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I'm recording a couple of cars for a film, and they need to sound like theyre on their last legs.

I don't actually know any mechanics, so does anyone know if I'll damage anything by .. pulling off a lead to one of the spark plugs for example? Or adjusting the timing?

Anyone else done something similar?

And is there a way to force a car to backfire?

Cheers for any suggestions!

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4 Answers 4

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I asked a mechanic, you can disconnect the power to one of the spark plugs to make the engine run with one cylinder less without risk of breaking the engine, and it will produce a very imbalanced sound indeed. Though running for thousands of miles like this is not such a good idea.

You can also disconnect / tamper with the oxygen and fuel sensors to screw up the mixture and make it run too rich or too lean.

You could hold metal plates or other stuff against the running engine or exhaust to create extra rattling.

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Thanks for that –  James Hayday May 16 '11 at 22:04
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I don't know if I'd go screwing around with unplugging/unscrewing too much of a big mass of running metal and gasoline without some proper supervision, nor do you want to pay for having to repair the thing if you make any big slip-ups.

That said, some amateur ideas:

  • Why not pop in to a local mechanic and ask what you can do to get the machine sounding a bit grimier at the front desk? If you tell them what it's for, they might be intrigued and have some great ideas for you.

  • Taking the muffler off would probably help to lend a little more of a raw, beaten-up sound to the vehicle.

  • Maybe also try poking your mic configuration around until you find one or two spots that rattles (if they're there), then make sure you bring that layer up in the pre/final mix.

Interested to see what everyone says in here! I'm editing/premixing a big chase scene right now and am becoming super interested in the mechanics of really nasty-sounding cars.

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Like Luca pointed out mic configuration and placement is key. Point around and look for rattles or elements you may want to layer in and use.

It could be great if you can try and find a car that you can abuse a little to produce some sounds.

Maybe loosen a heat shield covering the exhaust under the engine or add a metal plate to induce a rattle.

Pull one of the ignition cables going to one of the spark plugs so that one cylinder does not fire. This will produce a poor idle and engine performance.

Putting holes in the exhaust system and close micing the exhaust can add a layer too

Breaking off pieces of the radiator fan and leaving them hanging will make a horrible sound as well...

Depending on what the image calls for and how far you are able to go with it.

For a comedic engine effect, I often like to use a higher pitched and smaller, struggling engine sound to give off the impression the motor is trying much harder that it should be.

In one film, a character was riding a motor powered bar stool and we wanted to come across this way, so we recorded a small 49cc pocket bike with a heavy driver to give the impression.

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To force the car to backfire, there are several ways and you can google that. None of them will be very nice to the engines lifespan though. Advancing the timing and reving the car very high before dropping the cluth and pushing the engine hard should do this depending on the car in question. If it has a carb you carb you can adjust the air fuel mixture screw to make it run a little rich then drive it a a bit and kill the ignition and wait, bang! turn it back on bang! –  Matt R. Sherman May 16 '11 at 3:31
    
Nice backfire tips, Cheers Matt! –  James Hayday May 16 '11 at 5:53
    
These are great, bookmarked for later! –  Luca Fusi May 16 '11 at 6:09
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What about going to a scrapyard and record one or two of the cars there?

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