Nissan's VG engines made the Z31 the worlds first in many respects. Previously, all Z's had L-series inline 6 engines, making the Z31 Nissan's first V6 Z-car. But most importantly, It was the first mass produced Japanese V6 car, not only that but V6 turbo to boot. The VG30DE made Nissan's ''87 300ZR (Z31), in late 1986, the worlds first production car with electronically controlled variable valve technology (VVT). The Z31 line lasted from 1984-1989 and was succeeded by the Z32 300ZX, famously known for the VG30DETT.
The Z31 isn't limited to the "300ZX" name, but unfortunately, the 300ZX was the only Z31 available outside of Japan.. The names differ based on the type of engine it came with. It came with both VG and RB engines, from 2.0 litre to 3.0 litre engines, they were:
- 200Z/ZG/ZS: 2.0L SOHC VG20ET
- 300ZX: 3.0L SOHC VG30E and VG30ET
- 300ZR: 3.0L DOHC VG30DE
- 200ZR: 2.0L DOHC RB20DET
Also note that the Z31 names are related to the number of liters of the engine it came with.
The Z31 had 3 main types divided by periods of years. For clarity sake, we will name these periods Zenki, Chouki (USDM only), and Kouki for our discussion. These are Japanese words and literally mean First, Middle, and Last term in Japanese. The first term, Zenki, lasted from 1984-1986. There is some overlap with the Chouki, middle term during 1986, which was only available in the U.S. Domestic Market (USDM). Finally the last term, Kouki, lasted from 1987 to 1989.
1984-1986 aka Zenki Z31
The Zenki body type is the original design. The defining characteristics of it is the fenders, bumpers and tail lights. The fenders have a sharp defined crease from the body. The tail lights are large with two levels. They are joined by a center light bar that usually says the name of the car and or the words "turbo" for turbo models. The licenseplate sat between the tail lights, under the light bar and was illuminated by it. 
The Zenki bumper was sharper in nature compared to that of the Kouki design and a solid black stripe ran accross both front and rear bumpers. Turbo models also came with a front spoiler lip, foam rear spoiler, and a hood scoop for cooling the turbo. In Japan, all Z31's were turbo, and these spoilers and lips were an option. In USA, the Zenki body is found on '84 and '85 models of the Z31, while in the rest of the world, they were produced through 1986. Naturally Aspirated models came with 4 lug wheels, while turbos came with 5. The Zenki model was produced under both Datsun and Nissan badges, and only under Nissan badges in 1985. As far as I know, the 200Z's were limited to Z31's Zenki period.
1986 (USDM) aka Chouki Z31 (300ZX only)
The Chouki was only found in the United states during 1986, meaning that all "Chouki's" are 300ZX's. It had rounder wider flares compared to the Zenki, and came with sideskirts. The black trim around the bumpers was dropped. Fiberglass sideskirts and rear deck spoiler became standard on all trims of this year. 
Turbo models came with a redesigned sleeker front spoiler lip, and a water cooled turbo eliminated the need for a hoodscoop on turbo models. 5 lug wheels became standard on all trims, and Nissan brought back the AE turbo wheels for the 1986 turbo. With the exception of he hood scoop, the 1986 USDM Z31 Turbo is commonly mistaken for the 300ZX 50th Anniversary Edition of 1984 (AE).
Note: In the case of the '87+ 200ZR, it can also be considered a Chouki (middle term) of sorts but it is the exact opposite of the USDM '86 Z31. It came with Zenki flares but Kouki front and rear design. However, Unlike the USDM 1986 model, it did not pave the way to the Kouki model since it did notpreceed it. Therefore, the 200ZR is more of a Kouki just with Zenki flares.
Kouki models became available world wide in 1987. The Kouki also signifies the coming of the DOHC powered 300ZR in Japan. The Kouki Z31 is often seen as the Zenki counterpart. It had the wider fender flares like those found on the USDM 300ZX Chouki. The tail light was redesigned as a single bar. The sideskirts and front lips were less pronounced. 
The rear license plate was moved down and recessed in the rear bumper. The front and rear bumpers were much sleeker and rounded with less indentation lines. The hood was also rounded and less defined. 5 lug wheels became standard and had a swirl pattern similar to, but a little different to, those found on the 1986 300ZX Chouki turbo and the 1984 300ZX AE. Like the Zenki, the rear spoiler came on turbo models, but was optional in Japan including on the 300ZR.
Note: All Z31's in Japan, with the exception of the 300ZR, were turbo.
Special VG Powered Z31's worth noting:
1984 AE, 1988 Shiro "SS", 300ZR
More info:
For more information on Z31 differences, visit
http://az-zbum.com/information.differences.shtml (USDM)
http://www.z31club.com/datafile/grade.htm (JDM)
VGPowered.com's Z31 Forum
If anything is incorrect or if you feel something should be noted, please post in the Content Submission section of the forum or contact the author.
