P90 vs. P79 Cylinder Head for NA 3.1L

[kce]

Everything I've read here on the forum points to the cylinder head as being the place to make power with the L6 series. If I understand correctly there are a couple of important points: 1) The L6 series is very detention prone making anything above about 9.1:1 on pump gas (92 octane) dangerous, especially when running an engine without electronic fuel and ignition management. 2) The L6 cylinder head, regardless of their castings, in their stock form just don't seem to flow very well. And since getting fuel/air mixture in and getting exhaust out again is the name of game, this is a serious problem unless you are using forced induction to overcome these limitations. So if you are a fool like me and you are considering ignoring everyone's advice to "just do a turbo or V8 swap" and instead go NA (I'll probably do the V8 swap anyway!), the cylinder head seems to be the place to put your money. Am I on the right track so far?

 

All the stuff I've read for NA engines seems to point at using a P90 or N47 for a naturally aspirated L28 application... why not the P79? They look like they have a pretty good combustion chamber shape, and with some porting (at least according to BRAAP) they can flow well enough. If you shave .080" or so off the cylinder head and use flat pistons you should be able to get a decent compression ratio, run a nice lumpy cam, get good burn characteristics and decent CFMs once ported.

 

So why does it seem that there are not many of these around? Is it one of things where it costs a little bit more for the P90 so you might as well get it? Is it just an availability issue? Did I just ask a really stupid question?

Edited December 8, 2010 by kce

[cockerstar]

The P79 head has exhaust liners built into them, which on their own flow decently well. However, they have a habit of breaking off and making their way down the exhaust and wrecking that exhaust port. Some of them hold up just fine and others will fall apart. It's a bit of a luck of the draw.

 

This isn't the only reason, but it is a contributing factor.

 

 

Check out the sticky on this.

[kce]

The P79 head has exhaust liners built into them, which on their own flow decently well. However, they have a habit of breaking off and making their way down the exhaust and wrecking that exhaust port. Some of them hold up just fine and others will fall apart. It's a bit of a luck of the draw.

 

This isn't the only reason, but it is a contributing factor.

 

 

Check out the sticky on this.

 

First thanks for the reply.

 

Second, which sticky are you referring to? The L6 heads; pics and descriptions thread or the big and nasty headwork thread?

 

 

I read that the P90 has square exhaust ports instead of the oval shaped ones on the P79. Is that another reason to avoid the P79? If so, why? This is also the first I have heard of exhaust liner breaking... that does not sound good.

Edited December 8, 2010 by kce

[Lazeum]

What you've stated on the first post is about right.

 

Regarding the P79, I don't think there's anything wrong with it.

Myself, I have a P79 made by Braap that was showing drastic improvements over my previous stock N42 head. Camshaft were also different so it is not only an issue with the head, you also have to match the camshaft. I went from a Schenider symmetrical cam (Stage II @ MSA) to a Rebello Street assymetrical cam FYI.

My setup is a NA 2.8L, see my signature for some details.

I know I'm not the only one to have it.

 

Regarding liners, I believe it has been measured than P79 does not flow less than P90. Liners can fall down, worse case scenario, they end up on the car behind me with my current exhaust

 

Regarding price afterwards, you have to know where you're going, head jobs are not cheap (way over 1000$). I wanted to keep the L6 in mine for many reasons (stockish engine, L6 sounds, regulations, Car value ...) so I've taken this route and I don't regret it.

Edited December 8, 2010 by Lazeum

[Pyro]

how much compression a L28 can take on pump gas depends greatly on the cam timing.

 

I run 9:1cr with the stock cam, 9.5:1 with a 260 degree cam, 9.75:1 with a 270, 10:1 with a 280, 10.5:1 with a 290, 11:1 with a 300 degree cam. All on pump gas.

 

The reason is..... the later the intake valve closes, the less air is trapped by the rising piston on the compression stroke, so less cylinder pressure. This is why more compression is required for bigger cams (later intake valve closing points). The intake valve is still open as the piston is rising in the bore, thus reducing the length of the compression stroke.

 

So just saying you can't run more than 9:1 on pump gas is not the whole story.

 

I have run P90's and P79 heads and can't tell any difference. However, it is hard to modify the P79 exhaust port due to the liners.

[BluDestiny]

There is a huge misconception that the exhaust liners are "evil". Honestly the P79 and P90 have the same shape combustion chamber and from recorded flow test have nearly identical flow.

