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When it comes to making power N/A, displacement and RPMs
are the rule. As much of both as you can financially afford is the key. The
“bottom end” is where you build reliability and durability, the top end is
where the power comes from. As for displacement, up to around 3.0-3.1 liters
is doable on a realistic budget, but the cost curve starts a real sharp
shift upwards from here, (this is where the V-8 conversions start to look
real good. $$$ per HP, the V-8 swap is pretty hard to beat). As for RPM's
the weak link in the Datsun engine tends to be the crank, this is assuming
that you have already installed good quality aftermarket rod bolts and the
pistons are forged, not OE cast. Now if a good billet crank can be had, the
rods are next in line. A good friend did some testing some years ago, (Carey
M, if I misquoted you here, please jump in to clarify), and found that with
as light a flywheel as possible and with a quality damper, i.e. fluid
damper, the L-28 crank is good for 8500-9000 RPM, after that he was
experiencing crank breakages at the rear of the crank. A billet crank should
allow for more RPM.
Now, on to Datsun L-series cylinder heads…
The biggest air flow restriction in the Datsun L-series head is the valve
curtain area, i.e. the region between the valve head and the valve seat when
the valve is OFF the seat. In all my L series engine builds, unshrouding the
valves is always the first place I start. Even on mild street heads, I
perform moderate valve unshrouding. Next, if the head is a square port head
and will be used in a race only application I’ll widen the bowl to slow down
the flow of air in this region. This allows the exhausting gasses to
transition from the vertical plane out of the chamber to the horizontal
plane on its way to the header with less chance of loosing the laminar air
flow across the port floor. In dong this, the laminar air flow across the
port floor will remain, (keeps the boundary layer intact), if this laminar
flow separates from the port floor you will now have turbulence and this
will restrict air flow considerably. (you may or not be able to catch this
on a flow bench as a flow bench doesn’t measure the dynamic air flow in and
out of the ports. I’ll expand on that at the end of this post). If the head
has the round exhaust ports with the liners, all I do is blend the back of
the valve seat into the liner. All of the heads I do, whether for street or
full race, also receive a nice 5 angle valve seat, (sometimes only 4 angles
can be performed do to space constraints), that I perform in house here at
Rusch Motor Sports using Sunnen seat cutters, (I’ll be offering radius seats
soon), and the valves are treated to a 30 degree back cut and the exhaust
valves sometimes even get a nice little 45 degree chamfer on the chamber
side of the head. All this extra valve and seat work mostly benefits air
flow at low valve lifts, i.e. as the valves leave and return back to the
seat itself.
Here is my personal take on the L-series heads.
E-31 and the early E-88 heads with the E-31 chambers are decent heads. They
have the same potential as the other N-series heads when rules permit
extensive carving. My opinion is the best place for the E-31 and the early
E-88 is for a restoration project, or for a performance application where
class rules dictate no material can be added to the chambers, no carving can
be performed on the head and the update/backdate rules apply to the engine
as an assembly, then these heads are a great choice. Now if you are not
bound by those kinds of rules, you do have other choices available and being
as the E-31 is becoming so rare now, these other heads are an easier option
from a financial and availability stand point.
The N-42 head is a great maximum effort race head if class rules allow
extensive carving and welding to the head. In this case, weld the chambers,
open the exhaust ports as described above, perform some serious valve
unshrouding and “waa laa”, you now have a wonderful maximum effort race head
that would SUCK on a street engine. (Side note; If you intend on having your
valves unshrouded and have not done this type of work before, you are best
served leaving this to a qualified experienced engine builder, preferably
one that has been successful in extracting noticeable to impressive
documented performance gains. If you are not sure what you are doing, do not
attempt to unshroud the valves yourself. In experience can hinder the flow
worse than what the heads were stock.) Of course there is more to
building/machining/porting a maximum effort cylinder heading than just
welding a chamber and/or unshrouding valves. There are items such as setting
spring heights, clearancing the retainers, stem seals, and guides for the
mega lift cam, deciding on just how far to go with oversize valves even to
the extreme of offsetting the valve guides to allow even BIGGER valves if
the cylinder bore permits, etc.
The Z car N-47 head is a great street head. This head
becomes almost ideal for the mild to hot street engine and even the mild to
moderate race engine especially if the chambers can be welded up, (pretty
much turns this head into the Maxima N-47 head which is a slightly more
efficient chamber than the E-31). This “peanut” or “kidney” shaped chamber
when used with flat top pistons or matching* dished pistons gives the ideal
quench area which makes for a more efficient combustion process. To make use
of this “kidney” shaped chamber on an L-28 running pump gas you will need
matching* pistons. What I mean by “matching* pistons”, is a set of custom
pistons from JE or other comparable source that has CnC the dish directly
under the open portion of the “kidney” shaped chamber, not the entire
surface area of the piston as is the OE 1975-1980 L-28 and all L-28-ET
pistons. Any how, with this ideal squish, the engines optimum ignition
advance will be less than the open chamber heads optimum ignition advance
due to the more efficient chamber design. This happens because the flame
front doesn’t have as far to travel to consume the entire air fuel mixture
within cylinder, it now is in smaller area so the flame front doesn’t have
to travel all the way to the other side of the cylinder during the ignition
sequence. To better understand this concept, just visualize the open
combustion chamber as being a flat wide circular disc, the diameter of the
cylinder itself containing the air fuel mixture with the spark plug on one
side, vs a small kidney shaped ball containing the same volume of air and
fuel. Ideally a sphere shaped chamber with the spark plug in the dead center
would be perfect, since that isn’t realistic, tuners just try and get as
close to this ideal as possible. Some engines came for the factory much
closer to this ideal, ala HEMI heads, etc. As for those round exhaust liners
in the N-47 and P-79 heads, they actually flow VERY well. The liners offer a
nice gentle radius as the air flow transitions from a vertical flow out of
the chamber to the horizontal plane where it meets the header which keeps
the air flow moving undisturbed even at high velocities where as the square
ports with their really sharp short side radius doesn’t allow the air to
make that transition with as much ease. What happens is the boundary layer
of air along the port floor can and will separate causing the air to slam
into the roof of the port and tumble along the floor of the port when the
velocity gets high enough. This is not a good thing in the quest for power
as this turbulence is very disruptive to air flow. This why we make that
region of the port, the bowl, larger in an effort slow down the air flow to
keep that laminar air flow across the port floor. Of course this only
happens at very high velocities. Also, all L-series heads starting in 1977
have a slightly smaller intake port volume. What the engineers did was cast
one side of the intake port wall with a “flat” in it, “D” shaped if you
will. This reduction in cross sectional area starts approx ¾ of an inch into
the port. This port shape is supposed to help bias the air flow as it enters
the chamber more towards the middle of the combustion chamber itself,
steering away from the chamber walls is passes the valve head. In theory
this helps to reduce some of the air from slamming into the chamber walls of
the combustion chamber. I don’t have enough hard evidence to back up this
theory, but I do feel that this port bias does not detract from the
performance potential of the cylinder head one bit.
