$20 D-I-Y Detonation Detection Unit

While running the Vortech supercharger, I always thought that I had a fairly safe tune - relatively rich and relatively low timing and was safe in tuning the car. I did heard detonation a few times while tuning, but no shattered pistons and the overall performance of the car never changed to any noticable degree. It ran great even after pulling the blower off (310 rwhp). Well, after swapping in my new big bore engine and the original engine being torn down, I found that running well does not mean that there isn't damage. Below is a picture of one of the pistons from the stock engine. The broken section of ring land was still captured between the two copmpression rings when the engine was torn down.

As my new engine runs fairly high compression (11:1) and responds well to increased timing, I thought that an approach to detecting detonation with a high degree of resolution would be a good thing. Below is information on building a detonation detection unit that can be built for about $20-$35. I have to give www.autospeed.com credit as I came across this concept in an article that they published. At one end of an extension cable is a clamp having a microphone mounted on it and at the other end of the cable is a sound amplifier. To listen for detonation, the clamp is attached to the block and the sound amplifier outputs engine noise through either headphones or the car stereo (via a cassette adapter). This unit allows for detonation to be identified with far greater resolution than listening in an un-aided manner.

All of the parts can be purchased at Radio Shack. The primary component is a sound amplifier (p/n 63-1121), which costs $10. It includes a set of headphones. I opted for a cassette adapter which added about $15 to the cost. The cassette adapter allows detonation to be listened for via the car stereo, which is preferred as it is not recommended to wear headphones while driving. A two-conductor cable about 6-feet long is required (18 gauge wire is suitable).

Behind the battery cover of the sound amplifier is a screw that secures the two halves of the case together. After removing the screw, the two halves of the case are separated, allowing access to the microphone within the case.

With the case separated, the oe microphone wires are unsoldered and replaced by two wires from the extension cable. It is important to maintain polarity between the connections at the printed circuit board (PCB) and the connections at the microphone.

The extension cable is attached (soldered) to the microphone (maintain proper polarity between the wires and microphone connections). Route the cable through the clamp prior to soldering the microphone to the cable.

An appropriately sized drill bit is used to enlarge the microphone port to accept the extension cable.

With the extension wire positioned in the enlarged microphone port, the PCB is re-mounted within the case.

The microphone is epoxied FACE-DOWN (wires-up) to the clamp. Use a small dab of epoxy to cover the exposed solder connections on the back of the microphone. If a large enough clamp is used, The microphone can be mounted near the teeth for providing better listening capability. In this case, mounting the microphone near the rear-most teeth of the clamp serves to protect the microphone from accidental damage during clamping. If a relatively small size clamp (as sold by Radio Shack) is used, due to the size of the microphone, the microphone must be mounted on the inboard face of the handle rather than near the teeth of the clamp.

Together with a wideband O2 meter (FJO unit shown) and AutoTap unit (not shown), this detonation detection unit goes a long way to allowing a car to be tuned in a very safe manner. A safe tune is up to the tuner ;-)

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