Balancing Act

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Cold air intake systems and its effect on fuel economy

When I initially installed the cold air intake system by Injen about 2 years ago, I noticed a sudden change in fuel economy. I didn’t do any data collecting at the time, so I assumed it was due to the newly installed intake. I’ve recently removed the Injen system and re-installed the OEM air box and did some analysis. This entry documents my analysis and examines the myth that a cold air intake’s less restrictive air filter allows more air flow to the engine which then results in the engine burning fuel more efficiently, enabling better fuel economy as the end product.

With the Injen cold air intake system installed for a close to two years, I’ve been getting a consistent average of 300-325 miles off of a full tank of combined city and highway driving with a maximum recorded at 340 miles (since then, I’ve never been able to surpass 340 miles on a full tank of combined city/highway driving). I was able to get 475 miles off of a full tank from strictly highway driving alone and traveling at 75 miles per hour, however, during a trip to the borders of Oregon and Northern California.

I’ve recently removed the Injen system and noticed an increase in fuel economy, which prompted me to do this analysis. The analysis involves driving the same routes three times with a full tank of 91 octane gasoline until the empty fuel indicator is lit and noting the amount of mileage I’ve obtained.

Trials after trials, I’ve been consistently getting better fuel economy with the stock air box.

Stock air box fuel economy, combined city/highway driving from a full tank of gas

Trial #1: 360.6

Trial #2: 364.3

Trial #3: 369.1

The verdict

Cold air intake systems increases high end power at the cost of fuel economy.


Since more air flow is pushed into the throttle body through the cold air intake, fuel is forced to combust at a much faster rate, resulting in a burst of high end power. This faster rate of combustion translates into a decrease in fuel economy. On the contrary, a more restrictive OEM air box will not enable air to flow into the throttle body at such a fast rate. Fuel is then combusted at a slower rate and the burst of high end power seen from a cold air intake is omitted. Since fuel is combusted at a slower rate, this translates into an increase in fuel economy.

After driving the RSX with an intake on for nearly 2 years, it is rather refreshing to drive with the silence of an OEM air box for a change.

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