NA Head Choice Dilemma: N47 vs P90

[Oddmanout84]

(scroll down if you don't want to bother with words and backstory)

 

So, after a long hiatus from the Datsun world (marriage, relocations, etc.), I'm back (sort of). Not much time for working on cars, so the orange 280z has sat like a lump in storage for 5 years. Luckily for me, my wife has forbidden me from selling off the car for a down payment, so I'm trying to blast away a quick build to get it running again enough to get it into a new garage home in New Hamster. Usually I hate asking Hybridz technical questions when I can get my enjoyment from browsing for a few hours, but this one is more of an opinion poll, I suppose, so I don't feel as bad.

 

I'm trying to get a budget build together and I have a few options. Essentially I'd be fine if it just ran, but I don't want it to build a working slug either. This car is a toy, first and foremost, and I'll probably have home improvement projects that will put any major performance improvements on the backburner. I'd like to be able to have a little fun on the weekends with minor upgrades and tuning here and there. Now here are the important details:

 

 

 

-Going for an NA build. I have a full out turbo L28 mothballed in the back corner, but it requires quite a bit more work & $$$ parts before it's good to go, and I want a break from it.

 

-My base is a rebuilt L28 bottom end, with ARP studs & ZX flat-top pistons installed for a bump in compression.

 

-N47 head, with a very mild home port (rough sanded intakes, polished chambers, very light unshrouding around the valves). It does however have a very slight warp to it, and would need to be bent/milled flat. Stock cam. This would bump my CR pretty high (near 10, according to the calc but I'd also CC before assembly to confirm), and I'd probably (definitely) need a bigger cam to run it on pump gas. Keep in mind though, that our New English pump gas is a bit "better" than the left coast stuff.

 

or

 

-P90 head. Pretty much stock, only hit the ports with sandpaper flappy wheels. Zero warp, stock turbo cam. Very clean, and as such I'd hate to mill it down for a bump in compression ratio, since that's an area I could potentially save some cash and use it as a spare turbo head down the road. I have a friend who ran a similar setup on Webbers for a while, but the lower CR seemed to make it a pig. It may have been dialed in since I last saw it, though, and his may have been running dished pistons as well (don't remember, never saw the inside).

 

Triple Mikuni 40phh, and 44phh. The 40's are pretty minty, besides their gaskets, and have brand new rebuild kits ready to install. The 44's are in pieces and will require a lot more work, and matching outer venturis, but the rest is there minus additional jets. The choice for now is obvious, even though Todd (Wolfcreek) told me years ago in our conversations that the 44's are too big for an L28 and I should stick with the 40's... Even though everywhere else I read says the 40's leave the L28 a bit undercarbed. If Tony D is reading this, I know I COULD just press ctrl+shift right right up down, but I threw that one out with the bathwater. I know its not logical or as efficient, but I got both sets for a song years back and I like the idea of carbs.

 

Various odds and ends for completing the project are available, including two sets of headers (both square port though). I just need a few sections of pipe to complete the exhaust from there back.

 

 

The dilemma is the two heads. I know the N47 will give me the greatest bump in CR, which is "good" for NA power, but sacrifices ignition timing & has design efficiency issues. A few years ago another Datsun guy on the internets told me he was running a similar, higher CR setup with welded up combustion chambers just fine on pump gas and a better cam, but everything on Hybrid that I've read over the years says otherwise.

 

With the P90 I probably have the more direct route, but a hideously low CR unless I put some work into it. I suppose I could swap in the cam from the N47 and make up for the loss with ignition advance, but I don't want a gutless slug that's impossible to dial in. It is, however, probably the most logical choice for getting the car on the road. I'll also add that I'm not entirely concerned with a specific HP number, but rather one that feels right, sounds nice, and is enjoyable to drive in the interim when there's mortgage, home improvement, and little demons running around monopolizing the remainder of free time.

 

 

 

 

Ok, so that was a bit long-winded. I suppose, in summation, the question boils down to N47 vs P90 budget NA carburetor build.

 

Wat do?

 

[NewZed]

This is always an interesting topic.  I typed in "compression ratio benefits" on the Google and actually found what looks like a decent overview of the issue.  It doesn't cover "quench", which is another factor in your choice, but it does cover a lot.  Seems to be based on facts also.

 

Edit - actually it does mention quench, with a few tips included.  Your question would be about whether or not the N47 wth flat-tops is a good quench design.  Or even if the P90 with flat-tops is.

 

http://www.hotrod.com/events/coverage/0311em-power-squeeze/

Edited August 9, 2015 by NewZed

[ryant67]

The P90 would be fine for compression ratio if you run flat top 89mm pistons, an LD28 crank, and L24 rods

Of the two, on the bottom end you have, it's a no brainer for me - N47.  Use a Nissan head gasket (1.25mm) and you will be hopefully be bang on 10:1 compression after you skim the head to remove the warping.  Run a big enough cam, and detonation shouldn't be a problem at full timing.  From what I have gleaned from hundreds of hours of reading on here, 10:1 would only be a problem on a mild cam, or with crappy gas.  You shouldn't have any problems.  