Now we get to the P-79 and P-90 heads. The combustion
chambers of these heads are IDEAL, almost perfect once the valves are
unshrouded. The only down side I see to the P-series heads, (and this down
side is for the extremely radical ragged edge engines, not so much for the
milder even hot race engines), is since the chambers are taller, (valves are
now shorter as a result), the floors of both the intake and exhaust ports
now have an even sharper short side radius and as stated previously, this is
a detriment to flow as velocity increases. The transition from the
horizontal plane to the vertical plane on the intake and vice versa for the
exhaust, is not as smooth as the N-series and E-series heads. Now don’t take
this as I am bad mouthing the P-series heads cause I’m not. I really like
these heads for hot street and mild to moderate race applications. They are
an inexpensive way to get the ideal squish using an OE flat top piston with
a compression ratio compatible with pump gas. I have built several and will
continue to build the P-90 and P-79 heads for street and mild race Z’s. What
I’m saying in regards to the P-series from the stand point of building THE
mega extreme N/A performance power house that is at the ragged edge of
making useable power over a very narrow yet very high RPM range such as a
best of the best N/A drag racing engine, these items should not be
overlooked.
Maxima N-47 head. What a darling mild race head this is. For mild to
moderate race engines on a budget, using flat top pistons in an L-28, or
even opting for pistons with a slight dome for greater than 13:1
compression, this is my go-to head. Just have your machine shop cut out the
old intake seats and install the larger seats for the 1.73” or larger
valves, have a competent Datsun engine builder/tuner unshroud the valves,
blend the exhaust seat into the liner, and “waa laa”, a moderately high
compression ratio race engine that will perform rather well up to 8000 RPM
with the right cam, induction, and exhaust system. With a set of custom
pistons machined so that the dish is directly under the open portion of that
gorgeous peanut chamber, this head, in my opinion, makes for the perfect
street head or mild race engine that runs pump gas. At the moderate level of
race and above, I tend to prefer the N-42 with welded chambers and opened up
exhaust ports.
Now for that flow bench info I promised earlier on…
Since a flow bench cannot duplicate the dynamics that are happening within
the intake and exhaust tracts while the engine is actually running, i.e.
sound pressure waves, pressure surges, exhaust heat, fuel enriched intake
charge, valve overlap extraction, etc, flow bench numbers are nothing that
should be used for comparing one head to another or bragging how good one
thinks their head is! The best way to use flow bench numbers is using the
SAME flow bench to compare ONE cylinder head for improvement after
modifications have been performed, making note of any changes in air flow
whether improvements were made or lost, nothing more. Flow benches do not
represent what is actually going on within the intake tract OR the exhaust
tract as mention previously, i.e. Dynamic air movement, pulses, waves, heat,
etc!!!!. In real life, the valves are opening and closing causing the air to
stop and move, stop and move, over and over and being as air has weight and
is compressible, this constant surging will cause the pressures to rise and
fall FAR above and below ambient. Depending on RPM, runner length, runner
cross section, port shape, number of bends in the port and radius of those
bends, valve shrouding, air density, cam timing in relation to the piston
movement, (this has a HUGE effect on how the air gets moving within the
intake and exhaust tracts, hence lopey idles, hard hitting powerbands, etc),
there could be higher than ambient pressures at a particular RPM, i.e. a
natural supercharging effect, that is why intake and exhaust runner lengths
are TUNED! We have all seen where almost all Nascar engine builders have
achieved over 110% volumetric efficiency on a naturally aspirated 2 valve
engine by tuning the intake AND exhaust tracts to a specific RPM with
specific runner cross section and runner lengths. This tuning is taking full
advantage of the Helmholtz principle. Your basic garden variety flow benches
do NOT and can NOT duplicate this, I’m not even sure if there is a flow
bench made that can do this. The only real measure of how good a port can
make HP, is to mount that head on an engine and run that engine on a Dyno,
(and this is what we are REALLY after right, HP! Not just some arbitrary
static flow number through a head port)! An example would be if one head
that on one flow bench indicates it will outflow all the others tested, when
all are attached to equivalent short blocks, the high flowing head could
easily make LESS power on the dyno, but then again change the configuration
a little with a different cam, intake tract, piston some shape, etc, the
results would get even more confusing.
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