Thursday, October 29, 2009

Its got that Octane stuff! The myth of Super Unleaded

Last night in Reno I stopped by a "VP racing" gas station. And behold they had 100 octane at the pump for $7.59 a gallon. I did not know they sold racing gas at the local pumps. So for all you high boost and high compression ratio folks: eat your heart out.This is a good time to talk a little about octane. It is very misleading that higher octane gas is better for your car. This is a myth perpetuated by the fact that high octane gas is labeled as "Super". Why run regular when you can run super. Higher octane gas actually has less energy per mass then lower octane gas. The reason it is, is because octane is needed is to delay combustion, and absorb energy.

In an engines combustion chamber, or cylinder, a volatile mix of air and gas is introduced. This mixture is then compressed with the piston. As the piston compresses the gas mixture, it is adding mechanical work energy. If the engine is built as a high compression ratio (the amount of space in the cylinder when the piston is at the bottom of the stroke vs the top), say 10.5 to 1 and you are running low octane at sea level, the piston can introduce enough energy to the mixture to combust it prior to the piston reaching the top of the stroke, and being lit off by a spark from the spark plug when the engine is ready. This is actually the basic theory behind the diesel engine, with its super high compression ratios of 20+ to 1. Instead the piston is still moving up and has not reach top dead center when the mixture goes boom and wants to push the piston back down. This is very bad and usually blows hole in the top of pistons. I believe this is called pre detonation, and it is the worst thing that can happen in a not enough octane situation. Lesser degrees of this are known as detonation, and pining. Octane is added to allow the gas to accept more energy and not pre detonate, and only fire off when at the correct time by the added energy from the spark.

So if you do not have a turbo/super charged car, or one with relatively low compression ratio (8 to 1), getting high octane gas is a waste of money. At higher elevations lower octane is required as well, due to lack of oxygen making for a less volatile gas mixture. For example my 10.5 to 1 compression ratio car runs fine on 87 at 6000 feet. "What about the additives they put in the gas that are only available with Super?" one might ask, "aren't those good for my car?" My car runs on gas not additives. If I want fuel injector cleaner (which is what Techtron is) , I buy it at the store and add it to my tank at suggested regular intervals.

Based on the above mentioned fact, I did not fill up my truck last night with 16 gallons of 100 octane gas. Cause hey, if 91 is consider SUPER...


  1. Good stuff! I guess they are teaching you a thing or tow in traffic enginering. If nothing else it pays to be board enough to write up some sweet articles. So answer this. If my Tundra has a knock sensor why do I need to fill with super to tow?

  2. Traffic engineering has nothing to do with the internal working of cars. This is all good ol Cal Poly stuff, shade tree wrenching, and internet reading.
    As far as your Tundra goes, I assume that your Knock Sensor is throwing a code. Knock sensors are simple accustic sensors. Tapping on them with a hammer can also make them trip. KS's throw two codes. Knock detected and sensor not working. I have had to replace sensors before, but have never had an actual knock detected. If you are indeed knocking this could be form many reasons. Engine running hotter when under heavy load can add in Premature Detonation. Carbon build up on the spark plug tips can glow and cuase PD. Timing being off. But the main cause of PD is a lean mixture. This can be caused by a slew of reasons. Air leak in the intake downstream of the air mass sensor allowing extra unmetered air. Fuel system not deleivering: clogged injectors, low fuel preassure, clogged filtered...