It's finally time to hit the dyno to tune spark. Speaking with the dyno operator (MER in Cresson, TX -- big time Miata race shop), I asked about knock detection while tuning. His answer . . . "we'll just use the existing knock sensor on the car." Uh oh.
My car is a 1990. It didn't come with a knock sensor. We can add one, of course, but you don't have to search much around here before you realize that det cans and human hearing (evolved over millions of years to dynamically detect minor sound variations) is a much better way to go. So, time to build a det can.
Now the question is, which type. There seem to be three main variations:
1. "Sound-Powered Phone" type. These are purely mechanical devices using a crushed piece of copper tube, flexible (but not too soft) air hose, and hearing protectors. If you're not handy with a soldering iron, these are a good way to go.
2. Electronic Stand-Alone type. These are electronic and use an electret microphone. Usually constructed by purchasing an off-the-shelf personal listener and then modifying it by adding wire to remote-mount the microphone in a battery clip. You listen with an ordinary headset.
3. Electronic PC type. Similar to #2 with the variation that you use your tuning laptop's built-in sound card to amplify the microphone signal. Again, an ordinary headset is used for listening.
I decided on option #3. These are the advandages as I see them:
1. As I'm tuning MS, I'm going to have my laptop on hand anyway.
2. The sound card built-in to my laptop is (I think) more high fidelity than an inexpensive personal listener.
3. If needed, I can use my laptop to record and analyze the audio.
4. I can use robust, shielded manufactured cabling.
5. This will actually cost less and be less work to build.
So, here we go:
Parts (PNs and prices are from Radio Shack -- use the vendor of your choice):
1. 1/8" (3.5mm) Stereo Plug -- PN 274-1547 -- $4.19:
2. Electret Microphone Element with Pigtail -- PN 270-0092 -- $3.99:
3. Battery Clip (2 to a package) -- PN 270-0343 -- $4.29:
Total . . . $12.37.
First, solder the microphone to the stereo plug. The white microphone wire goes to the plug tip. The red microphone wire goes to the plug's middle ring. The bare shielded microphone wire goes to the plug's base and shield. See http://www.hobby-hour.com/electronic...microphone.php
for more information on wiring a microphone plug for a PC sound card.
Next, place the microphone in the battery clip and crimp the clip handle's ears around the plug's shield pigtail. This provides electronic shielding (grounded to your engine block) to reduce unwanted noise from things like your ignition system. It also provides strain relief.
Finally, pot the heck out of it with hot glue. You're doing a couple of things here: (a) strain relief/robustness/waterproofness/etc. and (b) helping the microphone to only pickup vibrations from the clip rather than the external environment. Note that because of the hot glue and the vinyl covers on the clip handles, this assembly should only be used on the cold side of the engine.
Finished microphone/plug/clip assembly:
Assembly plugged into a high-quality 20' 1/8" shielded stereo extension cable (PN 420-2562 -- $12.99):
Test run. Clipped to the engine mount bolt that secures the starter bracket. Note starter (left), oil filter (upper right) and intake manifold brace (far right).
The sound quality from this assembly seemed good on the initial test (engine started and idled with some revving). I could clearly hear the hydraulic lifters operating the valves. There was little, if any, hiss or external noise. Volume was good and not distorted. This should work well. I'll let you know after the dyno session on Monday.