I have no idea why there is not an aftermarket plenum....

[budget300zx]

i finally dove deep and starter on fixing my plenum issue. this picture should tell all the reasons why the #5 runner should be addressed...

[evildky]

Look at any intake from any factory turbo Z car from 81 to present and most of the n/a cars as well, you'll see that the intake furthest from the TB is smaller than the one closest. The same is true for most injected cars, more so on turbo cars, it has to do with air flow seeking the path of least resistance, and the one straight back is the least resistance so it's often restricted to prevent the rear most cylinders from leaning out due to getting more air than forward cylinders. The 81-83 turbo zx actually didn't have this taper and it causes cylinders 5 and 6 to lean out, the easy solution is to use the n/a intake which has a taper to the plenum reducing the amount of air that gets jammed to the back 2 cylinders. Keep in mind the Z31 and Z33 both feed from the rear so it tapers towards the front of the engine for the same reason.

There are plenty of big power Z32's out there making gobs of power on stock plenums, that's not to say there isn't room for improvement. The problem is twofold, first packaging, the z32 engine bay does not allow a lot of room to improve due to packaging constraints, I hear the Nissan engineers responsible for packaging the motor into the Z32 were none too pleased with the challenge, second Z32's are generally catalog built cars. In spite of being the poorest selling Z car chassis the Z32 was a darling of the aftermarket, i think timing had a lot to do with that. Most people building a Z32 just order the parts they need from a catalog or online, the S130 and Z31 guys had very little aftermarket and had to fab many of their own parts, on the intake that meant gutting and customizing the stock plenum or building from scratch, packaging on the z31 made this a challenge as well but not as bad as the z32.

I look forward to seeing your progress just beware that unless you do something to increase the airflow to the forward cylinders you could be causing a lean condition in the back cylinders.

[budget300zx]

i don't plan on modifying the other cylinders. i figured only remove the "splatter" from the welds to slightly compensate.
The removed original runner is slightly wider but volume seems to still be smaller than other rear runner. if in not happy with the results ill just throw another plenum on. I can now pull my plenum in 15mins so it not a major job.