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Old 07-18-2007, 06:11 PM   #1
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How to FE drive when OAT is hot?

As the summer season is in full swing, I got to wondering if there should be more of a driving method change when the Outside Air Temperature (OAT) is high?

I currently drive 55 mph on the highways. Should I reduce that to 50 mph to help offset the reduction in horsepower due to a narrower temperature difference as well as possible inevitabilities such as air conditioner use?

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Old 07-18-2007, 06:46 PM   #2
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My numbers go up considerably all on their own on hot days without me doing anything different. I've been averaging around 70mpg lately.

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Old 07-18-2007, 07:18 PM   #3
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I'd just recommend you use the best overall technique you can manage to do.
My reasoning...

Hotter air is less dense so there's less oxygen per cubic inch but the air mass meter does a decent job of measuring air mass anyway - so I wouldn't worry about that part. If you had a grossly underpowered car you might notice a difference on a really hot day. Power would be off but I don't think FE would really suffer. Less air, less fuel, less power. Oh well. You'd have other issues, like you'd be sweating buckets.

One benefit is the hotter thinner air will give less air resistance. You're pushing away less actual mass of air per unit of volume. Wheeee!

True that the overall efficiency of getting mechanical motion out of the combustion might be a bit less when really hot. But if I recall combustion temps are something like 1500 deg. or more so I don't think that 100 F OAT vs. say 70 F OAT would make much of a difference.

That's why I'd recommend you use the best overall technique you can manage to do. Just go for it.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 07-19-2007, 02:35 PM   #4
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Extra work, but worth it.

During seasonal changes, I've made it a habit to route Intake Air either from the outside (ambient) or inside the engine bay for cold outside temps. It requires some wrenching between seasons, but if IATs get too high or low, FE tanks.

I had an elaborate plan to switch incoming air on the fly, but it's still in the "planning stages"

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