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Detonation Detection and Recording

Using a cassette player, microphone and earphones to hear detonation.

by Julian Edgar

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At a glance...

  • Effective and cheap detonation detection during tuning
  • Engine sounds also able to be recorded
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This article was first published in 2005.

Detonation can destroy engines! That’s a problem – and it’s even more so when any mods designed to increase cylinder pressures (upping the boost, putting on an exhaust and intake) will make detonation more likely. Another reason that detonation might be a danger is if you’re running a load interceptor to adjust mixtures. Lean out the top end and the ignition timing will also advance – another potential cause of detonation. (For more on detonation, see DIY Detonation Detection - Part 1.)

The problem in detecting detonation by simply listening for it is that it's hard to hear the noise clearly from inside the car. The sound deadening that comes from thick firewalls - and wind noise that results if the windows are lowered - mean that it's often hard to even hear the engine, let alone any specific odd noises being emitted from it!

So to hear detonation, a way is needed of transferring the sound from under the bonnet to the ears of a careful listener.

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Over the years we've covered two audio-based detonation sensors. In November 1999 in DIY Detonation Detection - Part 2 we showed you how to modify a then commercially available listening device. In 2002 in DIY Detonation Detector - Mk II we covered how to build an audio detector from scratch.

So why write an article on a third approach? Well, this one requires no electronic construction, gives potentially excellent audio quality and allows you to record the sounds for more leisurely later analysis. They’re the upsides – the downside is that, depending on how you get together the bits, it can be a more expensive approach.

The Hose?
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Lots of people talk about using a copper lug bolted to the engine and a rubber hose connecting this to the ear of a listener. On paper it sounds fine – after all, voice-tubes have been used in ships for years and every time a doctor places a stethoscope on you, a similar principle is being used. However, when we tried it, we found the approach worked very poorly. The sound was muffled and the high frequencies restricted. Perhaps it would be better with a metal tube the whole way between the engine and the ear, but that’s pretty unwieldy.

What You Need

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To use this approach to listening to engine sounds, you’ll need a decent quality portable battery-powered cassette recorder. It helps if it has a level meter and an adjustable input level. You’ll also need to have bud style earphones, industrial ear muffs and a microphone that will work with the cassette player. Oh yes, and an extension cord for the mic.

Check that when the earphones and microphone are plugged into the cassette recorder, and ‘Record’ is pressed, the sounds picked-up by the microphone are amplified and can be heard clearly in the earphones.

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We used a good quality Sony tape recorder and dug out of the junk box a microphone that also proved to be a Sony. Weirdly enough, the ear buds were also Sony – from a Walkman. With this combination, the sound quality was excellent - clear, undistorted and with a good frequency response.

Engine End

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The microphone needs to be mechanically connected to the engine. We already had in place the copper lug (see ‘The Hose?’ breakout box above) and so we just cable-tied the microphone to the lug, with the sensor end of the mic in contact with the engine mount to which the lug was bolted. If possible, attach the microphone very securely to a casting or major mount bolted to the block – don’t connect it to the head, intake manifold, etc.

Using the System

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Use an assistant who can wear the bud earphones and then – over the top – the industrial ear-muffs. The latter prevent any outside sounds getting in – all that the listener can hear are the engine noises. Adjust the level input so that there’s no distortion or overload, and then go from a drive. Detonation most often occurs from peak torque upwards in revs, and is usually (but not always) most severe at full throttle. (Part throttle detonation can occur if the ECU runs timing that is too advanced at those loads.)

So what does detonation sound like? It’s hard to describe but firstly, it’s a new sound – not one that’s there at light load high revs, or on the over-run. Secondly, the sound that is heard is caused by the ringing of the block as the hammer-blow of improper combustion strikes the piston – that gives you a clue to how it will sound. It’s usually a higher-pitched ‘ting’ sound, a little like coins rattling in a coin tray. It can occur in very fast succession (ting-ting-ting-ting) or can be a transient noise that occurs, for example, ting!, only on sudden throttle applications. The big danger is when (1) it occurs at high loads and (2), it is continuous ie the knock sensor doesn’t pull back timing (or drop boost, or close the throttle, etc) enough to stop it.

Whenever detonation is heard, the driver must immediately back-off – don’t let it keep occurring!

The Guinea Pig

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The car on which the system was being used is a turbocharged Toyota Prius hybrid. The engine has a very high geometric compression ratio and with the addition of the turbo, there were some concerns about detonation. When held at full throttle up very steep hills, a noise could very faintly be heard in the cabin which might – might! – have been detonation.

However, repeated listening at a variety of loads with the audio system (and recording the sounds for later playback) showed there was no need for concern.


Cassette recorders - even good ones – are often sold at garage sales and secondhand stores for nearly nothing... and you’ll also often find a cheap microphone at the same place! With some good quality earphones, the components make an excellent, easily assembled system for listening for detonation while tuning or testing.

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