Compression ratio sets the level to which a piston compacts a mix of air and fuel before a spark ignites it. Make the ratio too high and the fuel ignites itself, as in a diesel engine. This leads to the formation of multiple flame fronts in the cylinder, as part of the fuel self-ignites and another part is lit by the spark. The resulting knock can ruin an engine.
Deci, compresia influenteaza usurinta aprinderii amestecului aer-combustibil. O compresie mare (ca la motoarele Diesel, care merg cu compresii de genul 18:1, fata de motoarele Otto care au 10:1 sau 9:1), duce la preaprindere si detonare.
La un motor clasic Otto nu poti sa cresti deci compresia la "cat vrei".
Links:
http://chevyhiperformance.com/techarticles/46778/
http://www.memagazine.org/backissues/june0.../otto/otto.html