By Ray T. Bohacz:
Numbers have meaning, and it is no different when it comes to octane. But how do those values relate to your engine and especially now that it is supercharged?
The definition of octane
Octane can be likened to a drag slick. A sticky tire does not allow the engine to make more power but instead facilitates getting the power to the ground.
The same can be said about the octane rating of gasoline; it does not represent the fuel’s ability to produce power but instead allows the engine to reach its full potential.
Octane is a fuel’s ability to resist combustion through pressure and heat and wait for the arcing of the spark plug. The higher the value, the more pressure and heat it can withstand before auto-combustion occurs.
A fuel’s energy content is rated in Btu (British thermal units) and has nothing to do with octane, though the process to increase the gasoline’s octane may lower the energy density.
A lower energy content will not limit the engine’s power, only the amount of fuel it consumes.
The goal is for the air/fuel mixture to wait for the arcing of the spark plug to ignite.
At that time, contrary to what some believe, the mixture does not explode but burns. The flame begins at the spark plug electrode and travels across the bore.
A reaction zone is established in the bore. This is where the heat from the burned mixture travels to the not yet ignited fuel. This is where the MON (Motor Octane Number) rating of the fuel is critical.
The expansion of the flame in the cylinder transfers pressure to the piston’s crown and pushes it down in the bore. Since the piston via the connecting rod is attached to the crankshaft, power is created.
This is known as chemical-to-mechanical energy conversion and allows an engine to run.
Normal combustion is an event that is initiated by the spark plug. The fuel needs a sufficient level of octane to resist auto-combustion through pressure and heat.
When the fuel has a sufficient amount of octane, it is identified as a normal combustion event.
If there is a “normal” event, then there must be an abnormal one too.
This is where octane comes into play.
A low octane fuel can be likened to a racer at the drag strip that red lights. Combustion occurred before the arcing of the spark plug.
When an engine pings or knocks, it is experiencing an abnormal combustion event.
When things go wrong
Abnormal combustion should not be confused with advancing the timing though that can cause it to happen.
Timing advance is measured against the piston’s location in the bore as referenced from the crankshaft’s position in rotational degrees.
For a quick review.
Timing at top dead center (TDC) means that the spark is initiated with the piston at the top of the bore. It would be zero degrees.
The term “After” TDC describes the flame starting when the piston is already heading downward; conversely, “Before” TDC has the flame start before the piston is at TDC.
Let us apply some numbers.
Ignition timing set a 10 degrees BTDC identifies the spark is created when the crankshaft’s rotation is ten degrees before the piston will be at TDC.
ATDC values are the opposite. The piston has traveled down from the top of the bore.
An engine needs to have the flame begin BTDC due to the piston moving faster than the flame propagates across the bore.
A sufficient level of octane allows the flame to expand across the bore and push against the piston.
If it were not given a head start, peak cylinder pressure from the expansion of the flame would occur too late.
There would not be enough stroke left for efficient energy conversion.
The desire is to have the flame expand for as much of the piston’s stroke in the bore as possible.
A general rule is that peak cylinder pressure from combustion (NOT ignition timing) should occur in the bore when the piston is around six to twelve degrees of crankshaft rotation past TDC.
When abnormal combustion occurs, multiple flames are expanding across the bore, fighting one another.
An abnormal combustion event has the ability to destroy the ringland area of a piston.
They are called rogue flames and create a shock wave that transfers into the piston and the connecting rod which causes it to rattle.
We hear that as engine ping.
It is the sound of the piston, rod, and connecting rod bearing vibrating.
In addition, a high temperature and pressure spike is formed, since instead of the flame burning, it is for the most part, exploding.
This can burn a ring land from the piston and bend a connecting rod.
You must recognize that the rogue flame resists the piston as it is trying to travel to TDC since it happened too soon.
Once the ionization at the spark plug occurs, the normal combustion flame then collides with the rogue flame, often blowing each other out.
If the abnormal event occurs as the piston is traveling toward TDC it is identified as pre-ignition. When rogue combustion occurs after the arcing of the spark plug it is detonation.
All of this is due to the gasoline’s inability to wait for the arcing of the spark plug and is rooted in the octane rating.
In addition, to those that need to pass a smog test on a dyno roller, abnormal combustion raises an emission called NOx (oxides of nitrogen).
An engine requires just enough octane to allow for a normal combustion event under all operating conditions.
An engine going down the highway requires less octane than one making a full-throttle pass at the drag strip.
You need to supply the octane for the most adverse conditions you will potentially expose the engine to.
Any octane above that does not hurt anything but offers no benefit.
The Cooperative Fuel Research (CFR) engine is the industry standard to determine a fuel’s octane rating.
Octane in gasoline results from the refining process, the crude oil employed, and the additives blended into the final product. There is no set recipe.
Some gasoline blends use octane-increasing chemical components that decrease some of the energy content and slow down the flame speed in the bore.
For this reason, many racers and enthusiasts tune their engines for one brand of fuel so that the air/fuel ratio and ignition timing curve can be optimized.
When a supercharger is added to any engine, it has an effect on the octane requirement.
Remember, the definition of octane is the fuel’s ability to resist auto-combustion from pressure or heat.
A supercharger raises the amount of air fed into the cylinder, volumetric efficiency (VE). The tighter the air is packed together; the friction of the molecules will raise the temperature and the pressure in the bore.
This is where the horsepower comes from.
For this reason, an engine’s appetite for octane will be increased under boost.
Do not look at it as unfavorable since you need to spend a little more at the pump.
That would be like complaining that your car no longer hooks up on street tires, and you need to buy drag slicks!
I call that a good problem!
For a more in-depth look at octane, other engine factors that impact it, and why some engines need more and others less, please listen to Episode #1 of the new “Beyond the Boost” podcast brought to you by TorqStorm Superchargers.