Regular or Premium: would this ruin it?

[The_Professional]

My dad just bought a car and he decided to use a premium gasoline instead. The car uses regular at the moment, if premium is mixed into it would it somehow ruin the engine? long term wise is this a bad thing?

[Cliff_J]

No. The cars with EFI (electronic fuel injection) have electronic computer controlled ignition systems as well and monitor if something like detonation, which premium helps prevent, occurs and adjusts the ignition to match. Most cars will show no benefit to running premium but some will make more power (because the computer can run a more aggressive program) and they are usually labeled as such (Corvette, Porsche, etc).

Most cars are designed to run on regular. Car & Driver tested a couple cars like an Accord and M3 on a dyno. The Accord actually lost a tiny bit of power on premium (but within margin of error so insignificant) and the M3 only showed a tiny gain but had other issues running on a dyno because of its programming.

On older cars, this is different because of different designs of engines and materials. If it were a late 60's muscle car its premium and lead additives to keep that thing happy.

Cliff

[hitssquad]

Professional,

You might want to point this webpage (http://www.howstuffworks.com/question90.htm) out to your dad:

What does octane mean?


If you've read How Car Engines Work (http://auto.howstuffworks.com/engine.htm), you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.


Sport Compact Car had a better article on octane a few years ago, but I can't find it at the moment. Helpfully, it pointed out that most articles on octane claim that the major problem is pre-ignition (igniting before the spark plug fires, as the above article from HowStuffWorks mentions) when in fact pre-ignition is responsible for only a minority of the bad ignition events in an engine running on too-low octane. IIRC the article in SCC said a majority of the bad iginition events occur after the spark plug fires. There are supposedly two types of post-spark events, but this article (http://www.superchargersonline.com/content.asp?ID=104) mentions only one, detonation:


What is detonation / knock?

Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the center of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a controlled burn. Detonation occurs when air and fuel that is ahead of the flame front ignites before the flame front arrives because it becomes overheated. Under these conditions, the combustion becomes uncontrolled and sporadic and often produces a pinging noise, or a "knock" noise when the conditions become worse.

So far, detonation sounds cool... why is it bad?

Detonation is definitely not cool. Detonation causes sudden pressure changes in the cylinder, and extreme temperature spikes that can be very damaging on engine pistons, rings, rods, gaskets, bearings, and even the cylinder heads. Even the best engine components cannot withstand severe detonation for more than a few seconds at a time. More severe detonation obviously leads to more severe forms of engine damage. If there is enough heat and pressure in the combustion chamber, detonation can begin to occur before the spark plug even fires, which would normally initiate the combustion. Under these circumstances, known as "pre-ignition", the piston may be travelling up towards a wave of compressed, exploding gas. These are the worst kinds of detonation conditions, and can bend con-rods and destroy pistons.

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[The_Professional]

This is very interesting. Thanks Cliff_J and hitssquad