In studio, I switched from a 0.4 nozzle to a 0.8 nozzle, keeping all other settings for my print exactly the same. With the 0.4 nozzle estimated print times are 20 hours, but with the 0.8 nozzle, the estimated time is 3 hours more? Is this normal? Am I doing something wrong with my settings? I was hoping for a reduced print time, by switching to a bigger nozzle.
You may not be doing anything wrong. There are a lot of things that the Bambu Studio code was clearly poorly or completely untested. That shouldn’t be too much of a shock given that the code itself is derived from Prusa, Cura and other open source snippets of code. It could be that 0.8mm nozzle parameters were never tested.
Here’s what I might recommend as an experiment. Take simple cube primitive and slice it and print it but also time it yourself. See how close it comes to the projected time slice. In my experience, the time estimates for the 0.40 nozzle have been wildly off. Who knows, these error could just be magnified with the 0.80 nozzle.
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If you only change the nozzle size, this is no surprise. Let me explain:
If you still use the same number of walls, you get much thicker walls and therefore use much more material. If you haven’t adjusted the filament settings, probably the max flow rate is still the same as for a 0.4mm nozzle. So the printer has to melt more material at the same rate which logically needs more time.
The key to faster prints includes two more steps:
1.) calibrate max flow rate with the 0.8mm nozzle. It should be able to put out a bit more.
2.) adjust the print settings. If you used 4 walls with the 0.4mm nozzle to achieve 1.6mm wall thickness, you now can reduce that to 2 walls with the 0.8mm nozzle. the same might apply to top and bottom layers.
Honestly, i fear the bigger stock nozzles don’t have as much effect as one would hope. I have bought all nozzle sizes and haven’t used 0.6mm and 0.8mm yet after reading some topics here. The problem is that throughput seems to be mainly limited by the heating, which perfectly matches the 0.4mm nozzle at the default speeds. So for the bigger nozzles, you would need a beefier heater.
What you can do is use a CHT style nozzle. That should noticeably increase the throughput.
I don’t need more speed, but if I did, I think I would grab an E3D Obxidian nozzle with 0.6mm. That is CHT style and has a stronger heater.
If you don’t like the price, there are other options from AliExpress. I think there are some threads here, which are good.
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I worked with nozzle sizes from 0.15 to 1mm in the past.
Let me tell you that time is relative…
Bambu isn’t well known for a great out of the box experience with any other nozzle size than 0.4.
And when changing sizes one has to factor in physics to understand what is and what is not possible.
Nozzle size and flow ratio are directly linked but in a linear way.
Don’t keep those nozzle diameters stuck in your head, use the area of the nozzle’s hole instead 
Like that you quickly realise the relationship and why flow ratios change so massively with nozzle diameters.
Having said that:
Just changing the nozzle to a larger or smaller one won’t do it.
You always have to calibrate the filament and you always have to tune the print settings for the task at hand.
If we stick with 0.4mm for the default that you could say a 0.2 nozzle is for much higher levels details and that you pay with more print time.
Jumping right onto 0.8mm is usually a matter of filament needs and a higher need for strength/print speed than quality.
Like when printing things in 0.8 in vase mode and 1mm or thicker walls to get strong and water tight prints.
The speed benefit only comes in once you changed the settings to match.
E.g.: 0.4mm layer heights, 1mm extrusion width and as few walls as possible overhangs allow for.
Same for the infill variations.
One has to keep the limits in mind though when going big!
A max flow rate calibration is as vital here as a proper flow ratio calibration.
And both heavily rely on what the hotend and heater is capable of, let me try to explain:
The area of a circle is define as radius squared times Pi.
And to get the volume of a cylinder we have have to multiply the result with the height = good indicator for flow rates…
With a 0.2mm nozzle we get and area of 0.126mm squared.
Changing to a 0.4mm nozzle gives us an area of 0.5mm squared - about 4 times more than for the 0.2 nozzle!
A 0.6 nozzle already breaches the digits with 1.14mm squared and for a 0.8 nozzle we get a whopping 2mm squared …
Again about 4 times the area we had for the 0.4 nozzle as the machine’s default.
Applied physics turns those numbers into more logical stuff:
To extrude the “same line” of filament after hanging from 0.4 to 0.8 the extruder has to transport the filament FOUR TIMES LONGER - four times the volume required.
Quite clear that the extruder has a speed limit, being a geared stepper motor.
You main problem though will be the flimsy heater.
While the hotend might be able to push four times the volume out at the same speed, the heater might not be able to properly melt the filament in the time required.
I know, physics are a pain …
I have no issues maxing out the speed limits of my P1S using a high flow 0.4mm hotend.
While this lets me save 50% or sometimes more on the print time for SOME models, there is little to no gain for others unless you want to turn the printer into a rattling mess.
Like some of these highly detailed Groot planters, dragon models and such.
For them the benefits of speedy hotends are rather relative.
Using a larger nozzle and doing this for speed always means reduced details and more prominent layer lines on the vertical surfaces.
Doing for strength like by still keeping low layers heights of 0.2mm or less means you usually reduce the number of walls.
Like 5 walls in 0.42mm for a strong model could translate into just 2 with just over 1mm wall thickness each - a massive time saver in some cases.
But all of this means one has to play with the settings and properly calibrate the filament…
Thank you for that insight. I just learned quite a bit from this post. It’s the main reason I come here. 
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