I've ignored all recent "pooled fuel can cause an exploding sump" anecdotes, as I wanted to do some reading first. After all, challenging the collective wisdom of the airport front porch is a task not to be taken lightly.
Pooled fuel in the intake plenum does
not cause sump detonations...at least not fuel pooled in the quantity imagined by most.
As noted way back in post #11, fuel in liquid phase has no practical explosion risk. The fuel of interest is that which is in vapor phase, above the liquid, and combined with air in any proportion within the combustible range.
Take a look at anyone's 100LL MSDS. It includes fire risk, expressed at Upper Flammability Limit (UFL) and Lower Flammability (LFL), said limits also describing explosion risk. The UFL for avgas in air is about 7.6%
by volume. The LFL is about 1.3% by volume. Mixtures with fuel vapor percentages richer or leaner than the limits do not support combustion.
Ok, so what does it mean in terms of
liquid fuel quantity? At standard pressure and temperature (call it 15 psi and 15C), the liquid-to-vapor ratio for avgas ballparks about 1 to 145...one unit of liquid, fully evaporated, becomes 145 units of vapor. That number rises with temperature; it would be about 1 to 180 at a temperature of 200F.
Assume a horizontal Lyc intake plenum is 10" x 10" x 4", a volume of 400 cubic inches. The UFL is 7.6%, so 30.4 cubic inches of vapor (400 x 0.076) is the maximum; more is too rich. 30.4/145 = 0.2 cubic inches of liquid fuel, assuming full evaporation. That's 0.11 fluid ounces, or 3.2mL...about 1/10th of a shot glass, for a sump at 60F. At 200F it's even less, about 2.6ml.
Now for the interesting part. One reason gasoline has been a practical motor fuel is because it is relatively safe to keep in tanks, large and small. At typical temperatures, its physical properties, notably vapor pressure, result in mixtures above the UFL in the headspace of a tank. It's why gasoline tanks rarely explode, regardless of what Hollywood portrays every time the bad guy drives off a cliff. The fuel/air mix above liquid fuel is just too rich.
Here's a figure from a university paper which nicely illustrates reality. The added red graph lines are mine.
The researcher carefully measured vapor percentage in standard 5 gallon rectangular plastic fuel containers, charting liquid quantity and temperature. The risky fuel level is not much more than a thin film, 15mL (half a shot glass) spread across the entire bottom of the jug, and then only at lower temperatures. 30mL pushed the mixture above the UFL at all temps above freezing. The 500 and 1000ml lines? They tell you anything more than a pint of liquid fuel in a 5 gallon space makes that space practically inert. There is not enough oxygen in the liquid or vapor to support fire or explosion.
So, want to worry about blowing your sump to bits? The risk is found when cranking at low temperatures, with very, very little liquid fuel on the plenum floor.