 

I feel it would only matter in an extreme case or road or drag applications with a N/A engine. Maybe someone could prove me wrong(is 250+ whp with an NA P79 head possible? probably with the right amount of cash)

 

Lazuem, have you had a chance to dyno your setup?

[kce]

Thanks for everyone's replies.

 

What you've stated on the first post is about right.

 

Regarding the P79, I don't think there's anything wrong with it.

Myself, I have a P79 made by Braap that was showing drastic improvements over my previous stock N42 head. Camshaft were also different so it is not only an issue with the head, you also have to match the camshaft. I went from a Schenider symmetrical cam (Stage II @ MSA) to a Rebello Street assymetrical cam.

 

Is there any chance you could post some more information about your Rebello cam (lift, duration, etc.)?

 

 

Regarding price afterwards, you have to know where you're going, head jobs are not cheap (way over 1000$). I wanted to keep the L6 in mine for many reasons (stockish engine, L6 sounds, regulations, Car value ...) so I've taken this route and I don't regret it.

 

So would $1500 be a reasonable amount to expect to spend for getting a head like a P79 ported? Does your figure include the price of the cam?

 

 

 

run 9:1cr with the stock cam, 9.5:1 with a 260 degree cam, 9.75:1 with a 270, 10:1 with a 280, 10.5:1 with a 290, 11:1 with a 300 degree cam. All on pump gas.

 

This is great info! Thanks! I know, at least in a general way, that duration and valve overlap play a significant role along with the compression ratio in determining an engine's characteristics, including how close you get to detention. Again, I have just read that these heads are *really* prone to detention...

 

Is there any chance you have some more specific information from your cam running in your 9.75:1 with a 270 setup?

 

 

 

 

There is a huge misconception that the exhaust liners are "evil". Honestly the P79 and P90 have the same shape combustion chamber and from recorded flow test have nearly identical flow.

 

Yeah and they seem to be substantially cheaper, hence my interest in them. If I can get a good P79 for $100 less than a P90 and for my planned application there's no significant difference, that's $100 dollars that could go somewhere else.

 

 

Braap had this to say in some older threads about the liners:

 

Yes, the P-79 is very close to the P-90 in port design and EXACT in chamber design, with exhaust ports being the prominent difference between the two.

In regards to the exhaust liners, you can spend the multitude of hours removing them, but if mega exhaust flow is your goal, you would be much better served just finding a P-90 head and porting the exhaust port by removing the step in the roof. Trust me on this. For mild to hot street applications, the round exhaust ports with the liners intact are more than adequate and offer great flow. Just blend that sharp edge from the back side of the valve seat to the liner using a die grinder or dremel tool.

If you want to really make it worth your while, you need to unshroud the valves as well. I wont go into details in this post, but if you perform a search, the details and pics are here on this forum…

Source

 

The P-90 has the SAME chambers and valves as the P-79. The difference between the two are the exhaust ports. The exhaust ports on the P-90 are square like the N-42 head where as the P-79‘s exhaust ports are the Round exhaust ports with liners like the N-47. For a turbo engine' date=' the liners are NOT desirable as they will come apart in short order and as those small parts of broken metal try to pass through the exhaust turbine it will cause problems, at best those broken chunks of exhaust liner will just block the turbo from spinning, or they could end up damaging the turbine… Either way, not good.

For a Turbo application, use the P-90.

 

I’ve seen a few liners on N/A engines that have come apart for one reason or another, including my second L-28 in my BRAAP car. A couple little pieces even fell into the chamber, had to remove the head to get them out and smooth out all the little divots in the chamber roof for those pieces bouncing around in the running engine. I suspect an overly lean condition over long periods caused the liner to run hotter than it was designed to and not to mention all the heat cycles that the liners go through over the 100,000+ miles that are on these cars currently… Any how, I don’t have any first hand experience running a Turbo on a round exhaust port with the liners, but my guess would be that the exhaust runs SO much hotter on a Turbo engine that the liners just wouldn’t live. The liners are sort of “free floating†hen they are cast into the head as and as such, they don’t have any place to allow all that heat to dissipate to, i.e. they aren’t touching the aluminum of the exhaust port itself which would help dissipate the heat.[/quote'] Source

Edited December 8, 2010 by kce

[blue72]

