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.

 

I’m surprised there is no interest in upgrading your Z31 N/A firmware.
If anyone is really interested just let me know and I’ll give the first guy an eprom for free as long as he is capable of doing some soldering to remove the old eprom and soldering in a socket to plug the new eprom into.
I would ask that he posts his results.
I’d like to sell the remaining EPROM’s for $35 each, which includes shipping to the lower 48 states.

 

I'm looking for one

 

What year Z do you have?
Is it a 5 speed?
Can you solder?