L28 rebuild!

[keepitcool12]

So I'm rebuilding my L28 and I want to run flat pistons in it, am I able to just run 280zx non turbo pistons instead of the stock 280z dished ones? Just curious will it be too much compression? I am running a earlier model head on the block as well. Just curious if any of you guys have done this or a similar build and could give me some info!

 

Thanks!

 

Mark G.

[datzenmike]

The zx L28 block should have F54 cast into it. The '81-'83 F54 block with P90  P79 heads have the flattops. Naturally if rebuilding it you don't want old used pistons but a fresh 20 or 40 over bored block with matched oversize pistons. Hone and ring is a poor way to rebuild as there is ALWAYS ring wear at TDC.

 

 

The head cc says everything about compression. But not saying much as... "I am running a earlier model head on the block as well." well what earlier head? E31, E88,  N42? .... E88 heads are the same combustion chamber size as the '75-'78 N42 and N47 280z heads. The '77 and up 280 N47 head has hardened steel valve seats replacing the softer bronze ones that relied on leaded gas to lubricate.

 

Some guesses and some math.... 42cc combustion chamber? and flattops makes 10.366 compression, slightly more when over size pistons used. Stock is 8.66?

[darrel]

F54 blocks with the p90 had dished pistons for the turbo applications.

 

F54 blocks with flat top pistons had the p79 head for non turbo applications.

[Bleach]

The 1975-80 L28 with dished pistons had 8.3:1 compression. Non-turbo 1981-83 with flat tops and the larger cc head had 8.8

 

So 10.3:1 compression by swapping heads? That's very high compression for that open chamber head design. I've heard of this being done before many times but I don't know how well it works.

[keepitcool12]

Yeah my bad the head I'm running is an E88, and yeah I plan to bore it out, the cylinders still look pretty good though, they still have cross hatch marks in them, I measured around the tops of all of them and they are all pretty much 86.mm but I dont have a way to measure them at different points on the inside, but either way I think ima do like you said and play it safe and just bore it out. So with flat tops would that be to high compression?

[datzenmike]

8.8 is alright. 10+ is a bit high needing higher octane rated gas. Increased compression increases the engine efficiency but by how much? Almost 9 to over 10 is maybe 6%? At max RPM and 170 hp that's just barely 10 more and less at lower RPMs. On a 'built' engine with cam and head work this would be higher of course. Is this really worth the bother and added expense when fueling? Being more efficient will increase mileage and help offset the price of gas.  

[CGraves]

This website is pretty good at figuring out what your compression is gonna be and lets you play around to find the best options. http://www.ozdat.com/ozdatonline/enginedesign/

[datzenmike]

Yeah I tried it for an L20B 85x86mm, 38.1 pin height, 227.85 block height, 145.9 rod rod length ad then it wants too much additional info. Wouldn't compute unless the redline was entered. Then wouldn't compute unless the piston diameter was input (isn't the bore enough??????. Finally the compression was...... n/a. Gee thanks.

 

 

 

Tried it twice now and got no answer. So what does it want that it's not telling me. Does it really need the number of valves????

[Bleach]

I would assume if someone does this that they will be always using the highest octane pump gas available in their country. No question on that.

[datzenmike]

As to the increased compression each point increase is about 5% but this is around an 8 to 9 compression range. As you get closer to the max ideal possible compression for a gas engine it becomes much less. Going from 8 to 9 is a much higher percentage than 12 to 13.

 

Detonation and 'ping' becomes a real problem with shitty octane gas and the timing has to be backed off, loosing any gains made. Higher octane allows you to run increased timing to get the most out of the gas used. Todays 'flex fuel' cars adjust the timing automatically to get the most out of the fuel used.

[CGraves]

@mike

 

I dunno it works fine for me? 

 

But if he's running 10~:1 compression 93 octane should be fine right?

[datzenmike]

Still nothing. Have the following

 

bore

stroke

rod length

pin height

dish volume

combustion chamber volume

gasket thickness

block height

 

What has piston weight, red line and number of valves got to do with CR????? What did you have to add to get an answer?

 

EDIT: OK added L20B and it came out 8.8 which is really wrong. It's 8.4 for open chamber and 8.903 for closed.

 

For some reason it thinks I have a closed combustion chamber head so what else did it input that is wrong??? Not liking this and will do it my old way with a calculator using only the 8 things really needed in my list.

[CGraves]

@mike

 

Yeah it's always gonna be better to do the math manually obviously but I assume it's useful for stupid people like me who aren't good at math.

 

Also I think it rounds down for some reason? I get 8.8 and 8.3 depending on which head I pick. 

[datzenmike]

Agreed.  I like seeing the math unfold, gives you a solid grip. I guess that computer won't work on anything but the Datsun/Nissan engines listed.