Octane ratings and what they mean:

Every once in awhile a member inquires about what other members are using in terms of gasoline. Do you guys use regular or premium? What are pros and cons of running one over the other? What happens to the car, will it blow up? But the manual says…

To put it straight with the G35/G37, it may really depend on your very own driving style to determine which gas you should use. We will now jump into the world of gasoline to discover the answers to these questions.

I believe that, after observing many of these threads over the last couple of years, it is safe to say that the majority of people on this forum use premium. I personally use premium as well for a few reasons. First, because the manual says so and secondly, because of my driving habits, but that is not to say that I am a rabid driver. There are many on this forum that seem to drive much more aggressive than I do.

So, what are the pros and cons to premium versus regular? To get into this, a little research is needed to understand why there are regular, mid-grade, and premium grade fuels, which are given the respective names to easily convey to the consumer the octane grade of the fuels. Many people use premium gasoline in the belief that it's better for engines than regular. They think that doing so gives them more power and their engine will run better. The only truth behind that train of thought is, “It depends.”

Octane grades don't represent a “good, better, best” choice; they simply measure the resistance of fuel to knocking or pinging, a condition in which gasoline burns uncontrollably in the engine's combustion chambers. Knocking and pinging can damage an engine. While high-octane formulations resist knocking better than lower octanes, most engines are designed to take regular gas. Engines requiring premium gas are typically the more powerful ones found in sports and luxury vehicles. Those engines use a very high compression ratio, making them more vulnerable to knocking, so recommended fuels have octane ratings of 91 or higher. Using premium gas in an engine designed to run on regular doesn't improve performance.[1]

So, the compression ratio of your engine should determine the octane level of the gasoline that you should use. However, this is only strictly true when your engine doesn’t have an engine management system. The ECU, the Engine Control Unit, monitors all aspects of the engine, including the burning of fuel. If it detects engine knock, it will retard the timing of ignition relative to the position of the piston in order to promote the optimal burn of fuel. When an engine experiences knocking, the fuel is burning uncontrollably, meaning it is self-combusting and not burning. The temperature and pressure build up too quickly in the combustion chamber and before the piston can reach the top of its travel, the mixture explodes. This explosion tries to counteract the advancing piston and puts an enormous amount of stress on the piston, the cylinder walls and the connecting rod.[2] With an engine management system, the computer can change the timing in order to prevent self-combustion.

So you know that a fuel-air mixture, under the right conditions, can spontaneously combust. In order to control this property, all gasolines have chemicals mixed in with them to control how quickly the fuel burns. This is known as the octane rating of the fuel. The higher the rating, the slower and more controlled the fuel burns. Here are a couple of charts that show octane ratings relative to compression ratios and how octane ratings change with elevation.

Compression ratio = Octane 5:1 =72 6:1 =81 7:1=87 8:1=92 9:1=96 10:1=100 11:1=104 12:1=108



The table of compression ratios and octane levels is based on compression ratios with no engine management system. With an engine management system, the octane levels drop 5-7 points. [3]

Onto the pros and cons of using premium or regular with your G.

The pros of using premium are that you don’t have to worry about engine knock, it says so in your manual, and you’ll have no issues with warranty claims if need be. If you drive aggressively all the time or even just on occasion when you need to let loose and have the engine in high RPM's, using premium would be the best choice. Another pro is that you maximize the engine’s potential to make maximum power and potentially maximize engine efficiency, or gas mileage. The cons are that it is more expensive than regular and may be unnecessary if you don’t drive aggressively and have it high up in the RPM's.

The pros of using regular are that it is cheaper than premium and is usable. You can use regular if you don’t drive aggressively and/or have the engine under high loads and RPM's. If you drive like a chauffeur who drives politely and doesn’t use all the power available, you can just allow the ECU to retard the timing to allow the use of regular. The cons to using regular are that you lose out on the potential to make maximum power and the possibility of a loss in engine efficiency. Some here may report no loss in efficiency, but the possibility is always there as you are changing the timing of the engine and therefore the optimal performance level. Sometimes the difference in efficiency isn’t noticeable and sometimes it can be quite large.

Using regular won’t necessarily cause your engine to blow up. If you use regular and have the engine under high loads such as quick acceleration, you may damage your engine over time. So if you decide that you want to utilize all of your G, go with the premium. If not, bump down to the mid-grade or regular.

1. Consumer Reports2,3. The Car Bibles

Written by: Eric Smock (smockers83)