Tumblroth

One of the great things about WIRED is that the place staffed by people who are authorities on (and obsessed about) a ridiculous range of subjects. That can lead to some surprising conversations.

Half the time I discuss a story idea with a colleague I end up learning a history lesson about some esoteric part of my article or taken down a rabbit hole I had no idea I was about to step into. Those kind of surprises are what makes journalism (alava shalom) so much fun; only here it happens in-house as well as on the road.

What got me thinking about this? Apparently, outside the San Francisco HQ yesterday there was a loud noise. Anywhere else that would have produced — if anything — a shrug. Here, the whole staff got this email from an editor (posted with the author’s permission):

From: Joe Brown

To: Wired Editorial

Subject: Backfires… What just happened

A lot of you have been asking about backfires (the loud noise that just happened), so here’s the basic gist of it:

A backfire is what happens when combustion—the explosive reaction of fuel and air that usually takes place inside your engine block—occurs in your exhaust system. It is normally loud, though I have never heard one as loud as the one that just rocked 3rd St. A backfire is usually the result of an overly rich air fuel mixture. When there’s more fuel than normal engine operation can consume, the unburned fuel is forced out of your cylinders with the exhaust gas, and just sort of hangs out in the tailpipe area. Until it blows up.

Judging by the level of that bang, I’m going to guess that the Toyota RAV4 in question was just recently started. Here’s why: If the car were at normal operating temperature, the gas, which is a liquid, would have vaporized as soon as it touched the metal. Gasoline’s flash point is 475 degrees F, and an exhaust manifold (the part that connects directly to the engine) runs around 1,500 degrees F. If the car were warm, any expelled fuel would have ignited instantly, creating a soft pop rather than that Baghdad-boom.

But if the car were cold, the unburned fuel could pool at the junction of the exhaust manifold and the flat part in front of the catalytic converter. Then, when the pipe heated up, the fuel would ignite, create a percussive wave that would be amplified by the restrictive pressure of the exhaust system, and scare the crap out of all of us.

Hope this helps.

JB