Race cars’ exhaust flames a cool waste of fuel
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
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.
Dear Car Talk: I have a 2003 Pontiac Bonneville that has not been driven since the March 2020
arrival of COVID-19. Long story short: It has 66,000 (accident-free) miles on the odometer and should have lots of around-town miles left.
It was well-maintained and running fine until it was parked in a back lot three years ago but won’t start now. Upon investigation, a toad leaped out from the trunk and there was leafy green growth in the engine compartment.
It’s accessible for towing. I want to tow it to a mechanic to see what it needs to make it road worthy again. What do you recommend I ask the mechanic to check to get a comprehensive evaluation of its condition?
Anxious to get your reply, and thanks for your sage advice. — Chris
Chris: Well, it needs a sign that says, “Caution, wildlife area.” And maybe a pass with a mulching lawn mower. But other than that, it may not need much of anything.
Your mechanic should begin by getting it started. If he wants to be ultra-safe, he can remove the spark plugs and squirt a shot of oil in each cylinder first. Then, with the plugs still out, he can hand turn the crankshaft a few times to get the rings moving and cylinder walls lubricated.
After that, he can put a good battery in it and fire it up. If it runs, he’ll know pretty quickly from any noises, smells or fumes whether anything else is seriously amiss.
If it doesn’t run, he’ll have to do some detective work and figure out whether it’s lacking fuel, spark or both.
It’s possible that rodents chewed through some wires or dined on a crucial sensor connection. Or maybe the fuel pump relay is rusted and stuck in the open position. Given the vintage of this car — and the fact that it was running well when you parked it — it shouldn’t take him too long to figure out what’s keeping it from starting.
Regardless, once he gets it started, he should test drive the car, to be sure the brakes are working well and that there are no snakes under the front seat looking for those toads. Then, he should check all the key systems — brakes, steering, exhaust, cooling, etc. — before he returns the car to you.
Then you drive it around, Chris, and see if you notice anything else misbehaving. You may not.
Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.