Why would a lighter flame go into an exhaust pipe?

Question

I just saw this image and I am wondering what is happening here:

The car was running and it seemed that the wind direction was out of the pipe?

Answer 1

This is a quick-n-dirty test done to see if there might be an issue with a burnt exhaust valve. Another similar test uses a dollar bill (or whatever currency you happen to have). What is being checked for is reversion. This is an issue where if the exhaust valve is open when the piston is on the intake stroke, there will be the situation where exhaust gasses are pulled back into the cylinder along with air/fuel from the intake tract (with a non-direct injection setup ... just air with direct injection). If there is reversion, the flame from the lighter will be pulled into the exhaust pipe when lit in front of it. In the dollar bill test, the bill will rattle against the exhaust pipe.

Reversion can happen in one of three ways (that I'm aware of):

• Burnt exhaust valve which no longer seals correctly.
• Stuck exhaust valve which doesn't seat correctly or gets stuck partially open (which will quickly lead to a burnt exhaust valve).
• Large amounts of overlap in the cam. The great the overlap, the more reversion (exhaust valve will stay open longer during the intake stroke). This is a common way to overcome the need for an EGR valve to meet emissions.

Answer 2

Most probably due to the velocity profile of the gases coming out of the pipe and any constrictions upstream of the exit - causing backflow at the specific exhaust gas speed.