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.
Over the years we've covered two audio-based detonation sensors. In November
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.
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
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.
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.
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
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
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
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.