With all the inquires resulting from the more affordable ZZP turbo kits I have been flooded with inquiries from non turbo experienced Slingshotters wondering about detonation and its associated risks. I have published my thoughts here with different information added from different articles I,m familiar with. If any other turbo owners wish to add some comments or experiences , please do.
Detonation is the Mother of all evils , when running a forced induction Slingshot , She will flirt with you and may destroy you ! But only if you let it happen. We all need to recognize her and better understand her, so that we can boot her ass right out with the trash anytime she rears her ugly head !
Even when running the prescribed higher octane fuels to combat detonation it will still occur in periods of high fuel loading particularly when combined with low RPM , more commonly referred to as long hard hauls at low rpm. When running turbo charged these have to be recognized as bad driving habits, turbos love rpm, detonation loves to destroy turbo charged engines at low rpm, and do so by way of detonation, so why would you instigate detonation by running at low rpm ? It,s suicide. Pamper your 2.4 Ecotec turbocharged by revving it up, shift down, before accelerating at low rpm's .
NORMAL COMBUSTION -
During the compression stroke the spark plug fires just before TDC ( top dead center) so that a small kernel of flame can form a flame front which grows outward from the spark plug propagating across the combustion chamber . Ideally, the flame front grows out smoothly, but at an exponential rate increasing the amount of pressure within the combustion chamber (cylinder) until all the fuel is consumed. By precisely timing the beginning of the spark event before TDC we can control when peak pressure is developed, and as a result provide a nice even push on the piston throughout the power stroke. Which optimizes the conversion of cylinder pressure into crankshaft torque. Figure 1 shows typical pressure distribution within the combustion chamber where the pressure is applied to the top of the piston in a long hard smooth push starting just before TDC, maxing out at peak pressure and continuing to push steadily downward throughout the power stroke.
Going back to the spark plug firing just before TDC, as the flame front propagates outward from the spark plug the pressures build throughout the entire combustion chamber affecting the pressure and temperature not only at the flame front but also at the unburned fuel not yet reached by the flame front. These unburned fuels are typically referred to as end gases. If the temperature and pressures reach a critical point before the flame front reaches them they can auto-ignite . Unlike the relatively slow burn of the flame front these spontaneously combusting pockets of end gases (explosions) referred to as detonation(figure 2) , cause a dramatic increase of pressure within the cylinder from a combination of the normal flame front and the unwanted explosions of end gases. this detonation in turn creates greater temperatures and pressures propagating more end gases explosions resulting in a sawtooth pattern of pressure oscillations on the top of the piston, at or near TDC. With detonation the majority of the energy is converted to heat and pressure at or near TDC long before the piston , connecting rod, and crankshaft are in a position to convert that energy into torque. Contrary to the analogy of a long hard smooth push on the piston, during the power stroke, with detonation we end up with multiple sudden and sharp impact blows on the top of the piston which can result in internal damages. (figure 3) You can see where the importance of higher octane fuels in a forced induction system is critical as they can withstand higher cylinder temperatures occurring as a result of detonation.
In laymans terms pre-igniton / detonation are commonly referred to as just detonation, but it is actuality pre-ignition which complicates matters even worse and is responsible for smashing out rod bearings and destroying engines. While with detonation alone , the sudden sharp impact blows occur on the top of the piston during the power stroke , with pre-ignition the sharp impact blows driving down on the top of the piston occur well before TDC while the piston is still travelling upwards . As long as this is instantly recognized , irregardless as to whether its missed diagnosed as detonation, and the throttle is instantly let off , the result is the same , the damage can be negated before too much is done.
Pre-ignition occurs when the fuel air mixture ignites before the timed spark, well before TDC. typically because of excess fuel air mixture coming in contact with a hot surface or too low an octane rated fuel. This is different than detonation in that detonation is the spontaneous combustion of the end gases. With pre-ignition once the fuel has been ignited it will propagate a flame front similar to that of the normal combustion process well before TDC. raising temperatures and pressures within the cylinder which inevitably will cause detonation and because these pressure increases occur even closer to TDC than detonation alone ,(figure 4 ) pre-ignition has the potential to do even damage to the engine. Detonation alone can cause increased temperatures and pressures meaning more hot spots within the combustion chamber which can lead to more pre-ignition , which in turn leads to more detonation and so on and so on . Again the highest octane rating fuel available is critical to be able to withstand higher temperatures without pre-igniting.
RECONIZING DETONATION AND PRE-IGNITION
The results of detonation , pre-ignition are easily recognized and terminated . When running forced induction ,If you are ever cruising in too high a gear at a low rpm, say 5th gear at 1900 rpm and when you accelerate even lightly without shifting down and you instantly feel shuddering and or hesitation maybe combined with spitting and backfiring, this cannot be misinterpreted as just bogging down and you simply hold your foot there till the rpm picks back up and she clears herself ! You must instantly recognize this for what it is, detonation, and back off the throttle , shift down and away you go, its as simple as that. Also at higher rpms if your messing about and running hard and too heavy a foot in and out of the throttle you can prime the cylinders for unwanted detonation .
There is a cost to forced induction ownership and one of those costs is having to know what your dealing with and driving it accordingly. with proper management of the throttle pedal and rpm the power is there, right there, anytime you call for it , just remember improper management will also call for detonation and she too will be right there to bite you in the ass , the easiest way to avoid her is to know here and recognize her, and just remember R.P.M. is your friend !