Delphi Flat Response Knock Sensors, $9.95, Ideal For Engine Experimenters

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The Delphi Flat Response Knock Sensor is a self-generating piezoelectric sensor. Commonly mounted on an engine block or cylinder head, it produces an output voltage in proportion to the engine vibrations caused by knock. When knock is present on the sensor signal, the engine control module (ECM) controls knock by retarding spark timing, or diesel injection timing. Benefits

Self-generating piezoelectric design requires no power to the sensor.
Flat frequency response allows the same sensor to be used on different engines and the sensor responds to knock frequencies higher than the primary frequency.
Ground-isolated design with two-wire shielded harness helps protect the sensor signal from electro-magnetic interference (EMI).
Large mounting hole accommodates mounting bolts up to 10mm in diameter.

Optional interface can be found here:
http://focus.ti.com/docs/prod/folders/print/tpic8101.html#inventory

The knock sensors are listed on Ebay with item number 260092197206, there are 68 of them listed on a dutch auction, so probably will not exceed the opening bid of $9.95. Should be a pretty good buy for that price. I have been told that these are ideal for engine experimenting.