Dear Car Talk:
I have exhausted all avenues but still haven’t found the answer.
As an avid fan of motorsports, I find very baffling as to why, when a car is decelerating, there is usually the presence of flames coming out of the exhaust. To decelerate, one cuts off the gas to the engine, but every explanation I can find says the flames come from unburned fuel.
That seem to be counterintuitive to me. Since you are the encyclopedia of things mechanical, the ultimate guru of gas, I was hoping you could explain this mystery. — Steve
I have no expertise in motorsports, Steve. The last thing I want to do after working on motors all day for a living is to come home and work on them for sport. But, it’s definitely unburned fuel being combusted in the exhaust system.
How does it get there? Well, race cars have different valve timing than your typical Honda Civic — on which you have to pay extra for the fire breathing package. Because its sole purpose is to run at high speed and full power, the opening and closing of the valves on a race car engine is optimized for those conditions.
To provide that maximum performance at high rpm, they increase the overlap between the intake and exhaust valves. So, compared to your 2014 Hyundai Elantra, for instance, there’s more time when both sets of valves are open.
When the engine is running at 5,000 rpm, pretty much all the fuel that’s pouring into the cylinders gets combusted and used. But, when the car suddenly decelerates, some fuel continues to pour in but can’t be burned fast enough.
So, that excess fuel ends up being sucked out the exhaust valves and ends up in the exhaust system.
Those exhaust systems are generally straight pipes, with little to no baffling or curves to muffle sound like we see on passenger vehicles, so a flame that gets ignited by the spark as the fuel leaves the cylinder can easily come right out the back.
While that’s a terrible waste of fuel and a source of smog, it does have the benefit of looking very cool.
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