N42 Head - Dished or Flattop Piston Advise

[S30Driver]

For the last couple of years, I have been planning to rebuild my 77 280z motor.  It is getting tired, running well but oil consumption is a issue.  I suspect rings & valve guides.

Last year I acquired a interesting 2.8 motor and am now getting around to looking more closely at it.  It is interesting in that it was claimed to have been rebuilt and never installed.  (kind of like ran when parked, lol)   It is a F54 block with a N42 head.  Turbo oil pan but NA intake & exhaust manifolds.  I had it sitting on a nice cradle & dolly, so easy to move around.

I pulled the plugs, which are new NGK's and never fired.  Got my laptop and borescope camera, inspected the cylinders and found it has dished pistons and what looks like a fresh hone.  All looked very clean.  Next, I removed the valve cover and lubed the valve train with some high zinc oil, added about 2 or 3 quarts more and turned it over by hand.  All appeared to be well.  Then bolted on a bell housing so I could add a starter and do a compression test.

The compression results were pretty consistent around 150psi.    151  152   151   150   150   150    cylinders 1 thru 6.

With the dished pistons, and Atlantic Z's calculator, looks like about 8.25 to 1 CR.   Going to flat top pistons, approximately 9.7 to 1 CR.    I am trying to decide if it is worth the trouble to source and install the flat top pistons.  From my quick research, it looks like this heads design is not the best choice with flat tops.    Since the head looks like it was totally redone - valve job etc. it would be nice to keep it on there as that is most of the money saved.     What do you guys think my options are..

From what I see so far with this motor,  it does indeed appear to show all the signs of a rebuild.   The head / cam area are very clean, manifold gasket is new, scraped some purple paint from a few frost plugs to reveal new looking brass ones.   Just transferred it to a engine stand and have not pulled the pan off yet to take a look at the bottom end.

My goals are not huge performance with this motor, a little bump up would be nice,  just looking to mainly maximize my time & dollars if I put this in my 280z. 

Already have a new Exedy clutch kit for it , and ordered a new Fidenza flywheel to keep up with Cliff & his Tilton.  

 

[Mark Maras]

If the issues with the old engine are it's tired and using oil. I'd use the rebuilt engine, as is. Would the flat top pistons demand higher octane fuel? Fuel prices could enter into the scenario if you drive it a lot.

[Zed Head]

Sounds like it is essentially identical to what you have in your 77 now, performance expectations wise.  If it's been assembled well it seems like money wasted to take it apart again.

You might check the wipe pattern and examine the quality of the rocker arm lash pads.  See if they've been reground or the builder just put old ones back on.  Cam lobe wiping seems to be the most common damage to newly rebuilt engines.

Mark just replied but I'm going in anyway!!  Submit Reply!

[S30Driver]

All good advice.   Yup, if I raised the compression, it would be premium fuel only time and I need to drive to Zcon this year.

At a minimum, I will do a block color change...

[madkaw]

Shave .030 off the head and call it good 

[gwri8]

If it passes Muster,  I vote for putting it in as is (with a color change of course).  Once it nestles in, you can go to town with your old engine and do anything you want with no time pressure.

[Captain Obvious]

I've found problems with the on-line compression ratio calculators, so I'm not confident in the numbers you mentioned.

I've done a little work on my own calculator and I come up with 8.3 for an N42 with dished pistons and 10.25 when used with flat tops. If you're really wanting accurate numbers, let me know and we can get into it here in the thread.

[siteunseen]

Are you gonna put that Fidenza in the glove box of a new Koenigsegg?

 

 

[Reptoid Overlords]

If the engine currently in your car is tired, then you will surely see some improvement in performance with the new one as is.  Like everyone else said.  

While you have it all out you could consider some exhaust options to spice things up a bit.  Not sure what you're running now, but if it's stock some headers and 2 1/2" exhaust is good bang for the buck. I'm running MSA headers and full exhaust except for Z story muffler.

[S30Driver]

Thanks for all the comments.   I'll continue with the inspection of the motor, cam wipe pattern, verify valve timing setup etc.

I currently don't have headers installed but do have Sean Dezart's Z Story system.   I'll report back with the progress...