| Comparing the plenums from top, front and back, either plenum should bolt onto any year head/upper plenum (dirty plenum=90-94 clean plenum=95+): 
The first difference we all know is the fuel rail/injector seat: 
Converting the 90-94 lower plenum to accept the 95+ injectors is just a matter of machining off a little of the seat so it is lower, and then machining an appropriate diameter seat. No problems so far... The tricky part comes from comparing the bottom of the plenum: 
Notice the very different design of the ports? The ports were designed differently for the injectors, the 90-94 injectors have a longer nozzle tip and a single pintle, while the 95+ injectors have a shorter nozzle tip and a 4 hole nozzle which should have a wider spray pattern: 
With the 90-94 injectors, the fuel leaves the injector a little lower, and has a narrower spray pattern. With the 95+ injectors, the fuel leaves higher up on the plenum, and has a wider spray pattern, so it was necessary to design a wider/different port to prevent fuel from hitting the walls of the plenum/head. A straightforward machining of the 90-94 lower plenum to accept 95+ fuel rails and injectors would seem to be less than ideal, as it is likely part of the fuel spray will hit the plenum walls. I have not seen any of the JWT/Z1/etc machined lower plenums nor have I actually tested the spray pattern with those plenums, I do not know if any work was done to the ports, hopefully someone can test this and post more info. I think a better option is to use the 95+ lower plenum and have the heads port matched and smoothed out to match the air and fuel flow from the upper plenum. I was talking to Miles earlier and he mentioned he did this on his Z, not sure if he would have any pictures to post. But this is a picture of the 90-94 head (taken from a tt.net member's post, unfortunately I forgot his screen name): 
For the other larger aftermarket injector kits available for 90-94 cars: Border 850cc injectors and rails - the injectors are still a single pintle/spray hole design, though the tip does not extend as far as the stock injectors. Sard 800cc injectors - these injectors have a 2 hole spray pattern, and IMHO would have a poor spray pattern into the lower plenum/heads.
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90-94 lower plenums and injectors vs. 95+ - 
