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N42 head with flat top pistons?


superfunk

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It will raise your compression to roughly 10.5 to 1. Some guys have said they get some detination with a stock cam....a higher lift/longer duration cam improves upon the situation. Plus, a thicker head gasket may help. I've contemplated flat top pistons in my N42 motor, upon rebuilding it, with the addition of a larger cam. I'm running SU's, 6-2-1 headers, and electronic ignition currently. However, I may just forgo the rebuild, spend the dough, and purchase a Rebello engine.....time will tell!!!

Tom

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Thanks,

Is the compression ration for an L28. I forgot to mention that this is an L24. Is there a difference in stroke of these two? Im pretty much getting it back on the street, I wont have time for a proper rebuild for a little while. At that time i'll definately find an L28 block to build.

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The differences between the L-24 and the L-26 & L-28 , is the length of stroke . the bore is larger on the L-28 and this engine has dished pistons . F-54s all have flat top pistons unless it is a turbo engine. Then they are dished . Gary

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Adam,

In order to put an N42 head on the L24 block you need to cut notches into the rim of the cylinder bore on the block to give enough clearance to the larger valves.

I did this on my 73 because I had a block with good compression and a good N42 head I found cheap. Using a compression calculator mine came out to a little over 9:1 factoring in some valve releif work and the notches in the block and using a 1mm Felpro gasket.

If you search this forum under "block notch" or "eyebrow" there is at least one thread on it, and there are some pictures in the member galeries somewhere (I couldn't find them just now when I looked).

This was just a means of getting the car on the road on a limited budget. In the future I do plan to put in an F54 block with flat top pistons. I think this would be a good high compression combo with the N42.

Hope this helps.

Steve

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It will raise your compression to roughly 10.5 to 1.

I'm not so sure about that.

According to the www.ozdat.com calculator:

http://www.ozdat.com/ozdatonline/enginedesign/

L24 w/N42, flat top pistons and 1.25mm gasket = 8.824:1.

L28 w/N42, dished pistons and 1.25 mm gasket = 8.257:1

L28 w/N42, flat top pistons with 1.25mm gasket = 9.768:1

And according to Lengine.exe with a standard OEM gasket:

L24 w/N42, flat top pistons and OEM gasket = 8.78:1

L28 w/N42, dished pistons and OEM gasket = 8.29:1

L28 w/N42, flat top pistons and OEM gasket = 9.82:1

So there are some slight differences which may be the gasket measurement.

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Check out MadKaw's gallery, he has pictures of the notches on his L24 (nice looking engine Steve).

My comp. ratio came out a little higher because the head had been shaved a couple times and I have a positive deck height, close to 1mm.

Steve

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Thanks,

I have been talking to a couple people locally who are telling me that with a stock cam interference is not an issue. I am looking at picking up a stock N42. They say that only when you change the lift that you have to notch the block. Anyone know about this?

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Not true. You need to notch the L24 block if you put on an N42 head. They may be talking about swapping on N42 head onto an L28 block. The valves are a larger in the N42 head than they are in your, presumably, E88 head. It's the larger valves coupled with the smaller bore in your L24 block that precipitate the need to notch the block.

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  • 4 years later...

I know this is an old thread, but just in case anyone checks this out, I have a couple of questions. I'm getting ready to start for the 1st time my rebuilt l28e with the stock N42 head and new flat top pistons. Looking into the #1 cylinder when the piston is at TDC, there looks to be precious little room for the spark plug. I'm assuming the rebuild shop (who have done lots of other Z car engines) would not have suggested the flat tops if plug clearance were to be an issue. The block was not decked, but the head was shaved .001". I have a Fel-Pro gasket under the head, which I'm assuming is 1 mm. I know my compression ratio will approach 9.9:1. So, here are my questions: Will I need to run higher octane gas? I'm assuming so. If this indeed the case, where should I set the timing? Will the stock 10 deg. BTDC be good enough with premium gas (93-95 octane), or will I still get detonation?

I'm looking forward to finding out how much more power she'll have with the extra compression and newly rebuilt fuel injectors.

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I know this is an old thread, but just in case anyone checks this out, I have a couple of questions. I'm getting ready to start for the 1st time my rebuilt l28e with the stock N42 head and new flat top pistons. Looking into the #1 cylinder when the piston is at TDC, there looks to be precious little room for the spark plug. I'm assuming the rebuild shop (who have done lots of other Z car engines) would not have suggested the flat tops if plug clearance were to be an issue. The block was not decked, but the head was shaved .001". I have a Fel-Pro gasket under the head, which I'm assuming is 1 mm. I know my compression ratio will approach 9.9:1. So, here are my questions: Will I need to run higher octane gas? I'm assuming so. If this indeed the case, where should I set the timing? Will the stock 10 deg. BTDC be good enough with premium gas (93-95 octane), or will I still get detonation?

