I hope this is the right forum for this, because I'm seeking a little feedback with a problem I'm having in one of my games.

Here's the situation:

• The system is Call of Cthulhu (fairly gritty, lethal) and set in the 1920s.
• The PCs (usually excellent, smart players) have gotten themselves in way over their heads by starting a fight that they have a very low chance of winning (this was not in any way my doing).
• They're by a truck that looks like this one: https://www.gracesguide.co.uk/...fe/Im1920-Albion.jpg

Here's the problem:

• PC 1 is standing next to a truck (engine still running), by about 13 of them. He warns PC2 to get away, then unscrews the truck's gas can and drops a lit match into it. He is operating on the theory that this will go kaboom and kill himself along with the thirteen cultists.
• PC 2 roleplays that he can't hear the warning, and gets into the driver's seat of the truck and tries to back it up. (the player privately believes that a lit match won't cause an explosion).

So...how likely is it that dropping a lit match into a half-full gas tank of a truck with the engine running will result in an explosion? I'll probably end up calling for a roll, but I don't know how low the percentage target should be.

Edit: I've done some research on this enough to give me a probably unsavory looking search history, but can't seem to find anything credible other than random Yahoo answers and Quora discussions.
This message was last edited by the user at 15:47, Fri 11 May 2018.

If I recall correctly, Mythbusters could not force it to happen at all (they used a gun, but the fluid dynamics of fuel/air mixtures don't care). It caught fire and not much else. In my games? It'll happen 100% of the time, unless the facility they're at, or some individual on the scene has a reason to have plot protection.
This message was last edited by the user at 16:00, Fri 11 May 2018.

The compromise is to start the truck on fire, rather than exploding it.  It's the gas fumes that go up, not the liquid gas itself, so the flame would catch on the mouth of the gas can and maybe start burning the wooden and cloth structure of the truck.  This would also give you the opportunity to distract/ignite some of the cultists without immediately exploding the PCs.
This message was last edited by the user at 16:24, Fri 11 May 2018.

I am hardly an expert but if I remember the science correctly your PC2 is closer to the truth.

• Gas might not ignite at all, whatever you are running on (diesel? kerosene?) is fuel which is not enough, to ignite you also need oxidizer... air will do but as PC1 only just opened the cap there might simply not be enough (especially since match will be burning out oxygen as it falls and might actually be extinguished before it touches the fuel)
• Assuming fuel ignites, it is fairly likely to go off before exploding - same issue there might not be enough oxidizer to sustain the fire.
• If the fuel does catch...  Explosion is by definition a rapid increase in volume.  Burning fuel will quickly create high volume of gases... but with cap off,  the time to explosion will be determined by the tag of war between gases generated by burning fuel and gases venting though the opening.
• Several other factors will be significant as well...  How full is the tank?  How long has the engine been running?  The more tank is filled with fumes rather then liquid fuel the more likely the explosion will be.

In the end you are GM, hard science and realism are subjects to your discretion.

Personally in this situation I would not bother with roll at all, I would consider explosion a 100% given, however will give several minutes before it happen (on one hand giving players a chance to get to safety, but on the other hand messing with their timing).

If I can get players to believe the plan failed and there will be no BOOM... so I can follow up with KA-BOOM as soon as they relax... So much the better. ;)
This message was last edited by the user at 16:40, Fri 11 May 2018.

Yeah, as mentioned, it's the vapor that ignites, not the liquid gasoline.  And the amount of vapor contained in a half-full ten-gallon-ish gas tank isn't sufficient to cause an explosion.  Even if your PC could get it all to ignite, you'd just get a sort of hissing WHUMP as all the vapor burned and then the fire ran out of fuel.

That's the sciencey answer.  The -correct- answer is that what happens is whatever the GM decides is most fun, or convenient to the plot, or in keeping with the spirit of the game.  If the game is "gritty and fairly lethal" then I'd say screw the science, that truck goes BOOOOOOOM and the cultists burn along with two of your PCs.

Thanks everyone! I think I have my answer now. Admins, feel free to close this as desired.

I definitely have a more gritty, pulpy, and cinematic inclination when it comes to games, so I think some things will be going up shortly.