1978 Polaris RXL Service Manual, Page 29Get this manual
COMBUSTI ONNORMAL and ABNORMAL (Con't.j )
Detonation occurs when th an ti-knock value of the fuel does not meet the engi ne require me ntsA portion of the airfuel cha rge begins to bu spon ta neously from incr eased heat and pressures just after ignition The two flame fro nts meet and th resulting "explosion" (as illustrated) ap plies extreme hammering pressures on the piston and other engine partsPiston dama ge occurs from the pounding pressures of severe detonation, and the increased heat factor can cause preignition Preignition is just what the term impliesAs illustrated, it is th ignition of the airfuel ch arge prior to the timed spark Any hot spot withi th co mbustion chamber can cause
preigni tion
Detonatio nd preignition are usually the resu lt of one or combinatio of the following condition sDETONATION:
Igniti on timing advanced too farFuel octane rating too lowLeanmixtu resPoor ca rbu ret ion and or leaks in ma nifo lds-cra nkcase andlor intake4Compression ratio increase due to co mbust ion chamber deposi ts or en gine modificati on 5Excessive intake manifold temperatu res6Lugging the engineThe piston in igure A, on Page 11, has been damaged by detonationBecause of the ha mmering pressures of detonation, piston damage usually appears as fractu res on or th rough he crownor in the skirt and wrist pin areas