The P-90 and P-90A page
I hope that on this page I can relay to you what I learned while researching and checking on the P-90 and P-90A heads. First, it is important to understand where these heads came from, and why they were produced.
First, Bryan Little has done an excellent job of providing a time line for the L6 heads. I do not wish to duplicate his work, so here is a link to that page:
Datsun Garage head page
There is information out there saying that the hydraulic head is the P-99. I cannot refute this claim, however, I have never seen one.
Those same sources have told me that there is no such head as a P-90A. I have one, therefore I know it exists.
This brings us to the heads in question. The P-79, the P-90 and P-90A. The P-79 was introduced on the 81 ZX motor. With the improved combustion chamber design, and flat top pistons, the Z now had
more power than it ever had,(although it also weighed more than it ever had!). When I can get some pictures, I will put
them here, but for now, you can go to the DATSUN
GARAGE and look on his head page and see the dramatic difference in chamber design. The later style heads have a "quench" area, which squishes the fuel mixture around the spark plug. This promotes better combustion, and as a nice side result, more power, more mileage, etc.. The first turbo motors, had N-42. I feel the primary reason was Nissan
wanted a square exhaust port,(for more flow), for the turbos, but had no recent head designed for such, except the older N-42. But they were quickly switched,(during the first year, so not many have it), to the newer P-90 heads. The P-90, the P-90A and the P-79 all have the same combustion chamber, in both design and size. The advantage of the "turbo" heads, or the P-90 and P-90A is that they have the squish combustion chamber of the P-79,(better power, etc...), and the same size SQUARE exhaust ports as the earlier, E-31, E-88, N-42 heads.The P-90A was introduced around mid 82, i think, anyways, later in the three year turbo run. (84 started the 300ZX).
As a side note, the first turbos were not available with a 5 speed because Datsun deemed that their existing 5 speed was inadequate for turbo use. So after adapting the stronger Borg T-5 to
work with the L6, the turbo was available with the 5 speed. However, as many people who have driven those gearboxes will tell you, they shift horribly compared to the NA 5 speeds. Balky, poor shifting, and vague. Almost completely
opposite of the NA boxes.
This doesn't mean you have to run the Borg Warner T-5 if you are running any kind of power, for
many people run the other tranny with lots of power and no problems. If anything, I would stay away from them. Just my opinion, but it is my web page!
To the best of my knowledge, the valve lengths are all the same on the later style heads,(P-79, P-90's), for I used some new valves I had laying around from an E-88 in my P-90A,(the E-88 had been fitted for the later, larger valves). As always, you
should have a machine shop check the length before you order any valves or try any mods, like the one listed on The Datsun Garage homepage, written by Bryan Little, where he talks about using longer valves on the P-79 head, and shaving to get higher compression. I see no reason why you can't do this on the P-90 heads.
As a quick comparison, the difference between the hydraulic heads and the non-hydraulic
heads is simple. The adjusting boss,(where we spend hours adjusting the valve lash on non-hyd. heads), is different.
The normal heads,
there is what amounts to three "nuts" if you will. There are two adjustment nuts,(one to move the
boss
up and down, which is actually a bolt which the rocker sits on, and one to lock that bolt down),
and the bottom
of the boss is nut shaped allowing removal of the entire boss. BTW, the boss is the part that the
follower,(under
cam, touches valve top), sits on top of. There are many names for that part, but that is what I have
chosen to
call it. Anyway, on with my book. The hydraulic boss, one, is physically larger, so there can be no
interchanging
of solid and hydraulic bosses, and two, it only has the nut that allows it to be removed from the
head. Beyond
that, it is really hard to tell the difference between the heads. No matter what anyone tells you, if it
says P-90A,
it is hydraulic.
Why do some people say the hydraulic head is not ny good for a performance
application?
Well, a while ago, I was talking with Ron at Nissan Motorsports about my car,(I was mainly
calling to check prices
on parts). I was asking about oil pumps and my hydraulic head. We got into the discussion about
my cam and whatI had found out about sizes, limits etc.. and I asked him if he thought I might have problems with
lifter bleed
down. Now, again, as some background, this is the reason many people say the hydraulic head is a
poor choice for
performance applications. What has been claimed,(and experienced), is that with a large cam,
when you run the motor
at high rpms for extended periods of time, the lifter "bleeds" down, in other words, it fails to keep
its pressure and keep the follower in contact with the cam, preventing the cam from reaching
Maximum lift, changing
the duration, etc.. lots of bad stuff. Bottom line, loss of power.
At this point, it helps to understand how the hydraulic head works differently from the
non-hyd head. I could
explain why we have such a thing as valve lash, but this is already VERY long. I can do that if
somebody really
wants to know, just ask. Anyway, the hydraulic setup eliminates lash and all adjustments for such.
