Atmospheric conditions and engines

Question: how does humidity affect engine power? Air that has high humidity versus dry air.

Less oxygen per given volume. So lower relative density. Less oxygen. Less power. Less chooch!

You have to lean out as the relative humidity climbs.

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There is a missing piece to your question. You need a second question before asking about power. Humidity and power are not necessarily related directly.

How does humidity effect air density?

Then you can ask how air density effects engine power.

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Is wet air denser? One would think density is good. More packed into cylinder.

Water doesn’t burn. So when you have humid air, the water molecules take the place of air molecules when filling the cylinder. Which is no bueno. Ideally you’d want cool, dense, air. Not humid, dense, air. It’s more about air quality. IE; go for a walk at sea level and go for the same distance walk at 5000ft. Which one is easier for you to breathe? Sea level has better air quality due to more air density.


Humid air is actually less dense unless it is SO humid that water droplets are actually condensing. Gaseous water is less dense than air at the same temperature, and, since water doesn’t burn, you get less O2 going into the engine because the water has displaced it.


Thanks all! Helpful.

You’re really asking about density altitude. Your true altitude, then temperature and barometric pressure are what’s most important. It’s then dew point (relative humidity?) that comes into play last.

Can anyone point to someone tuning by using RH as a decision factor in what mixture to run? Its elevation followed by temperature that rules the day.

In drag racing, we had folks with a baro altimeter and a device to measure dew point so they could dial in their next run to the hundredth (thousandth sometimes) of a second (they had a lot of prior data).

Does RH make a real difference in karting? I dunno, I’m new to karting.

That’s what set this off. My last race felt good, pace was good, but times and top speeds were off relative to dryer conditions. The weather was the dominant thing, so I was trying to understand if humid day equals less top end. My top speed into t1 was 46-47mph versus 48-52. Yet my pace relative to field was good. I wasn’t getting walked in straights at all.

The chart I posted above is my initial answer to “what difference does RH make?”. At 40C, there’s only a 5% swing on air density, so at first blush, a 5% reduction in power would drop a theoretical top speed of 50mph to 49mph. A drop of 50mph to 46.5 (average to average) would take a 20% power loss.

Looking deeper into it, lower air density also reduces drag, but that’s a linear change whereas power and top speed are cubic, so I think we can ignore that. This tell me that something else has changed, OR there is some compound effect. For example, a 5% change in O2 content might cause a 20% power drop if the engine is just hyper-sensitive to fueling. With my limited exposure (206 and C51), neither of those engines are anywhere that sensitive. I don’t know what you are running. I also don’t know the track - could it simply be that there was less rubber down on the penultimate corner, and this lead to the 2mph drop?

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Yeah. It’s a bit of a noodler.

Context: track dry but with standing water in braking zone of t1. Water inside apex 3, resulting in mud and dirt in apex of 3. Otherwise, track dry and clear.

Track is green. Rain had been heavy off an on for about 5 days. rain heavily earlier same day as well.

The standing water in t1 narrows the entry making it less fast. This means that I’ll be slower getting to 2-3 complex. Normal low speed here is 28-31 and I’m getting 26-27. My top speed entering two is 40 as opposed to 41.

Similarly, turn 5 normally has a minimum speed of 28mph. I was doing 26ish instead. The run from 5 through start finish and down straight to one is also compromised at 45 mph max as opposed to 48-50.

The water in one slows everything down the line a bit. The green track slows things down too. It didn’t seem like I had a weak kart as I would do fine in battles down the straights. Relatively, I drove strong (we won).

So, looking at my normal footage from normal day, dry races, this was slow. But it wasn’t slow relative to the rest of field. So it can’t be a crap engine. I did well so I don’t think it’s driver error. So, I’m putting it in atmospheric conditions.

Putting these two statements together doesn’t make sense unless “atmospheric conditions” is referring more to the track than how the engine is running

Yeah I meant something other than kart. The “weather” affected the track, making it green. Wet spots complicating things.

I was wondering if the generally wet air would also make things slower. I suppose the answer is yes but it’s not that significant.

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When the air density drops, the mass of the inducted charge is reduced so you get reduction in peak compression pressure too.

This is why you can run higher compression ratios at higher altitudes. While the volume has not changed, the mass has.

Yep, that’s the convenient thing about ratios. If your starting pressure drops 5%, final pressure drops 5% (which is a greater psi drop than the starting pressure drop). With engines, the big reason these simple ratios start to fall apart is that, when the initial conditions change enough, timing becomes a huge variable. Being able to advance the timing introduces a new variable that the simple ratio didn’t account for, but when talking 5% atmospheric changes, that doesn’t really account for much here.