How high, how wet, how cold, and why it matters.

By Jordan Cameron Remeljej, Owner, Modern Aircooled

This is the first of a two-part TalkingTech. This first write-up introduces the principles on how weather affects oxygen potential for your engine. In next month’s article I’ll examine three different Porsche engines and how they compensate for those conditions to make more or less power.

More unique to the automobile as a product is the prevalence of old wives tales. When it comes to our vehicles we tend to have a much stronger interest in the inner working than we do with most of our other equally utilitarian possessions. It’s this difference in interest levels that has spawned hundreds of car clubs, but far fewer dishwasher appreciation societies.

What is wonderful about this broader interest is a higher level of knowledge (or in some cases conjecture) that allows a much better discourse in discussing the different elements of automobile function. I’m sure we can all recall a particular heated discussion between friends, family, or colleagues about whether turbochargers or superchargers are better (turbos every day!), or whether pushrods have a place in the modern engine (they don’t!) for example.

Cold Weather Equals Power?

In this month’s column I’m going to weigh in on one of the more settled (as far as the engineers are concerned) topics that comes into contention with an almost seasonal regularity. Does my engine make more power when it’s cold outside? The answer, yes, yes it does, but not always! I will now try and explain why.

The internal combustion engine is commonly referred to, and understood to be an air pump. It can extract the chemical potential energy from a number of different fuels (petrol, diesel, vegetable oil, natural gas, e.g.) but the unifier in all designs is that power output is measured by how much oxygen, and how efficiently that oxygen, can pass from intake to exhaust. More oxygen allows more fuel to be burnt for a given period of time, which means more power.

Not All Air is Created Equal

Aside from pollutants, other environmental concerns etc., the air we breathe at sea level is generally only about 20% oxygen. The rest being mostly nitrogen (78%), with a couple of percentage points being argon, carbon dioxide and other gases. If we take a theoretical cubic foot of this air we can change the total amount of oxygen that can exist within that space my adjusting a number of environmental factors. These factors are: altitude, humidity, and temperature.

The Elevation Factor

Air has a mass, and is therefore subject to gravity, the weight of the air above, squishes down upon the air below causing it to compress slightly. The general rule here is we lose half a percentage point of oxygen content for every thousand feet of elevation we climb. Starting at sea level (20%) and driving ten thousand feet up a mountain we would be able to see a drop to approximately 14.3% oxygen. This means we would see 5.7% less oxygen in our cubic foot.

The Humidity Factor

Here in Houston we know all about humidity. Half an hour after a light shower we all feel the soupy air in our lungs. That water vapor in the air displaces other gases. This leads to a reduction in usable oxygen in our cubic foot of air. There are multiple standards in determining the amount of measurable moisture content in air, but the principle remains, the more water vapor that exists in our cubic foot, the less oxygen we have to work with.

The Temperature Factor

The last and most easily noticeable effect on the oxygen content available to us is temperature. It is not common for us to quickly scale or descend 10,000 feet, or drive a vehicle between 0% and 100% humidity. We do however often wake up to find a morning up to 40-50 degrees cooler than when we were driving the day before. Temperature affects the density of the air in our cubic foot.

The colder the air the less energy its molecules conserve, the slower they move, the closer spaced they can be in our container. Closer spacing allows for more molecules to exist in the same defined space, leading to more potential oxygen in our container. It is a hard and fast rule for a material to contract when cooled and expand when heated (assuming no change of state is observed, looking at your water with your magic expanding ice cubes).

Horsepower Potential

While the math of this density change is more difficult to follow, here is where we can start talking horsepower. On a typical naturally aspirated motor you can see a potential horsepower shift of 1% per every 10’ degrees observed change. Higher temperatures mean less power, lower temperatures mean more. Directly linked to the available oxygen that can be delivered to the combustion chamber per combustion event.

While these scientific principles and environmental conditions well deliver more or less horsepower on paper, in real life they exist as horsepower potential. What your engine is able to make of these conditions is what takes the statement about cooler weather making more power and shifts it into a ‘yes, but maybe not’ discussion.

Next Time

In the next segment I will extrapolate this topic to how these conditions actually affect the running of three different engine types. How these engines respond, and which, if any, bragging rights you might have as far as power on these colder winter mornings. I will discuss a carbureted engine, as those found in a 356 or early 911, a modern naturally aspirated engine with port fuel injection and a computer controlling individual ignition events such as out 996, 997, and 991.1 motors. Then finally how forced induction engines try to work around these principles but ultimately cannot defy physics, the 991.2’s for example.

A heat camera on a Porsche 997 Turbo's intake

With this camera we can see the heat of the intake surfaces and see the principles in action. This is an intake of a running Porsche 997 Turbo.