N42 with Flat Top Pistons

[Barefootdan]

I searched for a bit last night on flat top pistons in combination with the N42 head on my L28, but came up with some conflicting information. Some say this will detonate due to the increased compression and design of the n42 head. You can remedy this by running a larger cam and having your dynamic compression ratio reduce this.

The other side of the story is that they run this combination with excellent results. Perhaps these folks dont run their cars nearly as hard or dont notice the detonation? 

 

I will unfortunately be running a stock cam until I go aftermarket efi (leaning mircosquirt). I do plan to run premium fuel (well...Arizona 91oct) to help offset detonation. My head has been machined 8 thousands and I do have a set of MSA headers. Will I be able to get by with this combination for the time being? 

 

@Captain Obviouscame to conclusion that the compression ratio with flat tops and N42 will yield 10.25, but I have seen estimates as low as high 9s and as high as mid 10s. 

 

This all started because I scored a set of standard size flatops with con rods last night for $150! 

Edited September 1, 2021 by Barefootdan

[Zed Head]

That conversation has been going on for decades.  I think that regional fuel quality might play a part as well as altitude and temperature, plus timing and other tuning factors.

You won't really know until you try it, basically.  I'd just make sure you're conservative on your ignition timing, including the fine details like the rate of centrifugal advance and vacuum advance.  There are many variations in timing curves out there in the world of Z distributors.

[Barefootdan]

1 minute ago, Zed Head said:

That conversation has been going on for decades.  I think that regional fuel quality might play a part as well as altitude and temperature, plus timing and other tuning factors.

You won't really know until you try it, basically.  I'd just make sure you're conservative on your ignition timing, including the fine details like the rate of centrifugal advance and vacuum advance.  There are many variations in timing curves out there in the world of Z distributors.

Totally. Some of the forum posts were 2000-2002 era. I didnt see many recent posts about it. I think I'll play it safe on the timing and worry about making a fine tuned timing table in the megasquirt software when i convert. I didnt want to miss out on adding this now when the engine is out. Cam is easy down the line 🙂

[Jeff G 78]

The biggest issue I see is the 0.008" head milling.  An unsurfaced head will yield around 9.83:1 CR, but once milled, you will be over 10:1.  Even on 93 octane, that's pushing it, but with our 91 octane, I'd be concerned.  I have a similar build with an un-milled head and it did just fine in Michigan on 93.  I moved to Sun City West a few months ago and other than driving around a bit this spring, my Z has been in the garage awaiting cooler weather, so I haven't even had to fill the tank with 91 yet.  I'm hoping that I'll have time in the next month to get the car out and drive it more.  At that point, I'll know how it likes 91 gas.

[Barefootdan]

Welcome to AZ @Jeff G 78! Hopefully we'll start to see some cooler weather. All of the rain is nice this year. I dont have A/C in mine, so it wont see the crazy summer temps...unless I go crazy and want a sauna day 😄

[Jeff G 78]

2 minutes ago, Barefootdan said:

Welcome to AZ @Jeff G 78! Hopefully we'll start to see some cooler weather. All of the rain is nice this year. I dont have A/C in mine, so it wont see the crazy summer temps...unless I go crazy and want a sauna day 😄

I have all the OE A/C components for mine, but I haven't had the compressor mounted for 30 years.  I plan to get it operational at some point so I can drive it in the summer.  I can't even imagine it sitting in the sun and then trying to drive it without passing out when it's 110F or higher.

Good luck with your build.  Are you on desertdatsuns.net?  I like that there is a AZ group, but I struggle with the 20 year old forum software there.  It feels like the old Zhome or even IZCC days for the really old timers.

[siteunseen]

I read up on that before I got my 240. A thicker head gasket was the conclusion from what I remember.

Read some of these from a google search.

https://www.google.com/search?q=n42+head+with+flat+tops+head+gasket+classiczcars.com+&client=ms-android-americamovil-us-revc&source=android-home&source=hp&ei=3-8vYZ7FCK-WwbkPytyqmAY&oq=n42+head+with+flat+tops+head+gasket+classiczcars.com+&gs_lcp=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_F1iwkgRgsJwEaBNwAHgAgAHGAogB6EqSAQgwLjU5LjQuMZgBAKABAbABHg&sclient=mobile-gws-wiz-hp

[heyitsrama]

I run a n42 head with some flattop pistons on my L26 felpro headgasket.... Its been a couple years but the motor still works... has not self destructed yet.... I need to do an oil analysis at somepoint, i think i ****ed up the piston ring gaps.... I was a broke college kid when i put that motor together 😞

When i got the head it seemed like it was already shaved flat. You can find some more information about it under my `Flying Z` thread.

Funny story i paid ~1 bitcoin for that head!

[Barefootdan]

2 hours ago, Jeff G 78 said:

I have all the OE A/C components for mine, but I haven't had the compressor mounted for 30 years.  I plan to get it operational at some point so I can drive it in the summer.  I can't even imagine it sitting in the sun and then trying to drive it without passing out when it's 110F or higher.

Good luck with your build.  Are you on desertdatsuns.net?  I like that there is a AZ group, but I struggle with the 20 year old forum software there.  It feels like the old Zhome or even IZCC days for the really old timers.

I'll be sure to check them out!

1 hour ago, siteunseen said:

I read up on that before I got my 240. A thicker head gasket was the conclusion from what I remember.

Read some of these from a google search.

https://www.google.com/search?q=n42+head+with+flat+tops+head+gasket+classiczcars.com+&client=ms-android-americamovil-us-revc&source=android-home&source=hp&ei=3-8vYZ7FCK-WwbkPytyqmAY&oq=n42+head+with+flat+tops+head+gasket+classiczcars.com+&gs_lcp=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_F1iwkgRgsJwEaBNwAHgAgAHGAogB6EqSAQgwLjU5LjQuMZgBAKABAbABHg&sclient=mobile-gws-wiz-hp

Thanks for the tip. My question for this is, doesnt it defeat the purpose of running this high compression combo to use a thicker head gasket? If running a larger cam down the line that is...

1 minute ago, heyitsrama said:

I run a n42 head with some flattop pistons on my L26 felpro headgasket.... Its been a couple years but the motor still works... has not self destructed yet.... I need to do an oil analysis at somepoint, i think i ****ed up the piston ring gaps.... I was a broke college kid when i put that motor together 😞

When i got the head it seemed like it was already shaved flat. You can find some more information about it under my `Flying Z` thread.

Funny story i paid ~1 bitcoin for that head!

I'll take a look thanks! Better never sell that head now 😉 

[siteunseen]

Take the thicker gasket off when you go to the high lift cam.

To be completely honest a good friend on here, who's dissappeared for some reason, told me to do it and run 93 gas and not to worry. He must have done it or wouldn't advise me to. That kind of guy, no guessing. @240260280 where are you? We (me) miss your knowledgeable comments. To hell with the negative people. 

 

[Captain Obvious]

I haven't dusted off my compression ratio calculator for a few months, but if you want me to double check the numbers, just say the word. 

Just remember that I'm just a guy with a keyboard and an internet connection.    

 

[Barefootdan]

2 hours ago, siteunseen said:

Take the thicker gasket off when you go to the high lift cam.

To be completely honest a good friend on here, who's dissappeared for some reason, told me to do it and run 93 gas and not to worry. He must have done it or wouldn't advise me to. That kind of guy, no guessing. @240260280 where are you? We (me) miss your knowledgeable comments. To hell with the negative people. 

 

I was about to complain that doing a head gasket would need me to retime the engine and all that jazz. But I just realized the cam gear comes off regardless!