280ZX (S130) Forums Dedicated to 79-83 ZCars

new head

Old 09-04-2003, 11:16 PM
  #3  
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Here's an article I found at Zhome.com Kinda lengthy but lots of info.

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
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Old 09-04-2003, 11:30 PM
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Here is another link to Z head info that is helpful in deciding which head to use depending on how wild you want to go with your engine. Z head info
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Old 09-05-2003, 08:08 AM
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The first article is probably old, and they guy doesnt' even know when they changed to the P79 head.

Usually people say the P79 head is the best design, despite the round exhaust ports. The P90 head has the same design in valve and quench area, but with the better square exhaust ports making it the most desirable overall. Also, it is the most rare as it came on the turbo 280zx.
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Old 09-05-2003, 09:30 AM
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I would agree with Bleach. The quench area on the P79 and P90 is much better if you are running high compression and with the big valves are excellent flowing heads especially the P90 with the square exhaust ports.
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Old 09-08-2003, 09:12 AM
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I have also heard from some people that race competitivly, that the round exahust ports and runners really don' t make any difference in horsepower. They find the power on the intake side, not the exhaust side... so its a toss up. don't lose sleep over not having the square exhaust ports. What kind of power does a header and exhaust add by itself? Very little, so how bad could having round exhaust ports be vs square if the whole rest of your engine is built properly.... 2-5hp? maybe
We're talking about a normally aspired street car here.... not some 9K rpm race car
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