question on p79 head.
07-31-2012, 08:47 PM
#1
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question on p79 head.
so. I cannot seem to find a n42 head nearby, and most of the prices i have seen online run anywhere from $250 plus shipping (and need to rebuild) to $750 (completely rebuilt) plus shipping depending on condition.
So Sunwest automotive here in town will rebuild, port, install 1mm larger diam valves and shave/shim the head for $350~ in my p79 head. considering the price is comparable and i already have the p79, would i be able to get the same kind of performance gains with this modified head? would an n42 be able to get better performance per dollar?
i tried searching but this seems to be a pretty specific question.
So Sunwest automotive here in town will rebuild, port, install 1mm larger diam valves and shave/shim the head for $350~ in my p79 head. considering the price is comparable and i already have the p79, would i be able to get the same kind of performance gains with this modified head? would an n42 be able to get better performance per dollar?
i tried searching but this seems to be a pretty specific question.
07-31-2012, 10:14 PM
#2
https://www.zdriver.com/forums/240z-...79-head-35315/
[ cue music ] ... plug it in, plug it in ... The numbers that is... 
07-31-2012, 10:43 PM
#3
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problem is, i dont know the numbers i would get after the modifications to the head. not to mention that calculator only makes sense to MIT graduates and math whizzes. or people who have an addiction to z cars i suppose.
i also dont know the difference between the 2 besides the combustion chamber sizes are different. i guess my question is, can i get the p79 machined to be, for all intents and purposes a N42? or is there something else that makes it better?
i also dont know the difference between the 2 besides the combustion chamber sizes are different. i guess my question is, can i get the p79 machined to be, for all intents and purposes a N42? or is there something else that makes it better?
07-31-2012, 10:52 PM
#4
From that first link in my quote:
- The larger valve N42 came on the 1975-76 280Z. 48 state U.S. 280Zs didn't use a catalytic converter until 1977, so the head came with softer brass valve seats for use with leaded gas. Many people like these for performance because of it's square exhaust ports. And as a easy mod, putting it on a flattop L28 motor puts compression in the mid 10's. It uses an external oil spray bar.
- The P79 came on 1981-83 flattop piston L28 (F54) blocks. It has a 53.5cc chamber and must be used with flattop pistons or compression drops to 7.8:1 (with dished pistons). The exhaust ports/liners are identical to the N47, and used an internally oiled cam.
08-01-2012, 06:18 AM
#5
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the drop down will fill in the stock information, but which item do i change to make the compression chamber smaller?
if it isnt possible, then i wont pursue it. if it is possible, help with getting the numbers required to tell the machine shop would be enormously helpful.
08-01-2012, 09:34 AM
#6
Mr Z++ Wiki
Join Date: Sep 2009
Location: Victoria, BC
Posts: 2,195
OK first off are you using dished or flat top pistons? What head did your car come with?
Read these stickies
Read these stickies
08-01-2012, 04:25 PM
#7
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flat-top pistons, p79 head, f54 block. stock cam. this head has the sleeves in the exhaust ports, so we've decided to not bore the exhaust ports, but we will go ahead and bore the intake ports. from what i can tell the stock intake/exhaust sizes are the same between the n42 and the p79 heads. the only difference being the sleeves and the shape of the exhaust ports.
after doing some more research, this head has a better quench design than the n42, stainless valve seats instead of brass, and my machinist says it is a better flowing head than the n42 (which is in opposite land compared to some feedback i've seen online)
so yeah. im using flat top pistons. my calculations (which are probably wrong) are telling me that if i want the same size of combustion chamber as the n42, i would need to machine off 6.9 mm from my head. (not my hair, the engine's cylinder head)
i got that number by using a cylinder as a model (not perfect, i know) and reducing the volume inside of the cylinder, and finding the new height.
after doing some more research, this head has a better quench design than the n42, stainless valve seats instead of brass, and my machinist says it is a better flowing head than the n42 (which is in opposite land compared to some feedback i've seen online)
so yeah. im using flat top pistons. my calculations (which are probably wrong) are telling me that if i want the same size of combustion chamber as the n42, i would need to machine off 6.9 mm from my head. (not my hair, the engine's cylinder head)
i got that number by using a cylinder as a model (not perfect, i know) and reducing the volume inside of the cylinder, and finding the new height.