As for the carbs, I'd just refresh and run your 40's for now.  You can then rebuild your 44's at your leisure, then decide what to do with them later down the road.  I doubt 44's are too big for an L28 though, unless the L28 is bone stock.  If you are running a big cam, have done some head work, and have a decent header, then you should be able to take advantage of the extra capacity of the 44's...



 

[Oddmanout84]

Thanks for the replies and the link, a good read.

 

To a point, yes, that's the question. What do I gain with one head vs. the other in CR and chamber design, and which is currently the most economical. I do already know that the P90/F54 flat top is a decent combo already, as it is essentially what came stock in the NA ZX models (different number head, essentially same combustion chamber shape). But that was meant for the stock vehicle, and emissions concerns. I'm certainly not saying it's junk though, as it is widely regarded as the best designed and highest flowing of the bunch, and has that nice squish area, it's just not at the static CR I'd want it to be. And since I don't have access to the LD crank et al for a stroker, that may not be the best choice.

 

Ryan, I do like the idea of the N47 the most, so I can agree with that. I should mention that I currently have a felpro gasket on hand, so that will probably bump the CR up a bit more unless I buy another Nissan gasket. I really need to research cams a bit more, especially for what constitutes a "big" one. I know with a higher CR I should be bumping up the duration, that's a given. I'm a bit less clear on the lift, though. I know more lift = greater quantity of air via increased flow, but most of the aftermarket cams I've glanced at seem to pair the duration I want with a greatly increased lift, to the point of requiring a set of springs. I'd like to avoid that, as long as it's not a bad idea. I'm admittedly ignorant of cam design, as I all but neglected the consideration for my previous turbo builds. I'll have to talk to the cam grinders if it's possible, or feasible, to get a high duration/low(er than .460) lift cam. I suppose I'll find out how stupid my questions are when/if it comes to that.

 

Edit: And yes, I'm definitely using the refreshed 40's for at least a while. I'm not worried about them being tiny. Prettying up the 44's is a long term side project, especially given their worse shape. They were also bought piecemeal, not as a matched set like to 40's.

Edited August 9, 2015 by Oddmanout84

[Oddmanout84]

Classic case of me speaking too soon, I suppose. I just found the "right" article to refresh my limited knowledge of cam specs and feel a little bit more enlightened about what I'd be needing. I'll have to do a bit more research, but it's looking like Web Cams' 391 grind is close to what I think I'm looking at. I'll have to see if the .050 duration of 238 is close enough to what I'd be needing at nearly 10:1 CR. I've only heard positive things about their cams, and the guy who runs the Datsun Z Garage website (who was in the local Z clued I once frequented) had one of their milder grinds in his motor. Of course, he was running a non-stroked flat-top L28 with a milled P90 combo, so his experience might not be congruent with my own if I'm going a slightly different path.

Edited August 9, 2015 by Oddmanout84

[Leon]

I'd run the P90 and be done with it. You'll be at stock NA 280ZX compression with the flat-tops. Run the head stock and use it as a backup once you put the turbo motor in. This is the simplest and quickest path to enjoyment. Plus, the P90 has square exhaust ports which will match your header. It may not be a monster but it'll be a fun, torque-happy motor.

[madkaw]

I'm with Leon, go with the P90. Better combustion design than the 47 and the CR loss would be made up with running more aggressive timing on the P90. I'm betting your friend was running dished pistons. The progression to turbo would be easier. 

[Oddmanout84]

I appreciate the diversity of opinions. The P90 route is certainly the more practical one, and perhaps using the N47 cam and tweaking the mechanical/ignition timing might bring me close to what I'm looking for. I'm assuming it'll still be punchy even if I'm not milling it down? I should also mention that I have toyed with the idea of turning that combination into a higher comp turbo setup, but not for some time so that's really not on the table as a decision making point. Maybe if I win the lottery (in terms of finding, not the paying) and acquire one of those HKS units I can play with a carb turbo setup, but that's a when/if down the road.

 

I also have an old E88 head, but I didn't bother mentioning it. According to the identification guides I've seen it appears to be one of the 240z versions with the different chamber design that's similar to the E31, and would solve the square exhaust port issue. It is however caked in 1970's grime, corroded in a couple exhaust ports, and all the brass seats are beat to hell. I'm just sort of using it as practice and a possible resurrection project someday. Uncle used to race Corvairs back in the day has a TIG, so maybe its worth trying for S&G. If nothing else, it's porting practice.

