F54/P79 Swap Project

[Zed Head]

Some people think that the P79 head with flat top pistons has better quench/squish, which is supposed to offer better efficiency and detonation-proofing.  But others who seem to know a lot say that the squish effect, even if better, is still pretty poor for the P79/flat top setup.

Power measurements suggest that there's essentially no difference between the two L28 combos.

[240260280]

Maxima N47 is already done.

[Captain Obvious]

17 hours ago, siteunseen said:

Without much drama, which would be the easiest to "hop up"? 

That's exactly what I'm trying to do with this P79. I'm trying to achieve the "best bang for the buck" and pick the low hanging fruit. I'm trying to get through this without having to use shims under everything or put in some fancy aftermarket chain tensioner. I would like the piece of mind that I could run a tank of regular gas through it if I needed to without detonating. I'm not sure that would be possible with the common .080 shave off the P79 since that sends the compression ratio up to over 10:1

What I would really like to do is shave the P79 as far as I can and still be able to use the stock tensioner, no shims, and regular gas. In other words, I want as much of a performance gain I can without the next level of modifications.

With that in mind, I did this... I measured the thickness of the head gasket I took off the motor. It came out to be about .047:

Then I put the head back on the motor WITH NO HEAD GASKET and tightened the bolts just a little. I put the timing chain in place so I could see what that would do to the tensioner. The theory is.... This is what the chain tensioner would look like if you were to shave .047 off the head. With the head gasket missing, the head is about .047 closer to the block, simulating a .047 shave off the head.

So with the head gasket installed, the chain system looks like this:

Here's a close-up of the tensioner with a normal head gasket installed:

And here's what the chain system looks like when there is no head gasket installed:

And here's a close-up of the tensioner with a .047 shave off the head (simulated by putting the head on with no head gasket):

How does that tensioner look to the collective? I'm not thrilled. I think maybe a .040 cut would be safer.

What say ye?

[Captain Obvious]

I've run the compression ratio numbers through my spreadsheet and came up with the following...

F54/P79 stock - 8.83:1
F54/P79 with .040 shave - 9.37:1
F54/P79 with .080 shave - 10.0:1

Is 9.37:1 still in the realm of regular grade gas?

[siteunseen]

Wow, that made a big difference.

Anxious to read Hoover's MN47 offerings.

 

[Captain Obvious]

Yeah, that .047 difference really changed the position of the tensioner.

However, forgot to mention... The chain guides and tensioner shoe are worn in that pic. If they were new unworn, it wouldn't be quite so bad. I can also slot the slack side guide holes a little to bring it in closer to the chain.

I'm thinking that my sweet spot of "low hanging fruit" could be about .040 off the head, get new chain guides and tensioner shoe and slot the bottom hole of the slack side guide a little bit.

[240260280]

You can adjust the slack side guide inward at the top to take up all the chain slack so that the tensioner goes back.

My Blue 280z was shaved 2mm with no slack adjustment correction. I moved the chain 1 tooth to get the timing back so that the mark on the cam tower aligned with the sprocket triangle.

Edited September 13, 2019 by 240260280

[240260280]

[siteunseen]

33 minutes ago, siteunseen said:

Anxious to read Hoover's MN47 offerings.

That's the dumbest thing I've said today but it's still really early...

Captain you have a P79 already, not an MN47. Phillip was talking about square exhaust ports. Sorry!

Back to the Salt Mines for me. ?

[Patcon]

Yeah, not thrilled about the tensioner position with the .047 shave. Slotting might fix that but I was surprised how much the tensioner has to come out to remove that slack

[siteunseen]

If you get really bored try hole #3 on the cam spocket and see how much it pulls the tensioner in. Just for poops and giggles.

[Captain Obvious]

11 minutes ago, siteunseen said:

 try hole #3 on the cam spocket and see how much it pulls the tensioner in.

You were right.... It was still early. This ones better. .