I have a ported and polished P79 on my N/A car and its working fine for me. Just as long as you make sure the liners aren't loose you should be good to go. As an added side effect they might just burn off extra hydrocarbons and clean up your exhaust a bit. I had the head done by Bryan Blake (1fastz on here) and he even took time to blend the edges on the exhaust side and open it all up a little bit. He also set up the asymmetrical cam and all the valvetrain too. Going from a stock P79 to this thing completely transformed the experience of driving the car. Before it would run out of breath at about 5500 rpm and now I can take it past 8K if I really wanted to. Everything was much improved afterward. We didn't modify the combustion chambers or shave down the head and I run flattop pistons, but according to my calculations I'm at right about 9.7:1 compression ratio with a mild / medium cam and can run 87 octane in it no problem at my elevation (5600+ feet). That combustion chamber shape combined with the quench effect apparently helps reduce detonation too.

[BRAAP]

My personal list of preferred L-series heads for N/A L28 street applications.

For stock to moderate builds, regardless of casting #, the round exhaust ports with liners flow very well and are my preference, just blend the back of the valve seat into the liner. The other beneficial attributes of the liner were already covered. Another bonus for he round port exhaust is with the correct gasket will work with the round and square port headers if you that what you already have, sadly, not the other way around.

 

If using flat top pistons, the P79 is my first choice with the P90 being essentially equal. The only time I would prefer the P90 over the P79 is for very wild street an all out race applications that require crazy porting in the exhaust to match a cam and induction for an engine that only works wells above 4000-5000 RPM, idle at 1200+, etc, (.550”+ lift) other wise for N/A street applications the nod goes to the P79.

 

For the dished pistons, follow the same exact scenario as the flat tops above, just substitute the N47 and N42 in place of the P79 and P90 respectively.

 

For boosted applications, I lean towards the P90 as a first choice for its lower compression ratio, (the L-6 is hyper sensitive to detonation). The other square port heads, (E31, E88, N42) also work well and for the very custom turbo builds if trying match chamber size/shape to specific pistons.

 

Round port heads with liners on Turbo engines. Having seen and personally experience exhaust liners coming apart on N/A applications I don’t recommend using the round port heads for Turbo applications. That’s not to stay it can’t be done, some on this form have done it and with no issues. I think so long as you keep very close tabs on AFR’s and EGTs and keep the tune on the more conservative side, the liners should hold up fine.

 

Bryan Blake, aka 1 fast z, is an L-series guru. His advice is sound and his work speaks for itself. First rate and makes lots of power!

 

KCE, if you have a P79 and are building a mild to moderate N/A L28, by all means use the P79, it is wonderful cylinder head.

[Lazeum]

Regarding cam specs, Rebello is quite secretive about it. I don't have much info about it. Braap basically was in contact with Rebello, he described my setup, my goals (street car on twisty roads - target was torque all over the rpm range rather than max HP) and the work being done on the head. Rebello then has grinded the cam to match the setup.

 

I've relied on Braap experience regarding the setup. I have also to say that asking a head builder to help could also be a good experience. I've learned a lot in the whole process since a lot of info have been shared over time with good support afterwards

 

 

 

Regarding price, your best option would be to ask directly to Braap about a price range. I don't know if he wants me to share those kind of data... 

[Pyro]

 

This is great info! Thanks! I know, at least in a general way, that duration and valve overlap play a significant role along with the compression ratio in determining an engine's characteristics, including how close you get to detention. Again, I have just read that these heads are *really* prone to detention...

 

Is there any chance you have some more specific information from your cam running in your 9.75:1 with a 270 setup?

 

 

Valve overlap doesn't effect cylinder pressure. The engine isn't even trying to make cylinder pressure during that part of the 4 stroke cycle. The intake closing point is what effects "dynamic compression ratio"

 

what about a 270 degree cam do you want to know?

[(goldfish)]

..... We didn't modify the combustion chambers or shave down the head and I run flattop pistons, but according to my calculations I'm at right about 9.7:1 compression ratio with a mild / medium cam and can run 87 octane in it no problem at my elevation (5600+ feet).....

 

9.7:1 sounds a little high for a setup like that.

