You want colder diff temps? You want bragging rights? You wanna spend $250 on speedflow fittings and another $500 on LSD oil? Me too.
High Volume Finned Cover
The most common of the finned covers are the untapped US market variant. These covers come from post '87 US Z31 Turbos with an LSD differential.
These covers sport (?)L in extra oil capacity over the standard smooth covers and passively cool the differential oil by having both a larger mass of aluminium, allowing it to soak more heat, and fins for air to flow over, whisking the heat away.
If you plan on taking advantage of an external oil cooler then it would be most beneficial to pick up a factory tapped cover (seen below) as it has a baffled pickup. In saying that this cover can still be tapped for oil lines via the cast bosses; it's important to include an inline filter as there is no meshed pickup installed in the untapped covers.
European Finned Cover
This style of the finned cover comes from a European Z31. These covers were tapped from the factory and fitted with systems that trigger a cooling loop based on oil temperatures.
The covers feature an ring gear oil splash plate and a meshed pickup for filtering.
It's commonly accepted that the factory tapped covers are to accomodate for the periods of high speed driving often possible in Europe (Autobahn etc)
Air Cooled Nismo Cover
This is a Nismo branded high volume rear cover. Unlike the common ones that come out of the US this cover has no taps for an external cooler, has no oil pickup, no taps for oil temp sensors, or side fins.
Aside from the passive cooling from the cast fins this Nismo cover has an external air to oil cooler with the center shaft tapped right into the main diff oil pool.
Editor's notes: I've never seen this cover fitted into a car, so I have no idea what chassis it would fit in. As of April 2020 the cover was listed on Yahoo Japan for approx $5,000 AUD.
Early Nismo Finned Cover
This finned cover is one of the rarer of the high volume R200 backing plates. As far as it's currently understood this 'untappable' cover was never a factory fitted item on a public sale model and could only be attained through the Nismo catalog.
Not a lot of info about these is currently known but it is assumed that the production run was limited and short due to the part going into standard circulation as a stock item as early as 1979 in the 280ZX
J30/Y31 Finned Cover
Affectionately nicknamed the "Flounder" due to it's two vertical bolts, this oddball finned cover comes from the Nissan J30 and Y31. It's a factory untapped cover and is essentially the same as the common untapped cover aside from the obviously different mounting bolts.
This cover features some superficial differences when compared to other finned examples, such as the bossed section on the right hand side of the studs and the spacing between the side fins being equally spaced whereas some of the other covers have uneven spacing/missing fins.
Euro 280ZX Finned Cover
Editor's Notes: This was listed as a 280ZX cover but I don't think they ever had a finned rear cover. I've decided to list it here because its design differs to the typical covers. It doesn't look like a home job and there's no reason for the fins to be ground down in that fashion; this leads me to believe it may be stock.
This is the only cover of it's kind that I've ever seen for sale and until its legitimacy can be verified it will stay up here with a question mark.
Edit 23/07/20: This seems to be an item that was fitted to European 280ZX Turbos alongside the 3 piece accordian style turbo log manifold.
This section is to serve as a PSA; don't use a solid rear mount.
The way that the Nissan trailing arm rear suspension setup is bolted into the chassis makes it difficult to keep the differential from rocking along the length of the car when driving. Due to the this fact the differential mounts heavily depend on the flexibility of the mounts used.
When you solid mount the rear diff cover and the chassis flexes the first thing to give way is the weaker aluminium.