Oil is directed
up into the hydraulic boss, pushes the "lifter" up, making the follower stay in constant contact with
the cam and valve. As the cam starts to exert pressure on the follower, a small check ball closes up
the "out"
hole for the oil, locking the lifter in place. From that point on, it acts the same as a solid lifter, just
sitting
there supporting the follower. So why do it that way? It accomplishes several things. One, you will
never have
to open up the valve cover and adjust the valves again. Two, it is immensely quieter, for the cam is
no longer
slapping down on the follower every revolution, making that nice clicking noise we are so used to.
If you have
ever heard the turbo motor with the hydraulic head, all you can hear is the injector clicks, nothing
else, very
quiet. You also have the added benefit of less valve train wear. Because your cam is not slapping
down on the follower
god knows how many times every day, hence experiencing a high load every cycle, it lasts longer.
Lastly you, because
of that, you will have a small,(ok, VERY small), increase in power because you have reduced the
internal friction
in the motor.
SO WHAT DID RON SAY???
I started asking if I would benefit from running their competition pump. I was concerned that
on some of my
high speed runs, I might encounter the same problems many people had talked about with the
hydraulic heads, lifter
bleed down. What I failed to understand about the hydraulic lifter, is that once it has enough
pressure to "lift"
it, and keep it pressurized, it doesn't matter how much more pressure or volume you run, once its
up, its up,(unless as I talk about later, the lifter is worn). He
said DO NOT run the competition pump unless you have really big clearances, like in a
race motor. I could
run a pump that had 600gpm and 1000psi, but it still might not address the problem. It would blow
out your seals, but still might let your lifters bleed down!
What causes the lifter to bleed
down?
There are basically three things that cause the lifter to bleed down.
- You are not running enough oil pressure.
if you run the hydraulic head, you MUST have the turbo pump. Your oiling system now has the
added demand
of the lifters, and a stock pump will be marginal at best. The turbo gives you a higher volume and a
slightly higher
pressure. So the first easy fix if your lifters are bleeding down, change out the spring for a new,
higher pressure
one. If that fails, replace the pump. Of course, if your engine is worn, and your clearances,(i.e.
main bearings,
rods, etc...), are excessive, then the new pump won't help address the problem. All these fixes are
assuming the
motor is in good shape, i.e. rebuilt or new. So what else can cause it?
- Bad or worn lifters.
This is the most common cause of lifter bleed down.
They are a wear item, and over time, wear out. The only solution, locate the bad lifter, and replace
it. It is best to go with all new ones if your budget can afford it. However, they are $42 each through
Courtesy Nissan with my IZCC discount. They are not cheap, especially for all 12. So with a good oil pump and good
lifters, what else can cause the bleed down? It is partially from big cams. However, you can run as large of
cam as you want, and not have a problem with the lifters. But you need to address the last issue.
- Incorrect spring pressures.
This is usually up to the cam manufacturer. If you can find a spring that has the required closed and max lift pressure, to work with your cam, then you will have no problem. The key for the hydraulic head is to stay close to the factory specs, and the lifter "thinks" it is seeing the stock cam. Where people run into problems is they run, say a .480 lift cam, and put new springs on it.
However, the springs they put on, have a seat pressure of , say, 270-300lbs at full lift. The hydraulic bosses are not capable of supporting that kind of load, and they will bleed down. There is no problem when you are running a solid boss, other than decreased valve train life,(higher loads). There are many places that will custom wind springs to meet your needs, you just need to provide the specs for them.
So what does my thesis mean? You can pick the cam you want, but you must find springs that will keep the spring seat pressures close to stock
at max lift.
Theoretically you could run a .600 lift cam on a hydraulic head. However, if you found a spring that had close to stock seat pressure at that lift, it would not have enough closed seat pressure to keep the valve closed at higher rpms. So it is all a trade-off, however, for the large majority of sreet/strip drivers, you can make the hydraulic head work as a performance head with a little planning, and a little more knowledge.
Here are some general guidelines to follow when looking at seat pressures. I say general
because I am not a cam grinder. They will have to tell you what will work for your cam. The
stock specs are as follows:
Closed seat pressure,(this is with the valve fully closed), ~70-80lbs. Stock max lift pressure,(this
is the spring pressure at max lift), ~160-170lbs. As you go to an aftermarket cam, which will have higher lift
and a longer duration, you must usually change the springs to control the valve under high rpm conditions. This is an area that MUST be coordinated with the cam grinder. They know what is required for their grind. I will put out what the pressures are for my cam, but remember, it is specific to my cam. I would, however, say that these are close to the max you
want to run with the hydraulic head.
Closed seat pressure, ~100-110lbs. Max lift spring pressure, ~210-230lbs. Now I would say,(and remember, I am no expert), that much over that max lift figure, maybe up to 250lbs, and you are beginning to exceed the capability
of the hydraulic lifter and check ball. Again, talk with the cam grinder. Get to know them and their family, then, you gain two things, some new friends, and a new found respect for that cylindrical, lobed thing in your motor.
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Please don't e-mail looking for an immediate response. The Navy keeps me really busy, so it may be up to several weeks before I can respond.
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