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Saturday, April 16, 2005

Buy Regular, Save Money (Octane)

Gas is expensive right now, and where I live, the difference between regular and premium is about $0.20/gal. So to save money, I put regular gas in my car (Volvo, naturally aspirated). The owner's manual says to run premium. You can do the same thing, to save money.*

The main difference between "premium" and "regular" gasoline is the octane rating. The octane rating number is a measure of how resistant to premature detonation (knock) the fuel is. Some premium fuels may have additional detergents and other additives. For a more detailed article on gasoline chemistry, see Wikipedia.

Knock occurs when the air/fuel mixture in the cylinder detonates due to heat and pressure, before the spark ignites the mixture, or ahead of the flame front after a spark ignition. The result is faster than desired combustion, and higher than normal cylinder pressure, which can damage the engine. Knock sounds from the drivers seat like a metallic rattling or pinging.

High performance engines typically run higher compression ratios, or forced induction, since more air and fuel per L means more HP/L. The higher the compression ratio, the higher peak pressure and temperature the air/fuel mixture will reach during the compression stroke, and the more knock resistant the mixture must be.

Modern cars have an accelerometer (or two) mounted on the engine block, to detect knock. This accelerometer measures high frequency (5-20kHz) vibrations of the engine block. On a V8 engine, you may have one knock sensor on each engine bank. As your car runs, the engine controller (ECU) constantly monitors the signal from the knock sensor, and uses it to adjust the timing of the ignition spark. Under normal conditions, the ECU will advance the spark timing, moving it later in the compression cycle, until it senses the very early stages of knock ("borderline spark"), minus a safety margin. This maximizes the power output.

If the ECU senses knock, it attempts to prevent it by retarding the timing of spark, moving the start of ignition earlier in the compression cycle. This reduces the peak pressure in the cylinder, reducing the likelihood of knock. It also reduces the power output of the engine, because less pressure means less torque.

This is what will happen if you run regular gas instead of premium. You will pay for this with slightly worse fuel economy, as you might loose 5% of your power, but you may not even notice the difference. Labeled fuel economy is calculated from controlled drive cycles in a lab, and few of us drive consistently enough to measure small differences in fuel economy.

* To be safe, I would not run regular gas in a turbocharged or supercharged engine. It does not take much knock at high load and high RPMs to ruin your day. If you do hear knock, slow down your engine right away (tip out), drive gently, and go back to premium gas.


Racedriven said...

If your owner's manual says to run premium gas, then you should run premium gas. I would check with the dealership or Volvo themselves. I not an expert, but I have heard something about that.

I have a question: Does all supercharges and turbochargers require premium gas or does it depend on the engine? I am looking at a Subaru Baja Turbo..

The Auto Prophet said...

Yes, in general, engines that run forced induction require premium fuel.

The Angry Engineer said...

I've seen some dyno data from high-perf motorcycles that suggest that running premium fuel on a regular-fuel calibration can actually result in less power. That was interesting.

Those of us that use large V8s with 11.7:1 compression ratios and poor rod ratios will continue to use premium :)

It's also noteworthy that excessive carbon deposits (usually caused by a combination of fuel detergents and a lack of regular trips to WOT) can increase the octane requirements of an engine as it ages.

Anonymous said...

I just found out that my girlfriend has been puting 89 octane in my car instead of the required 91 for the last 30000K(she thought that was what i ment when i said the good stuff). It is turbocharged. Think that has done any damage.

Hudaman said...

Its a 2005 SRT-4 if that makes a difference

The Auto Prophet said...

No, I doubt it.

If you want to do a quick check, pull the plugs and take a look at them. Are they clean? Do they have any evidence of damage or overheating?

The Auto Prophet said...

Your engine's knock sensor has probably retarded the spark timing enough that you didn't have any sustained knocking, however, I would run 91 in a forcefed engine.

trevor.reichman said...

being that using premium gives you about 5% better fuel efficiency, which comes out to about a 15 cents saving on a 3 dollar gallon of gas...doesn't that about justify using premium, which is about 20 cents more expensive than the lowest octane ? In other words, isn't is worth paying the adjusted 5 cents per gallon for the recommended octane ?

Topher said...

Hello. I was looking for your opinion. I'm looking at buying a Subaru baja turbo 5 speed. 2004 with 106K. When talking to the seller they acted like it would be ridiculous to put premium gas in and told me it was only supposed to have regular (she is very obviously not a car person but seems to have had it well taken care of and had routine/recommended maintenance performed---she had several things done at 100k like new gaskets and radiator flushed, ect). I know that it should have premium and have been trying to figure out how big of a deal this is. She reports a lower mpg than others I've read online (maybe because of the lower octane?). She has also lived in warm humid areas (Florida and Hawaii) since I read somewhere that humidity changes combustion (not sure if it helps or hurts though...couldn't figure it out so if you can clarify then that would be helpful). So how big of a deal is this? If the car sounds ok can I just use this info as a bargaining chip to lower the cost? Can a garage look at the engine to see if any damage has been done or will they be unable to see it in the horizontal boxer style engine? I'd be gracious to hear your opinion. I love the car but don't want the engine to blow 6 months down the line. I'd move to the next car but these are hard to find (obviously the baja didn't sell many years because it was only for a niche market, but I'm one of those niche people! Ha).

The Auto Prophet said...

@Topher, the car is probably fine. Take it to a mechanic you trust and have him check out the sparkplugs (look for badly worn tips) also could do a compression check. Driving a car with lots of knock can do lots of damage, but I am sure the subie has a knock sensor.