I use to have an ‘87 NA 300ZX 5 speed that I bought new, sold it in 2001.
The local Z club I was in did a group purchase from a company that had a modified eprom that increased the power and raised the usable RPM up to 6000+ RPM. It wasn’t a kick in the pants increase, more like 5-8 HP more, but the additional RPM was the big benefit.
If I remember correctly with the stock eprom it was pretty much out of power around 5400 RPM.
The group purchase price for these was around $150 each.
Some of the guys decided it was too much work to replace the eprom because it was soldered to the PC board. Plus there was a coating on the board to keep moisture out. When these guys eventually sold their Z’s they gave me their eproms.
I was just cleaning out the garage and found them. If any of you are interested I’ll give you a killer deal on them.
I’m pretty sure I have more info about install the eprom on one of my old computer. I’ll see what I can find and add more info soon.
I used this on my ‘87 NA 5 speed and also on an ’88 engine. I’m not sure this will work on a Z with an automatic tranny. this isn’t for turbo’s either.
Let me know if you’re interested.

 

More info, here are the eproms I have as well as a pic of the ECU opened up.



Not a great pic, I didn't have a good camera back in the day. The eprom that gets replaced is the one on the left with the paper tag on it.


And a few pics of my old Z.



And a pic of it just before I sold it.