Detonation stumping me

[Tuner]

Have you determined the range of vacuum between start and total travel of the vacuum diaphragm? Measure in 2 in. Hg steps from 0 to 20 in. Hg exactly how much vacuum causes how much advance. Some vacuum advance diaphragms have a very soft spring and are literally useless for any application other than a late '70s stock production emissions engine that has an extremely retarded mechanical curve and a large amount of EGR. Many aftermarket adjustable vacuum advance units are similarly useless because they have the wrong spring and cannot be adjusted to provide a useful vacuum advance curve.

In the late '70s it was common to see a vacuum canister with a spring so weak it still has 20 degrees advance when the throttle opening is large enough the engine vacuum is down to 5 or 6 in. Hg. The adjustable OE vacuum advance units on Ford and Mopar engines would slowly back off the adjustment over several thousand miles and eventually they would ping a ton at part throttle and as you went to full throttle the timing would finally retard and stop knocking. They would knock at part-throttle but not at WOT. Many aftermarket vacuum advance units have the same problem, the spring is so soft it cannot be adjusted to provide a useful advance curve.

A quick web search didn't find a 351-W "AllPro" head. Can you post a pic of the combustion chamber in your heads? Nearly all cylinder heads manufactured in the last 20 years have very efficient combustion chambers and can be described as "fast burn" and do not need as much spark advance as earlier designs. If your heads are the FB sort, this advance setup should work well.

timing BTDC at 650 rpm in gear 18
BTDC at 1000 rpm 18
BTDC at 1500 rpm 18~22
BTDC at 2000 rpm 22~24
BTDC at 2500 rpm 24~26
BTDC at 3000 rpm 26
BTDC at 5000 rpm 30
BTDC at 7~8000 rpm 32

Vacuum advance
6" Hg 0 deg.
7" Hg 1 deg.
10" Hg 8~10 deg.
15" Hg 18 deg.

And, sorry if you already answered this, what heat range spark plug are you using?

[Tuner]

Have you determined the amounts of vacuum at start and total travel of the vacuum diaphragm?

[F-BIRD'88]

Tuner is on the ball here on getting vacuum advance right.

[My427stang]

Tuner is a sharp cat, but the OP did say that it happened without the vac advance too

Not saying to ignore anyone's advice at all, but did you ever verify the head gaskets were right? It overheated once, and Fords have a front and back of the gasket, if assembled wrong, the water will flow to the front of the heads not the back, which would get things very warm on that side.

[treyrags]

Tuner is a sharp cat, but the OP did say that it happened without the vac advance too

If it happened without the vacuum advance then it is coming in too soon and/or too much.

[af2]

Tuner is a sharp cat, but the OP did say that it happened without the vac advance too

If it happened without the vacuum advance then it is coming in too soon and/or too much.

Coming in to soon? It is what is when you set it the centrifical weights could be if it has it.

[treyrags]

If it's coming in too soon the spring rates need to be changed.

[My427stang]

If it's coming in too soon the spring rates need to be changed.

Of course, if that's what is happening, but I don't think he has found anything that simple yet. Unless one spring broke or popped off or something silly like that.

Should be easy to look in the advance mechanism

[treyrags]

Timing light and a partner to operate the throttle. Take timing readings every "X" number of RPM and graph your curve.

[madmooney]

Extended tip plugs were the culprit on my SBC.