David, I think that's a good idea on the timing. Interesting on the intake comparison. It would be great to see flow bench data for the two, more for curiosity at this point.

I probably rambled a bit there, didn't mean to distract from taking action. Dropping timing is a very straightforward test, as is forcing open loop by unplugging the oxygen sensor.

 

Every where I've looked, the Windsor head gaskets are .047 to .050 compressed. Which means if the pistons aren't at the deck or slightly above the quench is out of the good area. For instance, my Sealed Power book shows the 351w Lightning pistons that have the same style dish as the non-Lightning pistons... Have a compression height of 1.784 from center of wrist pin to crown. While the LD (150, 250, 350 HD is 450 and up in this book) has a compression height of 1.765.
Now that's a difference of .019 if they use the same gasket then I would assume the L piston is slightly out of the hole and that's where the added .8 compression comes from. I think they are 8.8:1 and the regular is 8.0, not sure. Still that would push quench very tight at .031 so maybe the STD 351 just has a very large quench from the factory, and that's why they seem to be gas hogs as large quench is also know to cause inefficient burning.
just food for thought.

 

Thanks everybody for their input. Today I did some test runs by retarding the timing to 8* as David and Goinboarding suggested and electrically disconnected the O2 sensor as Paul suggested. For starters, I filled up both tanks, one with regular 85 octane, the other with premium 91 octane. I left the timing at 10* with the O2 sensor disconnected, it pinged with 85 octane, but did not ping with 91. I then reconnected the O2 sensor and retarded the timing to 8*. It still pinged with 85, and maybe ever so slightly with 91. It's possible the balancer isn't reading accurately, I have yet to bring #1 piston to TDC and check where things are at.

Does this sound like a compression thing? Perhaps the compression ratio is higher than I expected and requires premium gasoline? This makes me question what Goinboarding said about his previous 351 although unlike his, mine seems to have plenty of power. Also, maybe it's my imagination, but when I had the O2 sensor disconnected it felt more responsive, it had more "pep." But I would think the O2 sensor disconnected set at 10* would be relatively equal to having it connected with timing set to 8*, air to fuel ratio wise.

 

My old motor wasn't quite like our builds, so it's not really the best comparison. E7 heads, stock intake, stock exhaust, mystery flat tappet cam advanced 4 degrees..
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I'm not sure about OBD-I systems, but my OBDII comes out of closed loop with not much load. So under the conditions you get det it is probably switching to open loop anyway, with oxygen sensor installed. More responsive might be because it's not lean as you first step into it with the sensor unplugged.

Have you checked fuel pressure under load yet? Verify that system is in good order. Might want to leave it at 8 degrees base timing, det isn't good for the pistons or rings.

Is it possible to easily watch oxygen sensor voltages as you drive? General OBDI question there. It's easy on OBDII. Don't know if it requires $$$ scan tool on OBDI or backprobing the wires.

I'm not sure about Mudsports specs. Ford lists these engines at 8.8:1. I measured 20cc dish pistons, and iirc E7 heads are ~64cc. My 96' donor was zero decked and the gaskets measured 0.040" when I took it apart for inspection. Stock bore and no indication it had been apart before. The specs I measured computed to ~8.8:1, matching Ford.

Either way, unless the pistons in your engine have some goofy low compression height you're probably not at unreasonable squish, and the issue may be rooted in going lean rather than assembly.

 

You're right, I misread somewhere on the compression numbers. But, the book definitely shows different piston height for 351 trucks and Lightning's

 

I haven't been able to check fuel pressure under load, yet. As for reading the voltage of the O2 sensor, the only way I know of is backproping the wires. I'll have to rig something up to monitor it while driving. I'll test these things out and will report the findings once completed.

 

Does it only do this under full throttle? Or does it do it at part throttle as well?

 

Hi Mudsport. It'll do it around 3/4 throttle or higher. It rarely sees full throttle because I'll back off when I start to hear it. Premium gasoline seems to be helping. Could it possibly be ignition related? Plugs are new and gapped at .055. Wires are new, 8mm. Some of them sit pretty close to the headers, but not touching. Wires 1/3 and 5/6 aren't touching.

 

Anything is possible, but before switching directions I would get around to checking fuel pressure under load. I suppose something you could try is pulling the vacuum line off the regulator, then drive around. It would be at max fuel pressure that the regulator can make, if it helps might be a direction to look if you can't get a gauge on it while driving. It wouldn't help full throttle but anything where it would usually see vacuum would be seeing more pressure.

 

You said this earlier:

I think you need to pull ALL the plugs & keep them in order. Maybe with pictures for the board.

You said you've got 12K miles on this & 12K is a LOT of possible detonation damage in a combustion chamber if it has done it the whole time to some degree.

I've got to wonder if you have a hot spot in a cylinder.

 

I can and will pull them all. Perhaps I should buy a borescope to see whats going inside.

 

Not going to tell you how to blow your cash, but...

There are clip-ons to smart phones out there I hear.

Just waiting on a guinea pig myself.

Look at the plugs first.

Sounds like fuel & timing more than anything, but you never can tell.

 

That's right. My neighbor just bought one of those you speak of. I'll have to get with him to see if I can borrow it for a day. I don't mind being the guinea pig!

 

So I made some interesting discoveries. I haven't been able to pull all 8 plugs as I am moving out of state this weekend and my special spark plug socket is buried in a box (because of the headers).

I pulled the ones I could easily get to with a normal 5/8 spark plug socket. Pictures below with cylinder number labeled in top right corner. Plugs 5 and 8 have two pics each, different angles. Those are white on one side of each plug, oil? fouled on the other. Plug 4 was cracked! These plugs were all changed this past December, about 6k miles ago. There's no way I wouldn't have noticed that giant crack when I put them in, so I'm not sure if that's something that happened when I was torquing them down or something that happened just now when I went to remove it. I replaced that plug with a new one and took the truck for a drive. I ran it pretty hard with several WOT pulls and could just barely hear something with the 85 octane. With 91 octane it was non-existent no matter how hard I tried to get it to ping.

I'm not sure whats going here, some of the plugs look ok, some look bad. Replacing the cracked plug has cleared a lot of the ping problem, but doesn't explain why some plugs look like crap.

Thoughts?

 

Perhaps 4 was bad the whole time and not completely burning the fuel. The o2 sensor saw this and leaned out all other cylinders.
I would still invest in a bore scope to check the pistons. That "white" looks more silvery to me, which may indicate aluminum dust from extended detonation.