SAVE YOUR MOTOR,by understanding whats trying to kill it!

[ChrisCarsonMarine]

Detonation and preignitioned defined

I see folks refer to detonation and preignition often,and even now and then someone suggests predetonation as the reason for certain engine failures.btw,predetonation isn't a condition that has taken place...sort of like a preheated oven,it's either heated or it's not,you've either suffered detonation or you havent.
We will start with the simple one and get it out of the way,preignition.This is also slightly misnamed,it would be more correct to be named pre-planned-ignition.Preingition refers to the ignition of the compressed fuel/air in the cylinder before the spark takes place that was meant to begin the combustion process.This is usually caused by a glowing piece of carbon or spark plug tip,and can be brought on by overheat,wrong plugs,or excessive carbon buildup.In certain rare cases it can also be caused by the hot exhaust from a nearby cylinder igniting the fuel/air mix as the ports are closing on the subject cylinder,this is usually encountered on highly modified high rpm motors.enough of preignition,and onto the common real problem,detonation.
Detonation,by definition,is simply a condition in which the piston is forced to compress an already expanding (burning)fuel/air charge in the cylinder,resulting in higher temperatures/pressures than normal,and the resulting shock and damage is termed "detonation damage".This damage can be insidious and slow,in mild detonation cases,or immediate and extensive in heavy detonation cases.
Detonation damage is usually most apparent in severe cases as melted and seized pistons,in mild or low-level cases the constant bouncing of needle bearings can cause chatter marks(the chipping away of surface metal) on the crankshaft journals,as well as the wrist pins and rod bearing surfaces.Piston ring land breakage is another common result of low-level detonation.While most folks understand the severe cases,the low-level damage is often attributed ot other causes.So...
To understand detonation,we have to examine its causes,and there are plenty of them.A motor is designed and tuned to capture the force created by burning fuel/air and put it to work,turning a propeller through a series of rods,cranks,shafts and gears.In order to maximize the useful available power,we must light the fire at precisely the right to get a full burn without developing excessively high pressures.Many factors come into play,but the main ones are compression,fuel octane,timing advance,engine temperature,and operating rpm.Realize this...probably the most important things for the boat owner to understand,is the fuel/air burns at a constant speed although the motor changes speed.Because of this we have systems to advance spark timing to assure that at high rpms the engine won't outrun the burn time of the fuel/air mix.The other factors that play heavily in this equation are fuel octane and engine rpm.Rpm is simply the speed of rotation...revolutions per minute,and octane is a relative speed of burn,the higher the octane the slower and more consistent the burn,or"push".
Realize that lowering engine rpm effectively increases spark timing,as it causes the piston to get to top dead center later,allowing time for the combustion pressure to grow,setting the scene for detonation damage.
This trouble can be avoided by using fresh fuel of adequate octane,maintaining a clean fuel system,doing proper routine maintainence,and propping to achieve full recommended rpm.Low level detonation is a lot like high blood pressure,over time it breaks things down.
Hope this helps,Chris
The chapter I left out!
PCRUSSELL50 asked...What about running an oversized prop,but not at wide open,say,4000 rpm instead of 5200.The answer is...low level detonation probably.The pressures increase as the throttle is opened,pretty linear curve.
Anyone tuning one of our 2 strokes has probably noticed full timing advance has been reached at around 1/4 throttle,1500-2000 rpm.Propped correctly,you still ask why no detonation.
The last(for now) chapter explanes this...Compression.At idle,or 1/4 throttle the motor is hungry,wanting to go...full timing,but held back by the throttle plates holding back the air,and effectively lowering the compression.The rarified (partial vacuum)air in the crankcase lowers compression resulting in lower pressures,heat and power.As the throttle opens the compression ratio goes up,along with heat,pressure and resulting power.
The bottom line is prop to attain full recomended rpm,with a full load,and then run at watever rpm you like.