Signal to Noise: Octane and Cheap Gas

- How Things Work / DIY Section -

Mercedes-Benz | Porsche | Motorcycles & Miscellanea | How Things Work (DIY) Section

Penny wise

Question: Is it safe to use 87 octane fuel, even though the owner's manual states 91 octane is the minimum requirement?

Answer: If you have to ask, maybe a car with different requirements . . . . .

Modern engines have knock sensors. When the ECU determines the engine is knocking, it retards the timing. Engines with retarded timing produce lower fuel economy numbers than engines with the factory recommended timing specs.

Lets do the math. Assuming regular gas is $3.30 a gallon, premium gas is $3.50 a gallon, and the car in question gets 25 miles per gallon. This means, premium fuel costs 14 cents per mile. If we lose 2 miles per gallon due to the timing adjustment for substandard gas, regular costs 14.35 cents per mile. There is no savings.

Hold on though, using regular gas can also trigger what's known in financial circles as a "balloon payment". For those unfamiliar with the term, a balloon payment is a large, lump sum payment made either at specific intervals, or more commonly, at the end of a loan. Balloon payments are most commonly found in mortgages, but may be attached to auto and personal loans as well.

In this case, the loan is the money someone thought they were saving at the gas pump. The balloon payment is made in the form of a blown head gasket, a valve job, or, in truly unfortunate circumstances, a full on engine rebuild.

"How?"

As stated earlier, modern engines have knock sensors. A knock or ping occurs, and the knock sensor sends a signal to the ECU, which retards the timing. However, the timing change isn't permanent. The ECU incrementally allows the timing to come back to the factory setting. Since the car is still running 87 octane fuel, it pings again. The knock sensor hears that ping, sends the signal, and the ECU retards the timing again. Once again, the timing is incrementally returned to it's normal setting, and guess what happens? That's right, another ping. This scenario continually repeats itself. The problem here is - the engine is pinging.

What exactly is pinging?

When the fuel is ignited with a spark, a flame front and a pressure wave start moving across the face of the piston. As fuel and air burn, heat and cylinder pressures increase. This causes the remaining unburned air/fuel mixture to become unstable. If the octane level is low enough, the remaining unburned fuel can auto ignite. Auto ignition is detonation. Auto igniting is when the air and fuel light off BEFORE the first flame front reaches it. This creates a new flame front. This second flame front has it's own pressure wave, which is separate from the first one. When the two pressure waves collide, there is a very high, short duration pressure spike. That pressure spike resonates throughout the engine, and it's audible. Pinging is the sound when detonation occurs. Rather than a steady, controlled push against the piston and the combustion chamber, the pressure spike results in hammered pistons, valve seats, and valves.

In case you think "hammered" is too strong a term, think about what you're hearing. It's a noise that originated in the engine block. A noise that's strong enough to pass through the metal engine block, through the engine compartment, and into the passenger compartment. Sometimes it can be heard over the sound of the radio.

Detonation is not necessarily destructive. Many engines run for years with light to moderate levels of detonation. Some engines can even sustain long periods of heavy detonation. That doesn't seem to be the case with high performance engines though. High performance engines are designed with less room for error. When a high performance engine experiences detonation, things go wrong in a hurry.

I know of three types of failures caused by detonation:

Mechanical damage such as blown head gaskets, broken sparkplugs, chipped valves, and breaking the ring lands on pistons
Abrasion. This usually appears as pitting of the piston crown, head and valves. An experienced mechanic who tears down an engine will recognize the Swiss cheese/sandblasted appearance of a piston as detonation.
Overheating, but not always the type that appears on the temperature gauge The piston skirts can get scuffed due to excess heat .

Some people will claim the octane requirement can be lowered by retarding the timing. That's a rather crude approach. Octane requirements are a function of engine design. Compression ratio, combustion chamber design, combustion chamber temperature, ignition timing, and engine load are all factors in the octane requirement of an engine.

So, "no". I wouldn't recommend anyone use 87 octane fuel in a car that states to use premium.

Home | Mercedes-Benz | Porsche | Motorcycles & Miscellanea | How Things Work (DIY) Section