http://www.dynotechresearch.com/blog/archives.asp?chosenMonth=2&chosenYear=2012another write up about listening for detno on the dyno:
How can we hear detonation occurring on the dyno? Sean Ray learned about listening to engine knock several years ago at Delphi where he is employed as a calibration engineer. Hyundai engineers came from Korea to Sean’s Delphi dyno cel with a roll of ¼” ID copper tubing that they attached to the engine Sean was calibrating for them. They drilled a hole in the control room wall, bolted the copper tube to the Hyundai engine and inserted the other end into the control room, just dangling in the air. Delphi engineers could now hear every click of detonation—which makes a loud snapping noise that emanates from the copper tube in the control room—sometimes even before the engines knock sensors could notice it! Sean brought the copper-tube deto sensor concept from his Delphi engine dyno cel to DTR several years ago. It’s an incredible tuning aid, and has saved many hundreds of pistons to date! We now listen for detonation here and usually can abort any test before engine damage occurs. Three or four audible clicks and we quit the test. On two-strokes we will never hear more than about twelve clicks of detonation—the engine will have seized by then! Last season Sean and Tim Bender ran over 700 max power dyno tests on one mule 600 race engine for Hentges Racing, and lost zero pistons because Sean could hear clicks and abort. During similar max power testing in years prior to the copper tube, Tim used to bring a box of pistons and a jug of muriatic acid to clean the bores after seizing. Back then it was more wrenching and less testing. But today it’s much more efficient thanks to Sean’s Korean engineer pals!
Four-strokes can be more forgiving—each detonating compression stroke is followed by a piston-cooling intake stroke. This means that a few more intermittent clicks of deto can be heard and tolerated by the engine before really bad things happen. Four-stroke piston seizures from deto are rare. Instead, when continual deto makes spark plug ground straps red hot, preignition can occur which can create the worst damage! Then, peak pressure will climb to the moon, which can crack ring lands, stretch the head bolts/ studs and either blow the headgasket(s) or just lift the heads enough to allow combustion pressure into the coolant passages, spewing coolant out the reservoir overflow. Too often people try to rectify detonation problems not by increasing octane or more conservative tuning, but instead by using stronger head bolts/ studs, O-ringing combustion chambers, copper headgaskets, etc. Then, instead of blowing headgaskets we might see studs being pulled out of the crankcase threads, conrods bend or break and cylinder and head gasket sealing surfaces fried into junk as though attacked by a plasma torch! Tune those high boost engines! Don’t scrimp on octane!