DAW Posted November 4, 2004 Share Posted November 4, 2004 McAdam, glad to see you finished that P47 head project. McAdam points out some important points about the Maxima L6 head. There are a lot of misconceptions about this head. First, it already has L28-sized exhaust valves which need nothing. It is the intake valves which need to be replaced with L28E valves; and new seats. E31 undersized exhaust valves can be replaced with L28 valves by grinding the seats out in an E31 head, but putting larger intake valves requires replacement of intake valve seats, be it an E31 head, or a P47. The N47 Maxima head isn't really a sister to the P79/P90 head because those have a higher roof to their combustion chambers and, accordingly, shorter length valves. The Maxima N47 is more like an evolved E31 as it has similar chamber cc and the same length valves and roof height, but a reconfigured combustion chamber design. It's easier to just call it "P47" though so that's how I'll refer to it too. The Maxima L6 cam is an economy grind with less intake lift and soft intake valve springs. The Maxima "P47," is a modern E31. It differs in that it has a true closed-chamber configuration, larger exhaust valves, f.i. provisions, round exhaust ports with stainless steel liners, and through-the-cam oiling instead of a spray bar. It is a viable route to a performance engine of 2.8L or less. When used on anything of greater displacement, it requires the use of dished pistons which defeats the advantage of the closed chamber head. The 0.080" shaved P79/P90 approach is more an alternative to using the open chambered N42/N47 head on larger displacement engines (that should be using unshaved P79/P90 heads and pop-up pistons), than it is an alternative to the "P47". DAW Quote Link to comment Share on other sites More sharing options...
Guest Magnum Rockwilder Posted November 4, 2004 Share Posted November 4, 2004 How about an estimated whp and/or 1/4 mile time for this setup: 71 240Z L28 block E31 head, ported & 280Z valves installed Comp 292 cam, 292º advertised, 246º @ .050, .480 lift Arizona Z intake w/ Holley 390cfm carb header, straight 3" single exhaust MSD ignition/280Z distributor R200 3.90 rear 5spd 280ZX tranny Is there anything special I need to do to the valvetrain? What's the RPM limit for the stock valvetrain? Anything need to be done to the bottom end? Estimated performance? I'm trying to gauge whether this setup is worth trying while I build up my Caddy 500... which may take a while due to high cost for parts. Quote Link to comment Share on other sites More sharing options...
Mack Posted November 4, 2004 Share Posted November 4, 2004 1. You CAN install the N47 valves in the P47 head by just cutting the seats to fit the N47 valve head, but you can also do it the way I did on my E31, by putting a larger seat in the head and porting the runner to match. The bigger valve in the P47 would flow a little better, but having the larger seat and port would do more. Ill have to check on this. This was a concern of mine while I had my head in the machine shop, that a bigger valve would really do nothing and that the seat had to be bigger as well. I recall the machine shop saying that it was the same size seat or some such thing.... at any rate, I have my old N47 and I will be pulling a P47 for someone else soon and I will compare the valve seat opening diameter on both heads and see whats what. 2. Unshrouding my valves took out 4 or 5cc IIRC on my E31. Not sure how much you took out on your head, but I'm guessing more than 1 cc. I suppose this is going to vary on who does the work and how aggressive they are with it. I thought I took a lot out, but I don't have anything to compare against. I also eyebrowed the block, which might have a tiny tiny effect too. Just for comparison, the P79 shaved .080 has about a 46 cc chamber according to this site: http://www.geocities.com/zgarage2001/head.html So if you or your machinist was as aggressive as I was, you might end up with just about the same chamber volume as the shaved P90. I dont think they got too agressive in unshrouding the valves. I noticed minimal change in the shape and appearence after the unshrouding of the valves. although they did take out some material. I woudl think that if they had to take out a lot of material, they would have charged me a lot more than they did, lol! 3. If you shave a P90 .080 and use the N47 valves, you don't have to mess with the lash pads. You do need to get cam tower shims. Last time I got a price on them they were ~$20 for a set, and that set was .015" thick. You could theoretically stack 6 sets of those, or perhaps find some others that were thicker to start. I didn't look for different thicknesses much at the time because I didn't need to. Thats still $120 just in cam tower shims. plus however much they charge to shave a head .080" also, the P47 is MUCH easier to come by than the coveted P90's. 4. The benefits of the P90 is that there is less work to do to it. The compression isn't as high to start with. Unshrouding the valves isn't necessary due to the fact that it already comes with a chamber properly shaped for the larger intake valve (there might be something to be gained here anyway, but it isn't a neccessity). Larger seats and intake ports. The only machine work necessary is the shaving of the head. I cannot argue with you on this, all I can say is that the P47 has a lower initial cost, as they are more plentiful. and also, even tho it sounds like more work (install bigger valves, unshroud valves) it is cheaper and all you have to do is droip it of at the machine shop and they will do it all at once. I think what you have done is very similar in the end result to the shaved P90 setup. The differences are that you have smaller runners, and that you might have a 2 or 3 cc smaller chamber. this is true. on a street motor this is an excellent weekend project! for an all out racer, maybe a P90 is the way to go. some other things to consider, The E31's used to be the head of choice for HI performance, all of the ports were designed for small valves, yet people still used them. at least the exhaust port on the P47 is designed with the bigger valve in mind. I think the intake might be as well, I will have to check. I know the beginning of the port where the intake manifold attaches was the same size diameter as my N47. this is good info and food for thought! keep it coming guys! and, as soon as my local car club http://www.az240sx.org has another dyno day, I will get it checked out for sure! of course, by then I hope to be running my M30 ECU/wiring harness with a colt cams 260 degree FI grind. McAdam Quote Link to comment Share on other sites More sharing options...
Mack Posted November 7, 2004 Share Posted November 7, 2004 well, I caught myself in a half turth. after beating on my car all weekend, I have come to the conclusion that the maxima intake valvesprings get airborne at around 5500rpm's under hard acceleration. When I said it revved nicely past 6K, it did! sitting in the driveway, lol! anything past about half throttle and over 5500rpm's and the engine feels and sounds like it cuts out on most if not all of its cylinders. so, within the next few weeks I will be ordering a reground cam and rockers at which time I will also install 280zx valve springs. I have heard that the maxima small valves tend to cause an L28 to fall flat on its face at anything much over 4750rpm's. when my valves do start to float, its still pulling HARD and it feels like someone slams on the brakes. this is with 280zx intake valves, which are bigger and have more mass than maxima valves, which only exacerbates teh problem caused by the weak springs. McAdam EDIT: GREAT thread with a TON of Maxima N47 info on it, including info on port and runner sizes! http://www.zcar.com/forums/read.php?f=1&i=501641&t=501546#reply_501641 Quote Link to comment Share on other sites More sharing options...
kalium99 Posted November 12, 2004 Share Posted November 12, 2004 The L28 valve springs are different to the Maxima head are they? I have a "Maxima" head as well (although mine is actually a skyline E88 head, but they are one and the same). I might put the 280zx springs back in it before I put it back on, if it is worth my time Quote Link to comment Share on other sites More sharing options...
DeusEx Posted November 25, 2004 Share Posted November 25, 2004 This is a handy thread I have a P90 that has been ported and polished and am looking to get it shaved very soon in a last ditch effort to gain power. Its running abit over 160RW HP (was 160 before the new cam). With my current setup, without stroiking the engine i believe that re jetting my carbs and shving my head is the last work i have to do. But i cant see this getting me o the 200rwHP mark. If there going to be a large NOTICEABLE advantage in shaving the P90 down (and if so how much?) as well as rejetting my carbs? it revs hard to 6 and will go to seven, but i think i should be getting alittle more out of it? Or am i deluding myself? For those of you not familar with my setup.. Triple 45mm Webers 32mm Venturis, 200 Air correctors, F16 Emulsion, 120 Mains. CAM Settings Valve Ex: .012 In: .010 Ex Opens: 66 Closes: 26 In Opens: 26 Closes: 66 Lobe Lift: .318/.318 bored to 2.9 Flattop pistons F54 L28 linaged crank blueprinted and balanced internals MSD 6A ignition 2.5" exhaust roughly 8.75 CR Flightened flywheel Thanks guys Quote Link to comment Share on other sites More sharing options...
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