[blue72]

That's what I would have said too if I hadn't done the measurments myself. I don't rely on the Z Engine Calulators out there on the internet, and that compression ratio is using the piston pop up measurement from my engine, the compressed gasket thickness that is consistently agreed upon, 1mm overbore and all other factors combined gave me that number. If the top of the block hadn't been cleaned up and was still stock height then I'd be at around 9.3:1

Edited December 9, 2010 by blue72

[johnc]

So would $1500 be a reasonable amount to expect to spend for getting a head like a P79 ported?

 

You also have to take into account the .080" shave a P90/P79 needs and the cam tower shims. IMHO, you're better off spending that $1,500 on a N42 head and running flat top pistons. But, I'm not an engine guy so I would defer to Paul's opinion if he thinks I'm a bit old fashioned in my thinking.

[kce]

First I would just like to express my appreciation for everyone who has taken the time to reply to my questions!

 

 

 

..If using flat top pistons, the P79 is my first choice with the P90 being essentially equal. The only time I would prefer the P90 over the P79 is for very wild street an all out race applications that require crazy porting in the exhaust to match a cam and induction for an engine that only works wells above 4000-5000 RPM, idle at 1200+, etc, (.550â€+ lift) other wise for N/A street applications the nod goes to the P79.

 

For the dished pistons, follow the same exact scenario as the flat tops above, just substitute the N47 and N42 in place of the P79 and P90 respectively.

 

KCE, if you have a P79 and are building a mild to moderate N/A L28, by all means use the P79, it is wonderful cylinder head.

 

Why would one prefer dished pistons matched with a N47 or N42 cylinder head over flat pistons matched with a P97 or P90 head? Is this the whole quench vs. non-quench debate?

 

At this point I haven't purchased anything beyond a rather clean '71 240z that's had some mild engine and suspension work done on it. I'm really just in the bench racing stage of figuring out exactly which kind of car I want to build and how I want to go about it. The underlying question for me really is something along these lines: if I did everything except the head porting myself could I build a 275-300hp 3.1L naturally aspirated L28 for around 5K? But posting such a general question seems contrary to forums rules or at least their intention; so at this point I'm just trying to fill in some gaps in my understanding (i.e., climbing the learning curve).

 

 

 

@Lazeum: Ahh, no worries, I was just curious. It sounds like you got a pretty sweet setup.

 

 

 

Valve overlap doesn't effect cylinder pressure. The engine isn't even trying to make cylinder pressure during that part of the 4 stroke cycle. The intake closing point is what effects "dynamic compression ratio"

 

what about a 270 degree cam do you want to know?

 

What kind of lift? Lobe separation angle? Overlap? It's not that I really have a need to know all about your cam? It would just be nice to put some numbers on it, that's all.

 

As for my comment about valve overlap and the compression ratio... you are absolutely right. I was thinking along the lines of compensating for a large overlap by raising the compression ratio so the remaining fuel/air that isn't inadvertently pushed out of the combustion chamber on the compression stroke is "compressed" further and more energy is thus extracted from it. (This may be a completely wrong understanding of engine dynamics.) I'm also under the impression that a lot these high horsepower NA L28 engines make most of their power past 4500 RPM, which is why I sort of mentally connect them with a high compression ratio and cam running a relatively large valve overlap. (This is probably completely wrong also).

 

 

 

You also have to take into account the .080" shave a P90/P79 needs and the cam tower shims. IMHO, you're better off spending that $1,500 on a N42 head and running flat top pistons. But, I'm not an engine guy so I would defer to Paul's opinion if he thinks I'm a bit old fashioned in my thinking.

 

So getting some mild "street" porting done, along with the machining and cam tower shims for $1500 is not realistic? I'm not trying to disagree with you, I honestly just don't know. In the SBC-world $1500 dollars gets you pretty close to nice set of aftermarket heads.

 

Why would you prefer a N42 with flat tops over a P79/P90 with flat top pistons? The advantages I see in the N42 are better CFMs with big "race" cams and the square exhaust ports... is that correct? I guess you get the desired compression ratio without having to machine the bottom of the head but you lose the combustion chamber shape of the P90/P97, which to my untrained eyes looks like it has better burn characteristics.

 

 

Thanks again for everyone's help.