08-01-2012, 04:54 PM
#8
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ok. news. i found a guy who will sell me an N42 for 75 bucks. i might just go with that. it will be 350 bucks to get the head rebuilt though, since it has brass seats. is it hard to replace the brass seats with stainless?
should i stick with the better quench/potentially higher flow head, and just get that worked on?
should i stick with the better quench/potentially higher flow head, and just get that worked on?
08-11-2012, 06:53 AM
#13
Big Poppa
Join Date: Jan 2005
Location: Mission, British Columbia
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because the head is decked .080" it slacks the timing chain and messes with cam timing and such. you lift the cam towers with shims and purchase .080" taller lash pads. it makes everything lovely
08-21-2012, 11:00 PM
#15
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p79 milled .080 question
hello again... so i got my head/block back from the machine shop. i got the head milled .080 as per shady280 and nismopick (i think) suggestion. now, the valves when they are fully open are extending past the bottom of the head by about 2mm...
will this be an issue? will the pistons contact the valves at any point? the cam tower shims were installed, and the machine shop did a great job, but i never noticed that the valves move so far. please let me know.
p.s. as a side note, on the cam timing gear, with this modification, what hole should i use? 1, 2, or 3? (the holes near the center that correspond to the holes used to align your timing chain)
will this be an issue? will the pistons contact the valves at any point? the cam tower shims were installed, and the machine shop did a great job, but i never noticed that the valves move so far. please let me know.
p.s. as a side note, on the cam timing gear, with this modification, what hole should i use? 1, 2, or 3? (the holes near the center that correspond to the holes used to align your timing chain)
08-22-2012, 06:12 AM
#17
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flat top pistons... aaaand... a 2mm ( i believe it is stock) head gasket. to clarify, the valves open past the head gasket as well. the measurement is eyeballed, as i don't have a tool that can accurately measure that. (except maybe a feeler guage, but thats basic addition... and thats hard math) i can look that up...
however, there is a few facts you should know. the head was a virgin head. the machine shop mic'd it and found that much out. so a p79 with .080" milled off should match what shady280 did with his n/a.
im sorry for making a seperate thread, but as the first thread was about which is better,and this is about what to do with the milled head, i thought it would make it easier to search for that way.
as i am in the rebuild process you might find i will be asking some more noobish questions, but i promise that they will be questions that the fsm and the haynes do not answer adequately. my apologies for anything that seems common sense. (such as which hole to use when installing the camshaft sprocket)
however, there is a few facts you should know. the head was a virgin head. the machine shop mic'd it and found that much out. so a p79 with .080" milled off should match what shady280 did with his n/a.
im sorry for making a seperate thread, but as the first thread was about which is better,and this is about what to do with the milled head, i thought it would make it easier to search for that way.
as i am in the rebuild process you might find i will be asking some more noobish questions, but i promise that they will be questions that the fsm and the haynes do not answer adequately. my apologies for anything that seems common sense. (such as which hole to use when installing the camshaft sprocket)
08-23-2012, 09:03 AM
#19
ZZzzz, set up your block to TDC on cyl #1, then set the head / cam to it's TDC cyl #1 mark. Look at the valves compared to the cylinders (pistons). There are also diagrams in the FSM / Haynes.
You will then see why open valves protruding 2mm past the deck isn't a problem.
You will then see why open valves protruding 2mm past the deck isn't a problem.
08-26-2012, 05:31 PM
#22
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so... the engine is rebuilt. everything installed, and the engine cranks over but wont start. a friend of mine said with the raised compression we might have to re-gap our spark plugs, and that could be why it wont start.
however, when we crank it over, the afm flap does not move and only a small amount of suction is present. we have spark, and we have fuel pressure, (no way to tell if the injectors are all firing) so i thought the air might be the issue. everything is installed correctly, as i went through the fsm and followed it step by step. please help!
however, when we crank it over, the afm flap does not move and only a small amount of suction is present. we have spark, and we have fuel pressure, (no way to tell if the injectors are all firing) so i thought the air might be the issue. everything is installed correctly, as i went through the fsm and followed it step by step. please help!
08-27-2012, 06:45 AM
#25
Big Poppa
Join Date: Jan 2005
Location: Mission, British Columbia
Posts: 4,499
take the dizzy adjust bolt out and get someone to crank it for you as you spin it around. if you have to go to far then its installed wrong. i was one tooth out once and it ran like a tractor. make sure your firing order is correct. most likely its the dizzy install.