 

The N47 w/ cam combo feels right to me, but perhaps I'm letting myself get caught up over compression ratios.

 

Madcaw, I've been following your Mikuni thread ever since I grabbed my triple sets. I also have the same Mikuni manifold (came with the 40's), how does it work out for your build? I'm under the impression that its short runners are more suited to the upper RPM ranges, rather than lower end torque? That's another thing I had been factoring in to where I wanted this engine to be, but I'd appreciate knowing if my assumptions are baseless and shouldn't be weighing my decision.

[HuD 91gt]

I hope you don't mind me hoping in on your thread but I'm going through a similar dilemma.

 

I have a fully working l24/e88 combo. A disassembled f54/n47 (flattop)combo which needs a new cam (smoothed intake), and the bottom end rebuilt.

 

 

There is also a L28et in the junkyard, that has been sitting there for two years. Apparently turns of freely. A turbo swap would be fun in the future.

 

A) rebuild the bottom of my f54, get a cam kit from rebello and call it a day

 

buy turbo motor, re-ring it and toss it in NA

C) put flattops into turbo block and run p90 head

 

 

Options B/C I would probably put in valvetrain from working l24 motor.

 

All the options cost money. What to do, what to do.

[NewZed]

Just a comment on combustion chamber design.  One of those small bothersome things.  The "quench" concept is supposed to give faster, more controlled "burn".  Allowing a complete burn with less ignition timing.  But the timing specs for 1978 (N47) and 1982 (P79/P90 design) are essentially the same.  29 degrees total centrifugal timing.  There's a +/- 2 degrees on the 1982 specs., but the max is 29.  

 

Just seems odd that Nissan would redesign a head for quench and fast burn and not take advantage of it.  There may be improvements but it doesn't look like quench is one of them.  Could be that flat-top pistons are just cheaper to make and the head needed some improvements anyway.  So they made the chamber bigger for that reason alone.  Overall, the two type of engines, early 280Z with dished, and ZX with flat-tops don't seem very different.

 

 

Your CR should be 8.8 with the P90, not really hideous.  You'll have a common L28 engine with carbs.  Should run great.

"With the P90 I probably have the more direct route, but a hideously low CR unless I put some work into it."

Edited August 10, 2015 by NewZed

[Oddmanout84]

NewZed,

Yes, I might just have been a bit hyperbolic with that one. I guess its not terrible when you consider it being only around a point of difference. Regarding the redesign of the chamber though, my understanding was that with the lower compression ratio comes lower emissions, which would be right about in line with what they were trying to do at the time: Power sacrifices in the interest of fuel economy and lower pollution. Adding a better working/flowing design retained performance.

 

HUD,

When I first started with my zed, I got it running on the stock N47/N42 dished setup for about as long as it took me to source an L28ET. I was lucky enough to find a new friend with a decently rebuilt one sitting in his garage with a Haltech E6K managing it. That motor was incredibly fun, until it broke due to the builder putting a wanky bolt stackup on the crank pulley and a low oil pressure issue. My fault for not looking at things hard enough. Bang for the buck, and the general consensus on Hybridz, tips the hat toward the turbo motor (if you're sticking with the L6) in most cases. Depending on how rusty/gunky yours is, maybe you won't need to re-ring it. It seems like when people crack open the bottom ends of these motors to "refresh" things, it opens up to more problems in the long run.

Edited August 10, 2015 by Oddmanout84

[HuD 91gt]

Thanks for the reply oddmanout.  That's the issue, i've already purchased one unknown engine and it is now disassembled ready for the machine shop.  Another one is a risky move.  A lot of work goes along with the L28ET for us 240z guys as well including the fuel system.  One step at a time.  I'll have to get more details on the turbo unit.  

[madkaw]

I wouldn't put to much weight on what the short runner will or will not do. My car felt plenty torquey down low and would pull all the way to 7K-and it's a 2.4.

The Mikuni shorty manifold is a bitch as far as mounting . It also has clearance issues with the thermostat housing which i believe I covered. I also would not worry about gasket matching the runners. The anti-reversion is there for a reason.

[Oddmanout84]

Madcaw,

 

Yes, I'm leaving the runners the way they are (for now) for that exact reason. Maybe I'll hit them with 80 grit to smooth out some of the casting, but that's about it. I hadn't mocked up the thermostat housing yet, so thanks for the warning. I certainly have had a problem with finding the right mounting bolt/stud that actually fits the manifold, however. They certainly made things tight on the upper side!

[madkaw]

I actually have the set that came with the Mikuni kit, but didn't use them. They are very short so starting a nut on them is a bitch. I shaved down some older studs that had shorter threads on one end of the stud. I probably could have used the shorter Mikuni ones if I didn't thread them all the way in, but I didn't like that.

 

Another words you will find a very small amount of error is allowed when sizing them because they will run into the runners .