[30 ounce]

I used the E88 head from my 260. Had a local race shop weld the chambers up to 39.5cc (finished) & reshaped, put steal valve seats and bigger (280z: I forget which one is smaller on the 260, intake or exhaust) valves, ported and flowed, installed Isky 490 lift 290 duration cam & springs for $1500. I used that for a year on my 260 block then decided to build a 3.0 liter. We (my cousin & I)then tore down the head, had new valve guides installed, cleaned up the exaust ports (had a nasty short side radious), unshrouded the valves because now we had 89mm bores to work with, measured each chamber to make sure they were exactly the same. We did not shave the head at any point as I didn't want any timing issues with the cam. Just cleaned everything and made sure it as straight. This time it cost $125 to install the valve guides and I forget how much the guides themselves were. So now I have a 3.06 liter (Rebello Racing 3.0 kit) JE flatop pistons (eyebrowed), Nismo .06mm head gasket, if I did my calculation's correct it should be at 11.5:1 compression. I can run on pump (91 octane here at 5280 ft. elevation) but it does run better on 93 octane.

[johnc]

So getting some mild "street" porting done, along with the machining and cam tower shims for $1500 is not realistic? I'm not trying to disagree with you, I honestly just don't know. In the SBC-world $1500 dollars gets you pretty close to nice set of aftermarket heads.

 

I didn't mean to imply that. You can probably get all of the above done on a P90 for your 1500 budget. Any head can be made to work well for your build with the budget you've got.

 

Its generally kind of pointless to compare modified cylinder heads and then try to draw a conclusion that X head is better then Y head. All you can really do is compare stock to stock. The top three in my opinion are the early, big valve, small chamber E88, the N42, and the P90. Those are listed in no particular order and my ultimate choice, given the $1,500 budget, would be the condition of the head before any work begins.

[BRAAP]

First I would just like to express my appreciation for everyone who has taken the time to reply to my questions!

 

If using flat top pistons, the P79 is my first choice with the P90 being essentially equal. The only time I would prefer the P90 over the P79 is for very wild street an all out race applications that require crazy porting in the exhaust to match a cam and induction for an engine that only works wells above 4000-5000 RPM, idle at 1200+, etc, (.550”+ lift) other wise for N/A street applications the nod goes to the P79.

 

For the dished pistons, follow the same exact scenario as the flat tops above, just substitute the N47 and N42 in place of the P79 and P90 respectively.

 

 

KCE, if you have a P79 and are building a mild to moderate N/A L28, by all means use the P79, it is wonderful cylinder head.

 

 

 

 

Why would one prefer dished pistons matched with a N47 or N42 cylinder head over flat pistons matched with a P97 or P90 head? Is this the whole quench vs. non-quench debate?

 

 

 

2 words, detonation threshold! If I had the time this morning I'd post all the details, sadly this morning. I'm short on time.

In short, the Datsun L6 is hypersensitive to detonate as mentioned in this thread already. On premium pump gas it does not like more than 9-9.5:1 compression ratio. Granted it should tolerate upwards of 10.5:1 premium on pump gas, but more often than not it doesn't and those combinations end up retarding the ignition timing so much that they lose more power from the ignition retard than they gained from bump in compression ratio. Mixing the flat top pistons with the N42/47 head generally results in engine that rattles on pump gas. There are many topic on that discussion in the L6 section.

[NewZed]

I was wondering when this topic would pop up again.

 

One question I've been left with in these discussions is, what are the specifics of the timing and where in the rpm range does the rattling occur, when it does? Since Megasquirt seems to be very popular, maybe a timing v. pressure and rpm map would tell the story. Or the basic distributor specs. if one is used.

 

I ask because I have been mucking around in the area of distributors and timing and ignition modules recently and have found a very large range of possibilities for variation in "stock" timing curves, mechanical and vacuum, from the variety of factory distributors used in the Zs over the years. So every time I see 28 degrees or 36 degrees total, I wonder "when"? When is the timing all in and when does the detonation happen?

 

It seems like, especially with something like Megasquirt, that instead of reducing your total timing if you were on the edge, you could move the curve up the rpm range.

 

Anyway, I think it would add some useful detail to this interesting and recurring topic, and maybe add some clarity too.

Edited December 11, 2010 by NewZed

[BRAAP]

I was wondering when this topic would pop up again.

 

One question I've been left with in these discussions is, what are the specifics of the timing and where in the rpm range does the rattling occur, when it does? ..

ALL RPM's. It isn't just a narrow band, or the upper range or the lower. The detonation threshold across the entire operable RPM range from below idle to well above redline, under all conditions drops when the compression ratio is increased.