Backfiring harmful to car? (I hope so)

There’s an idiot in town driving a BMW that, I guess, is supposed to be some sort of race car or something and the exhaust is annoyingly loud. It seems he likes to get on the gas hard at every opportunity, even when the traffic light a few blocks away is going to be red when he gets there. As he comes off the gas to shift gears, the backfiring (or whatever it actually is) is extremely loud. I saw and heard him yesterday driving with his window down, presumably to hear his own noise better since the weather wasn’t really “windows down”. Please tell me his is ruining his own car.

It’s probably post combustion in the exhaust system and not all that harmful.

Yeah, TriPolar is right. If it’s a loud snarling sound, then it’s not backfiring—that would sound like a gunshot.

That “snarl” on overrun is common in “racy” engine tunes. If the guy had a quieter exhaust system, that snarl wouldn’t be nearly so annoying.

I used to drive a Mini Cooper S with the JCW package. The ECU tune that came with it had the snarl-on-overrun effect intentionally tuned in. But the exhaust system was reasonable, and it sounded about as loud as someone clearing their throat.

The short version, removing all the car nerdery, is that your BMW driver has some kit installed to make that noise intentionally. It’s in the name of performance, but he’s not using it properly if all he’s doing is gunning it for red light stops.

It is likely ruining his turbocharger, that’s the “good” news. The bad news is that it’s likely just shaving a few years off something that would last for several years.

Let’s not remove all the nerdery. What exactly leads to to believe that this guy is ruining the turbocharger he may or may not have?

Performance cars often make fairly loud “pops” when decelerating.
These are not backfires* per se*, since a true backfire is out through the intake.
The pops are not doing any favors to the exhaust system, but they probably don’t hurt it very much (sorry to say(.


Yes! Backfire is through the carb, throttle body, or whatever else sits in the intake path these days. Afterfire is the loud bangs from the exhaust, although that seems to be a distinction preserved these days in British motor enthusiast mags.

As a point of reference for the amount of damage caused by the crackling and popping on the overrun when the throttle is closed abruptly, BMW sold an optional tune for my car that purposefully emphasized that sound. And that package didn’t void the warranty, so they obviously didn’t think much damage could be caused.

My motorcycle does it too, but if I understand correctly, on a bike it’s more a function of a very short exhaust path rather than a super aggressive tune.

This is like a gunshot. Two or three at each shift. LOUD. I thought maybe it was unburnt fuel igniting in the muffler and was going to break something. Oh, well. As the weather warms up people will start complaining and, hopefully, the police will cite him for something.

Apologies right off the bat, I’m going to put some nerdery back in.

If the guy had a performance-modified car, then he may have a larger turbo charger.

(For the less car-grogardy amongst us: a turbo charger uses exhaust gas–waste energy–to spin a turbine, which in turn drives a compressor to shove more fuel into your engine. It allows smaller engines to work more efficiently, and allows brawnier engines to be even brawnier than ever.)

Larger turbo chargers, obviously, are generally better at the job than smaller ones. But the larger they are, the more rotational momentum they have and take longer to spool up (get up to operational RPM) to deliver that power. In a performance environment, you want that power to be available right away, not “in some vague unspecified time”. If you read older car reviews, you may read the reviewer complain of “turbo lag”, and this is what they were complaining about.

The solution of yore in racing contexts (especially rallying) was an anti-lag system: when you lay off the gas, the ignition would be delayed so that the fuel charge would get ignited when the exhaust valves were open, meaning the shockwave travels down the exhaust system. Instead of driving the piston (and generating contextually-unwanted power), some of that extra energy gets to the turbo charger instead.

The end result of that is that the turbo charger is still spinning at high speeds even in off-throttle conditions, which means when you step on the loud pedal again, the turbo charger is at maximum efficiency and you have more power on tap right away, when you most need it.

The drawbacks are that you’re basically smashing gasoline fireballs against the exhaust-facing side of the turbo charger. The turbo was meant to handle (comparatively) cooler exhaust gasses–the extra heat and force will damage the turbine blades and greatly diminish its useful life. In the context of racing where you could just replace the parts after a weekend, that was just the cost of doing business and was no big deal. In the context of some guy in a street car, it’s just expensively obnoxious behaviour.

And to add final context, somebody on Youtube (I am entirely unsurprised) has a compilation video of anti lag systems on various racing cars: LOUD Anti-Lag & Backfires SOUND!! 206 WRC, Delta HF Integrale, Clio R3, LP600 GT3 & More - YouTube

That youtube video is exactly what I’m seeing/hearing. So, there is hope that this fool will be wrecking his own car. Thanks.

Wow that is obnoxious as hell to be doing in a populated area.