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Discussion Starter #1
hi. I've been searching around and haven't been able to find exactly what I need,so hopefully you guys can help.

me and my dad are working on a 240z project, and for the motor we've got a L28 from a 81' 280zx N/A. also, we've got an old N42 head laying around. basically I'm just wandering what kind of compression ratio we'd get if we bolted the N42 head to the stock L28 block?
 

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10:1
 

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Somewhere out there, there is a a program called Lengine. With this programs you can try all kinds of different parts combonations, including different heads, cranks, rods, pistons, etc. If you do a search on it you should be able to find it pretty easy.
 

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Discussion Starter #4
10:1?

with some other mods...like triple webers, exhaust, and hopefully a new cam, will we still be able to run on pump gas?(91 octane here in boulder, CO)

also, if its too high, is there any place to get thicker head gaskets? or is there anything real easy to lower it a little?

thanks for the very fast response!
 

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yes,
according to the L engine calc
to get 10:1 with that setup you will need to make sure you have flat tops and probably a 1mm Head gasket. If you don't have these then your looking at 8.3:1
 

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None of those mods you listed determine whether it will run on pump gas or not. If you get an aftermarket bigger cam it will lower the compression some. 10:1 is fine on 91 octane. The thickest headgasket you could get without getting an HKS 2mm (.080) gasket is a stock Nissan gasket. A felpro, and HKS 1mm gasket are thinner than the Nissan which will raise compression a bit more than the stock one. The HKS 2mm should drop the compression ratio by about .5. Even with a thinner felpro or HKS 1mm you will be fine on pump gas. I've heard horror stories from the Felpro's though. An HKS gasket is about $140.00 I think? Maybe more.
 

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I'm running that much compression on premium (93) pump gas and a 276 deg camshaft with 36 deg total ignitin timing with no detonation. I'm assuming the "81" engine you have has flat top pistons? These things get swaped around so much I never assume anything!

Also at the altitude you live at, you can get away with more compression than someone at sea level. You WANT about that much compression with a hot roded Z engine.
 

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Discussion Starter #9
but the more air going through the engine, the greater chance of detonation, no? well, whatever that doesn't matter

okay, so I'm kinda confused now about the comp ratio now, with the comment about having flat top pistons...

so, if I just take a regular old L28 block, don't change the pistons or anything, use a regular factory nissan head gasket and bolt on the N42, I'll be looking at 10:1 ratio? or is that WITH flat top pistons and a different head gasket?

EDIT: does the N/A L28 from 81' have flat top pistons?

any idea how much the ratio would change with a 1mm gasket?


and lastly, how would changing the camshaft change the compression ratio? If I'm not mistaken, the camshaft should only make a difference when the valves are open...errr... opening... which is not on the compression stroke... I just don't understand how something thats not actually changing the size or shape of the combustion chamber itself could effect how much the A/F mixture is compressed.

thanks for bearing with me guys. I'm new to nissan motors... used to working mostly on rotaries, and 4ag's, which are both very much different. lol.



Post Edited (Oct 14, 9:01pm)
 

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Discussion Starter #12
okay, so now, more question... all other questions have been answered!

if I do happen to have the dished pistons, can I use the pistons out of the old L24 in the L28 block.....?

and apparently the stock L28 has the N47 head. would there be any advantage to use the N42 over the N47? what would you guys do?
 

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I never go by the year of the car the engine came out of, many have had engine swaps and people throw anything they can get cheap into them.

With mild port work (cleaning up the valve seat area) the round ports flow the same or better than a square port with the same level of mods. The advantage the round ports have is they already have steel valve seats, the N42 most likely has brass seats.

On the camshaft, a hotter cam closes the intake valve later so it will tame a high compression engine down some. I stock cam might ping but a hot cam in the same engine won't.

To get high compression you either have to use the flat top pistons out of the 82 type "F54" bottom end with an early head N42/N47 or again usethese pistons with the P79/P90 head shaved .080" with the cam towers shimmed and use the longer N47 valves. The advantage to the later is a better combustion chamber shape, less detonation problems. Also the 81-8 maxima head is aprox the same thing as the shaved and shimmed head, just smaller intake valves. Lots of options, you just have to use flat top pistons to get there. Dished pistons just can't get you there and no the 240 pistons won't fit, they are too small.
 

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Discussion Starter #17
I'm kinda having trouble finding places that sell either of the 1mm gaskets. where do you guys go? HKS doesn't seem to even list them on their site.
 
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