Question 3 120mm or 2 140mm

Larger fans are generally quieter than smaller fans. They can move the same amount of air at lower rpms. See where the fans are blowing. You want them blowing directly on thr CPU and especially the gpu, not into the psu shroud, for example.
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As Bearmann says, usually larger fans are quieter as they work less for the same airflow. But this will also depend on the quality of the fan itself.

if you wanted to get mathematical:
An easy way of checking this is checking a fans CFM (if it has it on it's technical specification) - the higher the CFM, the more air it brings in per minute.
If it doesn't have the CFM on the technical spec, you can calculate it.

The combined CFM of 2 smaller fans might outweight the CFM of 1 large fan, so technically the small fans will bring in more air per minute, but then it's down to what you prefer, airflow, noise, size, colours etc.

Obviously also consider whether this is input/exhaust and how air is moving through your case generally.
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An impossible question and even harder to answer. If you assume 2 identical fans, same series, brand, model etc then you'd have to calculate the airflow for 1 rpm for both sizes per fan. CFM given by spec sheets is a maximum amount at full rated speeds, not the actual amount at used speeds which will vary greatly depending on the fans performance curve. This also depends on the fan blade itself, as fans get faster the blades twist straighter and you loose pitch, so the performance curve changes, it's very much not a linear curve.

I'm assuming it's for a case that fits 3x 120mm intakes or 2x 140mm, which is many of the new designs that no longer have 5.25" optical slots.

Generally speaking, with airflow equitable, the 140mm will be spinning slower and quieter. However, most 120mm will spin at max faster that a 140mm, so 3x 120mm can put out more cfm, if barely.

Mostly it boils down to aesthetics. Many ppl perceive having 2x 140mm in mid/low positions as looking like a cheaper case, like the cheap single fan Rosewill mATX cases, whereas fully filling the front with 3x fans looks more higher class. Then there's RGB, they prefer the fully filled look to something that's not quite full.

But performance wise, just assume 2x 140mm = 3x 120mm, especially with good fans built for silent operation since most fans rarely if ever spend any time at 100% speeds. Any decent fan under @ 900-1000 rpm is going to be very quiet, and idle speeds of 400-600rpm next to silent.

It's pretty much all about whatever look you are going for.

If the assumption is an AIO, then you can pretty much assume equitable performance upto a limit. Most 280mm rads have somewhere around 350w+ limits, and thats actual heat output from the cpu, nothing to do with TDP which got left behind as soon as you OC. 360mm rads upto 350w+ will have almost identical performance to the 280mm rads, it's only in the position of having a higher ceiling that changes things. 360mm rads are 400-450w+ capacity. All about the headroom.

That makes a difference on cpus like the i9-9900k which can easily put out 300w+ of heat in a high OC and full load situation.
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360mm x120mm x30mm = 1,296,000mm^3
280mm x 140mm x 30mm = 1,176,000mm^3

Relatively similar volumes, assuming the same radiator thickness between a 360 and 280 rad. However, still not certain if this is only about fans or about radiator sizes. If the latter, there are other considerations, as well, although fewer if we're talking AIOs.


Yeah, and thats just pure volume, whereas you'll also have dead space in front of the motor housing which further complicates things. It took Noctua 20 years to design a 200mm Fan, simply because they have standards of silence and performance and with motor sizes and blade composition tech they couldn't do it and still maintain a relatively small motor housing.