Maxima N47 (pros: small AND quench chambers, some have large valves, cons: some have small valves)
P90 (pros: sq ports, large valves and quench chambers, cons: large chambers)
N42(pros: sq ports, large valves, and small chambers, cons: open chamber)
P79 (pros: quench chambers, large valves, cons: large chambers and round ports)
N47 (pros: small chambers, large valves, cons: open chamber and round ports)
E31 (pros: small chambers and square ports, cons: small valves)
E88 (pros: small chambers and square ports, cons: small valves)
The round port vs square port is up for debate, I've read that the rectangular ports flow better due to lacking the exhaust liners, but I've also read elsewhere that due to the casting, the round ports bend at a less severe angle for better flow, and once ported can flow better than a rectangular port head.
The N42/N47s are nice because they have small chambers that can be used for easily achieved higher compression, but due to the open chamber design are less detonation resistant than the quench chambered P90/P79 heads, which unfortunately have larger combustion chambers and require shaving to achieve a high CR. I guess it all depends on your money situation: if your short on $$$ for machine work, I'd go with the N42 so you can just bolt it on with flat top pistons and have nearly 10:1 CR. If money is no object, go with the P79, have it professionally ported, and according to Brian Littles page, you can shave it .080", use early valves (which are .080" longer) and shim the cam towers.
I agree with zmonster. However, I would place the P90 at the top. It was Nissan's best engineered head for the the L6. It has been confirmed that the heads are already ported extremely well and to exacting specifications. The head is so well ported that often extra porting if not done extremely well can actually lead to less performance. If you decide to port it, the metal should only be removed enough to smooth out the surfaces inside the ports.
My favorite head is the P90a. It is the rare Hydraulic head made on 83 zx turbo models. They don't make the lifters for this head anymore. But removing them and dissassembling them and letting them soak in gasoline to clean out deposits can restor them to like new conditions. Plus, there is enough metal to shave them a little bit for excessive cam profiles (granted it is costly). The P90a allows for much higher rpms than the typical spring lifters.
I don't think there is any ideal head. No matter what kind you use, you have to do something to it to get the maximum improvement. p-90 and p-79 have to be shaved, and while the round ports flow well on a mildly modded motor, I have yet to see flow bench results that would indicate they would work on a really wild motor. I would love to see the flow tests done on a p-79 that has had the liners removed and the exhaust ports smoothed out and polished up....I suspect they would be very good......
The head you have is not as important as the knowledge of what you need to do to each kind...any of them can be made to perform very well with modifications.
1 fast z recently put a square port and a round port head on a flow bench (not just paroting internet hype) and found (when both heads had a minor clean up in the seat area) the round port flowed at least as well if not better! So much for the P90 being the best head. Sure the P90 etc LOOK like they should flow better, but unless you're going to have a professional port it, the round ports work fine. I guess the "square port is better" is never going to end even though people have posted info proving this is urban legend? Another advantage the round ports have is: with a decent sized header, they have a nice anti-reversion quality the other heads don't have.
The reason they used a square port on the turbo engine was the liners wouldn't be able to take the heat from the turbo, not because it's a "better flowing" head. The early turbos used the old N42 head (with it's crappy combustion chamber) for this reason. I'm using a P79 on my Z engine and it's -making- power (not just spinning..) well past 6500RPM with a round port head. I have a P90 head and not sure if it's worth swapping for the P79 unless I wanted to have it ported. To do this right, isn't cheap and most home porters would likely make it worse doing anything other than a mild clean up.
IMHO for someone on a budget either the N47 head or a shaved and shimmed P79 is the way to go. Already has steel seats, big valves and either one will get you the compression you need and flow WAY more than most people ever need with a mild valve seat area port job. Most people already own one or the other as these came on most of the later 2.8L engines, just there isn't much bragging rights to owning a "non-turbo" head?
As far as a P79 with the liners removed, they would be way too large for the valve size these engines will support and also too large in relation to the intake port. The P90 or N42 would be a better choice for a race ported engine.
"Plus, there is enough metal to shave them a little bit for excessive cam profiles (granted it is costly). The P90a allows for much higher rpms than the typical spring lifters."
Where did you come up with this info? How is a hydro cam going to "allow for much higher RPMs" over a solid lifter cam? Funny that the F1 and Nextel cup engines builders seen to disagree.. And where are you talking about 'shaving for excessive cam profiles'?
oh yeah I forgot to mention I'm using a .040" shaved/shimmed N47, ported, with exhaust liners removed. I'm using it because I had one, and I didn't have the money for all the machine work that would be needed on a P79, nor did I have a P79.
The way I evaluated all the heads was stock for stock. If you had the money or skill to have the machine work done right, I would switch the P79 with the P90 and the N47 with the N42. I would still keep the Maxima N47 at the top though, because it has quench chambers, small ccs (so it doesn't need to be shaved), and later models can be found with the large valves, I think making it the perfect combination between P and N series heads. I don't remember the port shape on the Maxima N47, but in my opinion with the above mentioned characteristics I think either way would still put it on top.
Although merely flow numbers is not a good way to evaluate a head, I do agree with Steve that the round port heads can be made to flow just as well as the square. Like i said, the round ports don't bend as sharply on the port floor.
ok so... first let me say pardon me for being a complete newb at this.. ive been reading all night and i have been readying over the last couple years this debate on heads.. it seems evertime i turn around somthing has changed first.. let me tell you what i am planning.. what is available.. then i want to hear some of your input on the best decision.. i work for a race shop.. so lets assume that money is not a huge hurdle for me.. ok the engine set up im going with.. im going with an f54 block bored to 89.5 with an oversized ka24 piston... l24 rods... either shot peened or custom forged.. have decided yet (any input is welcome) LD28 crank.. micropolish and balanced(if not already... ill have then check it first) 0 deck height with 1mm head gasket.. (im gonna experement with .02+ mm deck height and see if it will clear but im almost positive it all depends on the head choice) I want to build an NA fairly high compression high reving (at least 7k) pump gassable SU powered motor... max compression im going to toy with is 11.0:1 (with the .02+mm deck height and 1mm head gasket if all clears well) soo the head options.. .... available are.. N42 N47 and im almost positive its a P79 ( i dont feel like walking out to the shed right now) I hear many debates on the flow properties.. does the P79 flow better than an N42... im either going to go.. N42 cleaned up ported polished(valve job?)... or P79 shaved, cam towers shimmed ports cleaned up and polished(valve job?)... i have an aftermarket cam for it (ill post the specs as soon as i find them) and i am going to go with MSA springs and lifter kit (unless someone else has some better options) Tony D... Blue... Norm... 1 fast z... you guys are gods on this site (imho) (sorry if i forgot someone) so ... lets hear what you got...wich is the best way to go.. small chambers, large chamber, quench chambers.. blah blah blah..
Ok found the cam specs.. Erson Duration 280/280 intake exhaust, gross lift .424/424 intake exhaust