Both Area-51 and Aurora R12 power supplies are ATX format where the height is 86mm, the width is 150mm, and the length can vary. For the Area-51 PSU the length is 200mm. For the R12 OEM PSU the length is 140mm. So while the PSU should work, the 200mm length will use up a lot of cable management space. Photos below compare a 140mm PSU and 200mm PSU in the swing-out contraption.

Re: the question is, is it possible to use DELL PSU 850W (from Area 51 R4) in Aurora R12 instead of 550W PSU

Dell ships R12 (and R11) with either 550w or 1000w psu, but the Dell 850w is also compatible w R12.

D850EF-00, HU850EF-00, HU850EF-01 etc. are Dell oem 850w modular psu used in multiple Alienware desktop models including Aurora R5-10 and Area 51 R2.  Area 51 R2 and R4 use same chassis and psu.

If that Area-51 R4 850 watt PSU went in the Aurora R8 then it must be only 140mm long. My Area-51 R5 1500 watt PSU is 200mm long, and I thought the 850 watt version was the same size.

I'm pretty sure they never included a 190mm long PSU in the Aurora R5 - 10.

Thanks for all the replies guys.

So after this I have a serious dilemma what to do next with the 850W PSU.
Since the PSU 850W [HU850EF-00] has dimensions of 190x150x85.

There are more options:
1. Area 51-R4, PSU 850W with i7 13700K, RTX 4070ti max 4080.
2.a Area 51-R4, PSU 1000W with i7 13900K, RTX 4090.
2.b Aurora R12 will receive an 850W PSU, which is very long, but also necessary for a new upgrade.

I'll see which way I go.

That's what I thought too, but apparently there were two different dimensions of the 850W PSU.

There are also more of those options:

1. CPU - AIO, RTX 4090
2. CPU - air cooler, RTX 4090
3. CPU - air cooler, RTX 4090 liquid cooling
4. CPU and GPU liquid cooling

I don't want to rush the ALIENWARE AREA 51 R4 project, I want to win with it as best as possible.

An i9-13900K plus an RTX 4090 would be quite warm with only two 120mm rads. Even the popular 360mm rad mod would require high fan rpm. Have you considered going external?

If you mean something similar, definitely not.

Yesterday I was surfing the Internet and came across some nice liquid cooling designs from two Chinese guys.

But as I wrote, today anything is possible and AREA 51-R4 has enough room for creativity... I don't see a problem in this.

Aurora R8 PSU part numbers below = (source Dell-Chris M)

9XG5C 850W Huntkey Gold
J2GNJ 850W Huntkey Bronze
48Y6D 850W Huntkey Bronze
NJVDN 850W Delta Gold
D1J02 850W Delta Bronze

Area 51 R2 Dell validated OEM PSU = (source Dell-Chris M)
N1WJD Delta 850W
48Y6D Huntkey 850W
9XG5C Huntkey V2 850W

N1WJD is a Dell oem psu for Aurora R5

HU850EF-00. Specifications Model Type 048Y6D Dimensions (L x W x H)19cm x14.9cm x 8.5m.

R10 validated oem psu

RGJRX 1000W, Gold, E-Star
0PDJK 1000W, Gold, E-Star
0WTGN 1000W, Gold, E-Star
9XG5C 850W, Huntkey, E-Star 2
0VMT9 550W, Gold, E-Star
FKY8W 550W, Gold, E-Star
RN3F8 550W, Gold, E-Star
YXK65 550W, Gold, E-Star

This shows Area 51 R2 and Aurora R10 share same 850w psu 9XG5C

XPS 8930 uses same PSU 9XG5C 850W Huntkey

9XG5C 850W PSU For Dell 80+ Gold Power Supply HU850EF-01, c/w Area 51 R4

dimensions L 7 1/2 x W 5 7/8 x H 3 3/8

there are two versions of Huntkey 850w psu: HU850EF-00 and -01.

48Y6D Huntkey 850W
9XG5C Huntkey V2 850W

both versions used in Area 51 R2, only V2 used in Aurora R10.

PS
R5-8 Aurora gen 2 chassis

R9-12 Aurora gen 3 chassis

R9 psu spec: 460w, 850w

R10 psu spec: 460,550,850,1000w

R11/12 psu spec: 550,1000w

The Chinese builds look nice with the lighting, but there is not much radiator there for modern day component wattage. If you can't provide enough radiator for liquid cooling, you would be better off sticking with air.  You don't have to go industrial grade, there are more low key alternatives.

Absolutely take you time, enjoy your Area-51 project, and make it your own. I spent 2 years on my project and it is still not 100% complete.