<div class="section abstract"><div class="htmlview paragraph">THUD—a noise due to high rates of pressure rise in the absence of surface ignition—is the subject of the investigations described in this paper. The tests were performed on 12/1 compression ratio engines.</div><div class="htmlview paragraph">The authors found that thud could be obtained both in engines with clean combustion chambers and those with deposits. Thud is most likely to occur in 12/1 compression ratio engines at least — at wide-open throttle, high speed, and high spark advance.</div></div>