F54/P79 Swap Project

[240260280]

I had 2mm shaved off my N47. I simply lined up the sprocket triangle to position #1. To do this I ended up moving 1 full link.  The tensioner took up the slack. It was on a well worn engine.

I would recommend repositioning the slack side chain guide to take up some slack if possible.

[Diseazd]

I didn’t know that you could take 2mm off of an N47 head. Are you sure you got valve clearance with an .080 cut? 

[240260280]

51 minutes ago, Diseazd said:

I didn’t know that you could take 2mm off of an N47 head. Are you sure you got valve clearance with an .080 cut? 

It runs nice.  Pistons are dished. 32cc Compression with C Cam is ~ 190psi.

 

[Captain Obvious]

Now that the dust has settled with my broken camshaft project, I'm trying to get back to my F54/P79 motor.

I was messing around with the handy-dandy Atlantic Z engine calculator app found here:   http://www.atlanticz.ca/zclub/techtips//calcs/engine builder/index.html     and I can't get it to give me the correct compression ratio. The FSM says the F54/P79 combo (used in the later ZX NA cars) yields a compression ratio of 8.8:

But using the calculator app, I don't get that result. I get 8.43 instead:

I get 8.43 instead of 8.8. What am I doing wrong? I know some of the numbers (like the head gasket thickness) is off a little, but not enough to explain my delta.

[Zed Head]

Where'd you get the chamber volume number?

[Zed Head]

Here's another calculator.  Gasket height has a bigger impact than I thought it would. Try some small changes there.  0.85 gets you there.

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

[Zed Head]

It does look like an error in the FSM though, the calculator is close for the dished pistons, using 1.25 mm for the gasket.  7.353.  One of those CR's has to be wrong.

[Captain Obvious]

I got the chamber volume "off the web". That 53.6 is the number that "everybody uses". Of course, because of that, I cannot attest to the accuracy.

So I've been messing around with the calculators a little more, and I believe they messed the math up some. I've got an excel spreadsheet with my own measurements and calculations and I get pretty close to that 8.8 using data that I measured or derived myself. When I use my measurements in my spreadsheet, I get 8.86. I have not measured the chamber volume myself, so some small differences there could explain the remaining discrepancy.

I do not think they are calculating the uncompressed volumes correctly. Also, my measurement of piston stick-up above the block deck back-calculates to a significantly different pin height than what they're using.

Edited September 11, 2019 by Captain Obvious

[240260280]

I just did the calc and it seems very close to the tool.

Some subtle points are:

• On compression stroke, the volume of the gasket is reduced by the piston protrusion into it.
• On the Intake stroke, the piston top is not 79mm down from the block but 79 mm down from its top when protruding.

Here are my calcs. Attached is the XLS.

P79 Comp Ratio Calc.xls

Edited September 12, 2019 by 240260280

[Captain Obvious]

Yes, I understand those subtle points, and that's where some of the problems are. For example... If you change the pin height spec in the calculator here>>   http://www.atlanticz.ca/zclub/techtips//calcs/engine builder/index.html 

The compressed volume changes, but the un-compressed volume does not. It's a bug. It's not accounting for the fact that the piston height at the bottom of the stroke is raised as well.

I'll clean up my spreadsheet some and maybe post my numbers.

[240260280]

Good find. I'll fix it.

[Captain Obvious]

Well if you're going into the code... Here's the other things I came up with.

First, I think the pin height used for the L28 flat top pistons is incorrect.  When I put my pistons at TDC, they stick-up out of the block by .021 inches. I'm assuming Nissan's target was 0.5mm there. So whatever you need to do with the pin height to make that piston pop-out number work out to 0.5mm.

Also, I measured the thickness of a used head gasket to be 0.047 inches (1.19 mm). I measured the thickness of a brand new one to be about .052 (1.3mm), so there's some crush there when the head is torqued down.

Other thing I found is that the piston holes in the head gaskets are elliptical, not round. I measured the holes on a used gasket to be approx 3.440 by 3.505 inches (87.4 x 89mm) . Sure you can average those two numbers to something in between to make the end result close to correct, but I think it would be better to use real numbers.

I also measured the holes on a brand new L28 head gasket and got approx. 3.447 x 3.516  (87.5 x 89.3mm), so the piston holes close down some as the gasket gets crushed. That makes sense too.

Anyway, if you're going under the hood, there's some other things to think about.