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. Here's a good head timeline to use./p>

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 high-quench/swirl 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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  EXHAUST RUNNERS
  The P90 square exhaust runners are slightly different compared to the N42/N47square runners. Because of the deeper chamber it creates a "straight-shot" out of the combustion chamber. Compared to the N42 with its larger short-side radius bump, you can actually put your finger straight down the exhaust runner straight into the P90 combustion chamber. If this doesn't flow better I don't know what does (!)

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?

MY NEW HEAD MODS

My P90 mod below followed the same mod-path as the one I used on my shaved P79. The engine is a L28 F54 with .50mm oversized flapttop pistons. SU carbs, SM needles, ZTherapy 1.0 nozzles and N36 intake.

• - Mild porting, contouring of underside of valve seats and runners
• - shaving .080" to raise the compression to 10:1
• - 5-way valve job intakes, 3-way exhaust
• - SI stainless valves, swirl polished, undercut stems
• - Webcamshafts cam 230/230°@.050", 430 lift
• - new valve seals and rocker arms of course

Well, on the road this mod exceeded my expectations!

My previous F54 flapptop engine with shaved P79, badly tuned by me,with iffy fuel pump turned a 13.9 1/4 mile a couple of years ago at the track. This new engine with 10:1+ compression, paired with better valves and better cam and simply screams. The car runs like it's at Lime Rock now, with serious power coming in at 4k and instant response all the way to 7k. It bellows like a IMSA car and runs fine on pump gas.

Rather than a more detailed description, all I can say is: higher compression plus a great flowing head equals fiece acceleration.