I'm looking forward to finding out how much more power she'll have with the extra compression and newly rebuilt fuel injectors.

If it's a plug meant for the L (e.g. NGK projected-tip plug), you'll be fine.

With a compression ratio close to or at 10:1, you better be running pump gas, and even then there is a chance of detonation depending on timing. We can't tell you whether or not you'll get detonation without being there, nor can we tell you where to set initial timing without knowing what distributor you're using.

A stock-headed L28 tends to make most use of the mixture with around 34-36 degrees of total timing, as demonstrated in dyno testing. The total timing depends on your initial timing plus the maximum centrifugal advance of your distributor. At your compression ratio, you may or may not need to back off on timing. There have been people that needed to go to 28BTDC to ward off knock, and there have been others that had no problems at 34BTDC, all with flat-tops and N42 head.

FWIW, upping the compression ratio alone doesn't produce much of a change in torque, maybe a 1-2% gain, which likely won't be noticeable. Compression ratio is usually increased in order to keep cylinder pressures up when using a more aggressive cam. Without swapping cams (and matching intake/exhaust manifolds), the gain from a bump in compression is minimal.

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It's a D6F4-01 distributor. I also have a spare engine out of a 280ZX with it's stock distributor, but haven't changed it over to my older engine. There seem to be a lot of people that recommend the swap, but I don't know just what I'd gain out of it. I was planning on using the same NGK plugs I was using before, which are the ones spec'd by NGK for the engine. I had also considered a set of those gimmicky E3 spark plugs, but they would definitely have to be indexed to work properly I think.

BTW, will a normal inductive timing light work on a stock '76 Nissan ignition 280Z system? I need to go buy one anyhow, so if there's some really good one to get let me know.

Thanks for the advice.

Ken

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For timing, we can't really help without also knowing the cam spec. I could assume you've got the stock unit with your EFI system.

For your reference, I was having Flat top & N42 head with MSA stage 2 cam (Schneider 274/274). I had to run 28-29° TOTAL advance Max without detonating.

I have DCOE carbs that needed some initial advance too; around 10-15° with no vacuum adv.

You have to make sure you can setup properly your timing then :)

With stock cam it could be even worse but injection might make things less difficult than carb setup.

Edited by Lazeum
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It is a stock cam. I have never made any internal adjustments to the distributor, either. I think at first I'll worry about just getting it running stably, then tweak it. The FSM provides decent instruction on distributor adjustment. I've also considered looking for a whole new ignition system for the future, probably a distributorless coil-on-plug system.

You know, it used to be that I had all the time in the world to work on my vehicles, but never enough money to buy the parts I wanted. Now it's the other way around. Can't win.

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I fully understand you're point of view. It is the other way around for me... Time but no money (mortgage)

My point is you might never be able to get your engine running well without detonating with your current setup; either it will run very poor at idle because of advance too low or it will ping at higher rpm.

One easy way to overcome this would be to use an MSD 6AL-2 unit but it you could keep your vacuum advance effective it would be a big plus.

DIS systems are the best but you would need an ECU to manage that. If you choose one, you'll be able to tune your engine for new cam, fuel and so on, so making your engine well shouldn't not be a problem anymore.

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Well, it ran with the dished pistons, bad rings, a slightly (.001") shaved head with a 5 angle valve job, dirty injectors, and 10 deg initial advance. Now I've got pefect rings and flat top pistons. So, other than extra compression and new injectors, nothing has changed. Should run all that bad. I guess I'll see...

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Fuel mixture ratio's will influence your engines knock thresholds.

The stock cam won't help your knock situation, too much cylinder pressure because of a lack of valve overlap unless you use high octane fuels, water injection, retarded camshaft timing or run very conservative ignition timing which leads to.........

Get a 'dial back' timing light. I bought an Innova from ebay and it's tops. You can plot the advance curve verses the rpms to create a simple 2d map of your distributor advance curve. A good starting point if/when you later run an ECU controlled ignition setup.

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I was planning on running premium fuel, which around here is about 93 to 94 octane, although it will also contain ethanol at about 15%. I will certainly see if I can find one of those timing lights. I don't mind paying for features in a tool if I'm actually going to use them. As far as timing, I'm hoping the stock 10 deg will be ok. If not, I'll either dial it back to 9 or bite the bullet and get a thicker head gasket. The engine builder seemed to think it would all be OK, and they had built a fair number of L engines for satisfied customers. I'm really hoping they were right.

If all else fails, I do have a spare engine out of a 280ZX that was running fine when I bought it. I can always put that in if I need to.

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