The Datsun/Nissan "L Series" In-line 6 Cylinder, OHC Engine Heads - Which One To Select For Your Application
Publication Contributed By: Carl Beck, IZCC #260 Technical Write-up's and Discussion Contributed by The Members and Guests Of the IZCC
The Purpose of this page is to give the reader information related to the OHC Cylinder Heads available for use on the L Series of Datsun/Nissan In-line 6 cylinder engines. This information has been gleamed from the IZCC's on-line discussion group and has thus been reviewed by thousands of readers.
Last up-date- 20 Jan. 99..cjb
Please Send Comments To Carl Beck at address above.
Discussion #1 - contributed by Jerry Jones, IZCC #1129
Selecting A Head For Your L Series Engine:
It seems that the reoccurring question in this thread is " which is
the best head", and ,as has been stated before, best is a relative
term. What is best for full on race car is not always best for a
street car or even for a streetable " hot rod". There are many
factors involved and for a "blueprint" to be successful, all of the
factors involved need to be considered. I have built a few "hot rods"
for customers over the years, and have done all aspects of race engine
preparation from porting and dyno work to machining my own dry sump
pump. There are a few key considerations to consider when selecting
or modifying any head:
COMPRESSION RATIO
The first thing one needs to consider when selecting a head is the
fuel you are going to use. High compression is one step toward H/P
but I'm not to sure a trip to the airport every week is very
practical. I prefer to stay between around 8.5:1 and 9:1 for todays
pump gas with a stock cam. Of course if you're going to run a "cam"
then you can up the compression ratio as you kill off the dynamic
compression ( but that's another story)
PORT CONFIGURATION
Port configuration is pretty good on all of the Z heads but the round
port heads have a liner in the exhaust port that kills the flow.
QUENCH AREA/DESIGN
I'm not to sure how important this is on a street car but in a racing
application it is critical, and this is one of the main differences (
next to compression ratio) in the Z head.
VALVE SIZES
This is not a critical consideration as all Z heads can be fitted with
the larger 280Z seats and valves. This is relativly inexpensive and
can be done by almost any machine shop.
PLUG LOCATION
Only important on initial engine consideration, and since the Z's are
all the same or nearly so, enough said.
OTHER DESIGN CONSIDERATIONS
This gets into things like valve shrouding; castings; seat and guide
material; cam location/oiling ; and a few other thing I can't think of
right now.
With the above taken into consideration I will give some Z head facts
as I know them. I have to admit that my flow bench knowledge here is
in terms of "relative" as opposed to the 6 months of research I did on
my 2.2 235H/P NA Dodge Daytona. So I can't give any specific flow
numbers here.
VALVE SIZE
The 240Z had a 1.65" intake and 1.3" exhaust. This valve size stayed
the same till the 260Z when they went to a 1.38" exhaust ( same as the
280). The 280Z got a bigger 1.74" intake and the 1.38" exhaust. The
valve sizes remained the same till the end of the L28 in '83.
E31 HEAD '70-71 240Z
This head has a 9:1 compression ratio. The quench area is the best of
the Z heads with the exception of the P79. As stated before 280
valves are an easy installation and recommended. A note of caution
here , however, if the 280 valves are used with a stock 240 or 260
bore ( they're the same) you will need to notch the block for the
intakes. They JUST touch and make a hell of a noise ( take my word on
this I know). If an oversize bore is used here then clearance is not
a factor ( even a .010" overbore will do). This is considered by many
to be the "best" head because of the "better" quench area and higher
compression.
E88 HEAD '72-'74
The compression ratio is around 8.7:1 for all of these years. This is
where things get confusing. There are three E88 heads, one for each
of the three years. The 72 differs from the '73-'74 mostly in the
quench area. This is significant as the '73-74 head has a raised
quench area that increases chamber temperatures for improved
combustion. Contrary to what you might think this is not good for
H/P. These are the least desirable of the Z heads and the only way to
tell them apart is to identify the combustion chamber. I think the
'73-'74 head only differs in that the 260 has a larger exhaust valve.
The '72 combustion chamber is similar to the E31 with the
hemisphereical quench area a little deeper than the earlier head. Some
IT competitors prefer the early E88, sighting that its valves are a
little less shrouded than the E31. I've run both and saw no
difference in ET's.
N42 HEAD '75-'76
This head is drilled for both carbureted and injected manifolds. It
has the bigger valves and the same silicon/bronze seats as the earlier
heads. The combustion chamber is very similar to the '72 E88 head (
including size/cc), and the 8.3:1 compression ratio is accomplished
with dished pistons. For a straight bolt on this is arguably the best
head by virtue if the reasonable combustion chamber and valve size (
remember this head will render an 8.7:1 ratio on a 240).
N47&P79 HEAD '77-'83
I lumped these together partly because I can never remember which year
they changed, and they are probably the least desirable of the
heads. The "round port" N47 head came out in '77, although I could
swear that I've seen some early '77's with N42 heads. They had the
same 8.3:1 compression ratio as the N42, and they were the first to
have the hard intake seats and round port/lined exhaust. Unlike the
earlier heads, flow bench tests yeild a 60% exhaust to intake figure,
well below the 70% that most head porters consider to be acceptable.
I have not done dyno work in this area but I've found most of the "
rules of thumb" in the porting world are reasonably accurate. Of
course there is always someone coming along changing the rules. The
N47 had a real problem with cracks from the exhaust seat and I have
taken to using the P79 head as a replacement. The one good thing
about the P79 head is the quench area ( closed chamber). It is flat;
large; and very near the piston. Unfortunately this is not enough to
overcome the bad exhaust port. The later of these heads have
longer valves. I've seen this screw up more machinists!!
For street use the best all around head ( if I had to choose
one) is the N42 head (as delivered). Reasonably equal performance can
be had from the E31; E88('72); and N42 especially if the 280 valves
are used. While this opinion may stir some debate, the differences
would not be noticed by most drivers and could only be measured on a
dyno.
I hope this helps.
Jerry Jones