Using Knock Listening Devices Part 1: Listen To Your Motor

I’ve spent a lot of time in labs and on dynos, trying to understand the relationship between what you see in recorded data and the performance of an engine.   To me, the difference between a tuner and a calibrator is the use of your senses versus instrumentation, and the best guys use both.  The longer I’ve done this, the more I’ve learned to appreciate the importance of my sensory inputs.

Cylinder pressure graphs

Cylinder pressure at various timing values, courtesy MotoIQ

Today we’re going to talk about listening to your motor.  The biggest role this plays is in tuning to prevent detonation, but the ability to understand what you’re hearing withing the glorious symphony of a well-built engine is an important tool for mapping.  This is when hearing becomes listening.

I’m not going to spend a lot of time in this article describing the how’s and why’s of detonation (or knock, if you prefer); there are quite a few that have covered it and there’s plenty of great reading on the subject.  What I’d like to discuss are two important and all too often underused tools when tuning your engine – knock amplifiers and your ears.

But first, some history.  Back in the day, I was a lab assistant at the University of Wisconsin-Madison’s Engine Research Center.  I worked with heavily instrumented engines, and it was here that I learned about cylinder pressure measurement and detonation.  To be able to watch a scope trace of cylinder pressure versus crank position was fascinating, especially when timing got too advanced.  Later on, I started recording knock sensor outputs to my laptops, using frequency analysis tools such as FFT’s to determine the predominant frequencies at which det was occurring.  The results were instrumental in setting up more complex knock systems.

At the same time, I was also a British car mechanic, which was very much a tuner’s job.  One could sync SU carbs as well with a stethoscope as a vacuum gauge.  Occasionally the dipstick on my MGB GT would pop out of the tube – I’d know right away by this very slight but definitely out of place rattle that happened only on deceleration.  I didn’t know it at the time, but late nights in basement of the Engineering Research Center and on the backroads of Dane County were grooming me for future dyno sessions.

Today there are many tools for listening to a motor, from det cans (made from coffee cans and copper tubing) to directional microphone systems.  We’ve become partial to the Phormula Pro Tuning Solution.  It combines a high-quality audio amplifier with a good basic knock determination system that can easily be datalogged.  The system is rugged, robust and easy to use.  Installation is simple – mount the supplied knock sensor on the engine block, preferably in the stock location, then connect up to the box.  You can calculate a predominant knock frequency and use their analyzer to view knock levels on a display or output to a data system.

Phormula Knock

Knock Detection System by Phormula

But as I said, the hardware is not the end of the story.  Listening is the place to begin.

I don’t want to hear anything besides the engine.  I wear in-ear monitors with a set of pistol-range earmuffs on top of them.  Very little exhaust noise and certainly no conversation comes through.  Ah, but the sound of that engine.  Turn the ignition on; you hear fuel rushing in to pressurize the rail.  Start the motor, listen to the starter clutch disengage and wind down.  Row through the gears and into some boost and a rhythmic tick builds in the background – that’s the boost solenoid starting at its base duty cycle.  As you are doing pulls, you hear more and more in that motor – variable cam actuation, blowoff valves, an occasional light missfire, the rush of boost forming…. and then you add some timing and a new sound appears – sometimes like a hammer or sometimes crackling paper…. when you figure out its knock, you’ve just added a new tool to your arsenal, conveniently carried around somewhere between the top of your head and your chin.

Its important to learn these sounds.  Frequency-based knock detection systems are all about using the right filtering techniques to gleam the knock from background engine noise.  It just so happens that our powers of perception work in quite the same way.  Much has been written about how human beings process sensory information, and the ability to apply one’s attention to the most relevant information is vital to our survival.  The same can be said about the survival of your engine.

Thankfully, there are ways to make your engine knock that minimize any chance of damage.  This is a very useful (and dangerous if not wielded properly) tool for learning how a knocking engine sounds.  If it sounds like I’m advocating making your motor knock, its because I am.  Know the risks before you try, and tread carefully.  The reason why is that the next step in learning about detonation control is to record the sound of your motor knocking so that you can get information needed to setup a knock control system.

As I said, being good at mapping means being a good tuner and calibrator, so there’s a point where recording and analyzing data becomes important.  In part II, we will discuss methods for characterizing detonation, by learning how it sounds and recording it to determine frequencies.  We’ll also talk about knock windowing and other common knock control software setups.






About Neel

President of Apex Speed Technology
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