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High Compression L28 Build Question


Boog

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Hi everyone, long time lurker and very scarce poster. I had an oil pump failure on my 76 280z and will be rebuilding with a donor F54 flat top block. 

 

Here is the plan:

 

N42 head (it had "stage 2" scrawled on the front when I bought it)

stock N42 cam

F54 flat top piston block

Car is already on Megasquirt II with a wideband and 280zx turbo dizzy

headers into twice pipes exhaust

Most likely a fel pro HG

 

After calculating the compression on the atlantic z car website calculator, it spat out a 9.9:1 compression ratio. I live in Washington with I believe 92 octane gas available. My biggest worry is pinging. I honestly am just trying to make a decent NA motor out of what I have. Many people have stated that this high comp combination is good for power, many say that it is a waste of CR and will require too much timing advance to be worth anything. 

 

Who is really correct here? Given the above setup, 9.9:1 CR, and megasquirt to keep it in check, will I really need to worry about pinging?

 

Last question, would going to a OEM head gasket and dropping CR to 9.7:1 make a big enough difference to fix it, if necessary? I would really just like a decent performing stock motor for use in spirited driving and the occasional auto cross. I also would like to make use of the flat top piston block I have, as well as the N42 head. I know the head, block, and pistons are in great shape and would like to use them. I would be very happy with 200 hp to the wheels, or somewhere in that ballpark. 

 

Thanks in advance for any input, you guys have been very helpful to me in the past and I appreciate any and all help I get!

 

P.S. I attached a picture of the car as it currently looks, just for fun

post-29296-0-89954700-1482042799_thumb.jpg

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N42 head (it had "stage 2" scrawled on the front when I bought it)

stock N42 cam

 

You should have that head checked out, at least get the chamber volumes measured.  It might have had port and/or combustion chamber work done, along with surface milling.  It's probably not a stock head anymore so the CR calculators won't work.  And choosing a proper cam will be worth much more than a bump in CR.

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You have Megasquirt and live in the same state as Delta Cams. It would be a waste to use the stock cam. Has the head been ported that you can tell? Again, if it was me with that setup, I would pay the extra $100-$150 for a different cam. You'd probably be nearly at your 200HP mark with a proper tune.

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^ What they said. A nice cam in the 270 to 280 duration with .480 to .500" lift really wakes these engines up.... big time. I have a F54 L28, flat tops and 280 cam. Engine has a nice lope at 1,000 rpm pulls like an SOB all the way from approx 3,000 right past 7,000. Stock cam falls flat on it's face around 5,500 to 6,000 max. Keep the lift under .480" and you can use stock springs ( New of course )  . Just change to shorter valve seals.

 

I calculated CR at around 10.3 with my engine using online calculators. I head has been shaved about .050". BTW: Oz-Data calculator is off for L28's. Calculator has wrong Deck height for flat tops. Flat-Tops are +.022" to +.024"  above deck height. Not -.020" as OZ  Data calculators is configured for.

 

My car runs fine on Washington State 92 Octane with 32 to 34 degrees total mechanical timing. Get about 25mpg on highway with 5 speed and 3.90 gears.

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I don't think you can make that blanket statement about deck height. I measured mine at .019

 

The OP could fatten things up in the prone areas of detonation with some more fuel. Beauty of MS. Non quench head makes for a challenge but there's enough success stories around . I'm going for 11:1 at least with 93 octane and a high quench head

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Thank you all for the input, I think my first step is going to get my head checked out. If it is high compression than expected, I'll go with dished pistons and aim for compression in the 9's. If not and it is stock, i think i'll give the flat tops a shot and call up delta cams to see what they recommend. I'll update when i get the head looked at.

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I don't think you can make that blanket statement about deck height. I measured mine at .019

 

The OP could fatten things up in the prone areas of detonation with some more fuel. Beauty of MS. Non quench head makes for a challenge but there's enough success stories around . I'm going for 11:1 at least with 93 octane and a high quench head

Can't like your Post 'cause I exceeded my Like quota??

 

Anyhoo you are correct. I'm just going by what I've measured and others have posted previously. I think Brap was one who head a threda about the L28 Flat Tops being around .024" out of the block. 

 

Main point was that some online calculators are incorrect. OZ Data being one of them. If you configure with their  piston deck height specs, it calculates that L28 Flatops are .020" below deck height... which is wrong. You have to manually over ride their " calculated " data.

 

Best to measure everything yourself. That's what " Blueprinting " is all about

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Thank you all for the input, I think my first step is going to get my head checked out. If it is high compression than expected, I'll go with dished pistons and aim for compression in the 9's. If not and it is stock, i think i'll give the flat tops a shot and call up delta cams to see what they recommend. I'll update when i get the head looked at.

That's the best way. I wish my engine had a bit less compression. It's a bit touchy on fuel. Pretty sure I'd have an issue in California. I'd have to dial the timing back a bit.

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I used this online CR calculator which seemed to give calculations more in line with what some Forums post up for L28 with FT pistons and N47 heads. Note: Measurements have to be entered in Inches and to get a positive Deck height ( Piston above deck ) add a minus sign to Deck Height figure.

 

http://www.wallaceracing.com/cr_test2.php

 

Calculation of the following figures gives this result. Seems a bit high, but head gasket diameter and thickness I inputted may be a bit low.

 

Your displacement is 167.93 CID
The Compression Ratio is: 10.61

 

post-44147-0-45992500-1482437185_thumb.jpg

Edited by Chickenman
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Recalculated from the above site using a head gasket diameter of 3.5" and a crush thickness of .047" ( 1.2mm ) which seems to be a common figure for Nissan factory head gaskets. CR came out to 10.35... which sound more in line with other forums such as the link below. Your thoughts?

 

http://www.viczcar.com/forum/topic/4286-overview-of-l6-engine-heads/

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