Nissan made only three US 280ZX heads from 1981-83, the P79, P90, and P90a.

- The P79 came on the non-turbo L28s
- The P90 and P90a came on the 1981-83 turbo ZXs.

All three heads used the same valve diameters (44m intake/35mm exhaust), combustion chambers and intake runners. While I don't have numbers, one would have to assume that Nissan designed its turbo heads for one thing: to flow lots of air. The important difference was that the P90/P90a came with square exhaust ports instead of the lined "diamond" shaped ones on the P79.

The P90a is identical to the P90 except for it's unique feature of having hydraulic lifters under the rocker arms which no other Z head had. This made for a nearly quiet engine needing no valve adjustment, but could not use high-lift aftermarket cams because of the lifters and was costly to rebuild. P90a replacement lifters were about $50 each, but have been unavailable for years. This made the P90a a great head, but not the best choice for modifying. Keep in mind that the a P79/90/90a on a flattop piston L28 will give 8.8:1 compresion, on a dished piston motor 7.4:1.

THE P90 PROFILE

- square exhaust ports with no liners (like the N42/E88/E31 heads)
- .100" deeper chambers than the N42/47
- nearly straight exhaust runners and high-quench/swirl combustion chambers
- steel valve-seats

• INTAKE RUNNERS
  The P79 and P90/P90a combustion chambers are deeper than other L28 heads. By making the chamber deeper, it lines up the valve bowls with the intake runners and greatly straightens out the short-side radius "bump". N42/N47 heads have a sharper "bend" as the runner transitions to the bottom of the intake valve.
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• FUEL PUMP HOLE
  Strangely, while the P79 has the mechanical fuel pump hole sealed over, the P90 actually still has it, the ZX power steering pump flange covers it. I'm guessing the same casting dies for the square port N42 were used by Nissan. Why else would they leave the hole there?
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