What causes exhaust snap, crackle and pop?

Geoff Fear, Hyundai’s official N tech explains to the CarAdvice podcast panel
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You’ve probably heard it before: that pop and crackle in the exhaust note of a performance car. A number of manufacturers produce cars with bi-modal exhausts allowing the driver to select what kind of noise they’d like their car to produce, and often the louder, sportier noise includes an armada of racy cracks on the overrun.

Speaking on the CarAdvice podcast, Hyundai’s official N technician from N Performance Australia, Geoff Fear, explained how the exhaust crackle was created for the Hyundai i30 N.

“What we’ve done with the N is played with the cam timing, ignition timing, and the volume of fuel. So normally when you get off the throttle, you turn off the fuel and revs come down,” he told the podcast panel.

“So what we’ve done is tuned it to give a little extra fuel at that point, advance the exhaust cam timing so that the flame actually comes out of the exhaust, pops out past the valves, and retard the ignition timing to start the flame later so what it actually does is create a jet in the exhaust manifold.”

Have a listen to the exhaust in question above.

While (spoiler alert) the pop and crackles don’t occur naturally, Fear said “it’s a little tool we use to engage customers. It gives you goose bumps; you just want to hear it again.”

However, in development, Hyundai made efforts to ensure that the noise wasn’t too over-the-top.

“They actually open up for a pretty set time, which is three, occasionally four pops,” Fear said.

“To allow for customers we can’t control... that is, customers that might not have any concern about upsetting people, you need to back it off a little bit, so you don’t become something that is annoying to somebody.”

Fear also addressed rumours on some Hyundai N forums that the pops and crackles were in fact anti-lag from the turbo.

“It is actually a mild form of anti-lag,” he said.

“Just to give you a bit of an idea on what it actually is; anti lag is a system they use on race and rally cars to keep turbos on boost, so you can have the engine boosting while virtually at idle about to take off and in between gears.

“So in anti-lag what you do is open up the throttle a small percentage.... retard the ignition timing quite a lot, maybe 20 degrees retarded from where you’d normally be, and give it a lot more fuel,” Fear explained, “and you actually create a jet that propels the turbo and creates boost.

“So this is a form of [anti-lag], but it's an extremely mild form. That way its not actually done for the boost side of it, because its detrimental to turbo life, but its done for the noise,” he concluded.