variation were termed "wow" and "flutter". Wow arises due to low-frequency variations in the tape speed. This is typically due to variation in the speed of the motor itself and/or eccentricity of the pulleys. Flutter is a high-frequency modulation and arises due to eccentricity of the capstan and pinch roller." source
"The LFOs actually modulate the tape speed. It’s the same as attaching a modifier to the tape speed. This is because in an actual tape deck the tape speed is what is typically subject to variation. Wow occurs due to variation in motor speed (or belt/pulley, etc.) and flutter is due to capstan and pinch roller. Wow varies the tape speed slowly, flutter is a high-frequency variation in speed. The actual delay time is very stable since the heads are stationary and the tape is taught between the heads. Delay modulation due to tape stretching is negligible."
"Originally all the delays "chirped" when changing the tempo (like many other effects processors). I rewrote the primary delay types so it cross-fades between tempos when you tap a new one to avoid this. The tape delay type doesn't do this so that you can use it as you would an actual tape delay and get all those cool effects by changing the head position or motor speed." source
"That algorithm does not support bit reduction." source
Also known as: backwards delay. It's a setting in the Delay and Multidelay blocks. Check out the Axe-Fx II stock presets "bass ackwards" and "regnalf esrever".
You can also use Pitch Crystals or Multi Delay Plex Shift, with pitch set to 0, and direction set to Reverse.
A little Diffusion helps to smear the initial attack points.
Keep feedback (number of repeats) at 0 for best results. To get multiple reverse-only repeats, send the wet signal through a separate forward delay. 
The parameters are also explained in the Owner's Manual.
The Delay "type" is just a "meta-parameter".
Config is the critical parameter in terms of specifying the base algorithm.
This parameter has been removed in the Axe-Fx III.
Time and Tempo
There are two ways to set the delay time:
- Enter a specific delay time in milliseconds.
- Set the tempo in relation to the global preset tempo (Tempo button), say 1/8 or 1/8 Dot etc.
Important: if you use the Tempo method, the menu won't let you enter a delay time in milliseconds anymore. To do that, first set Tempo to None.
The Delay block contains a two-band parametric EQ for the wet signal with low cut and high cut with adjustable slopes for the cut filters. The EQ stuff is on a separate GUI page with a graphical display.
"IMO, what people like about analog delays are the narrow bandwidth. The noise, aliasing and crud is debatable. The early analog delays typically used fixed anti-aliasing and reconstruction filters. These filters were designed for the worst-case scenario: maximum delay time. Typically at maximum delay time the clock frequency was only 4-5 kHz IIRC which means the filters need to be < 2 kHz. One popular pedal had the filter at 1.75 kHz. They filters also have a very steep cut-off, typically at least 4th-order but usually 6th order. The slope is given by order x 6 so for 6th-order you would set the slope to 36 dB/oct. For those who are interested there's a paper on DAFX about it" source
The Drive parameter allows adding distortion to the echoes when using the block before an Amp block. Turning up Drive will decrease the level of the repeats.
"If it didn't do that the repeats would get louder until they started clipping." source
The Time Offset parameter (mono delay only) allows adding up to 100ms of delay to the right wet signal which can be used for widening effects.
When Hold is activated, the wet input to the block is muted and feedback is set to infinity. This can be used to achieve pad sounds and drone notes/chords.
"To gain basic understanding try extreme settings in an experiment. Start with a "hot" delay mix of about 50%. The first way you might do this is to set DUCKER ATTENUATION to 80 dB and then adjust the DUCKER THRESHOLD, from the top down—starting at 0.0 and lowering it to hear how ducking kicks in. This will teach you where your threshold is most dynamic. Another good experiment is to set the THRESHOLD all the way down to -80dB and then increase the DUCKER ATTENUATION from 0-dB upwards. This helps you understand what a reduction of "X" dB sounds like. Once you have extreme ducking dialed in, you can play with RELEASE to see how it works. After this, you should be able to set all three controls so they suit your needs." source
By attaching an external controller to Hold, an external pedal or switch can be used to control this "freeze" feature.
Goes up to 200% for oscillating delay effects.
Tips and tricks
Turn off the delay with trails fading out
By attaching an external controller (connected to a pedal or switch) to Input Gain, you're controlling the delay level at the input stage. This will make delay trails fade out nicely when muting the delay.
Delay settings for leads
For a lead tone with ambient delay, try this: Stereo Delay, Tempo 1/4, Right Time Ratio 61.8%. You can use these settings with a stereo or mono rig.
Use a fixed number of repeats
Use Band Delay or Tap Ten Delay in the MultiDelay block to specify an exact number of repeats.
Add an effect to delay trails only
To add an effect to the delay trails only, not to the direct tone, place the delay in a parallel row and add the effect after it.
Auto-activate the Delay block
When the parameter Auto Delay in the Tempo menu is set to “ON,” any delay blocks that are bypassed will become active whenever a tempo is tapped in. This allows you to set the tempo and un-bypass your delay block(s) from a single footswitch. Read this: Tempo and Metronome.
No delay with 2290 Delay
The 2290/mod delay type has Phase Reverse set to Right. Because of this you'll hear no delay when the signal is summed to mono (for example when placed before the Amp, or when using a mono cab), due to phase cancellation.