Muggy high humidity days significantly improves performance - But how?

[vlad01]

As per title, why and how can a muggy warm and very humid day cause a significant uplift in performance above baseline, more so than cold winter days?

This is something I instantly noticed during the last year or two over summer and the warmer months as we have had 2 years of wet and warm summers which hasn't happened since 2010/2011 here in Vic.

I googled this thoroughly and only found the opposite effect, but is this the case of all of the armchair mechanics on the net being all wrong and just regurgitating the same misinformation?

The basics in physics and metrology, gas law etc... also counters this effect I am seeing. So is there something that I may have missed as well as 99.999% of the internet?

The basics that appear to be correct, although, same armchair forum augments back and forth, didn't seen any actual papers on the science, that is that humid air is less dense and also has some of it's O2 displaced by the moisture.

I am lost to why this effect happens and I seem to be the the very tiny minority to have this effect on my car.

Has anyone else seen this and even if its possible, had dyno data to prove/disprove humidity's effect on power production?

[immortality]

As you say, moisture in the air is supposed to be a bad thing but then why does water injection increase power?

I remember my old 308 was really happy on cold mornings, especially when there was a bit of fog about, you could hear it in the exhaust note.

Maybe it's the way you have it tuned? Factory cars tend to be on the rich side, maybe because we tune more at the Stoichiometric or slightly lean side the moisture in the air is having more of a cooling effect and because the MAT sensor is showing a cooler air temp more timing is added?

I know I run more timing on the MAT table on our L67 than the factory tune which only took timing away.

It's probably really hard to prove or disprove on the dyno as the environment is so controlled and not like real world conditions. Dyno's will only get you so far in simulating real world driving conditions.

A old work mate's inlaw worked for GM/Holden and he drove around in test car's fitted with huge sensor packages to datalog real world conditions, stuff you just can't simulate on a dyno or test track.

[brindo]

If its high humidity and raining, isn’t the atmospheric pressure low? So if its high humidity, but not raining, then is the atmospheric pressure high? Because if the pressure was low it would be raining?
No idea, but could be part of the answer.

[antus]

Everything I read about water injection says its cooling, so then more timing or boost can be added for more power. But the ECM wont know about the humidity so it wont increase timing any differently, and I think you're talking about an NA engine, so no boost.

You almost need to wait for these weather conditions, have the car warmed up, and log a 2nd gear pull to redline. Then wait for the rain to hit, clear the humidity, road to dry and repeat the test. And look at acceleration/speed vs time to prove the difference exists and look to MAP and timing for any differences on the two runs. If its noticeably faster see which, if any, sensors read differently and what the ECM did with it. Proving the increase is probably the harder part, and I think a real world scenario might be easier to prove than on an indoor dyno.

[immortality]

Probably need to measure EGT too to get a better idea of what is happening in the cylinder.

The other thing that comes to mind is moisture changing the burn of the fuel. It's been a long time since I've looked at it but they used water injector on aircraft and it changed the fuel burn characteristics which helped in parts to create more power, as well as lower temperatures allowing more turbine rpm.

[vlad01]

All valid points, but all of these other's have mentioned are counter to what physics are saying to the best of my understanding.

The point on combustion speed might be the best explanation, but it doesn't make sense if the engine is already tuned at MBT in more typical conditions.

The cooling effect of water injection is well understood as it is a phase change that happens within the engine, allowing denser charge, however a muggy day with high humidity and high temps has already undergone phase change and already absorbed heat. This then enters the engine without those specific benefits and from my reading, it should be a loss in power due to less o2 per volume. So I am stumped why there is a very noticeable bump in performance.

The gains of a cold rainy day for my car is basically baseline, it's barely any different to a hot dry summer day other than some small loss felt when totally heat soaked on a 40+ day in traffic.

Some say that the gain is from the higher effective octane from humid air, but my timing is not under knock control and using 98 didn't yield anything that I notice, other than less sulphur exhaust smell.

One thought perhaps might be the speed of sound but I expect that to be negligible, but it's a possibility as the speed of sound change would change the induction and maybe even exhaust tuning.

[vlad01]

I found this.

Still suggests a reduction with high humidity, but there is a decrease in internal EGR and intake pressures also increase with humidity, but peak pressure of combustion and it's timing significantly slows down and retards.

These engines are quite knock limited, so perhaps the high humidity could be helping in this way. But they mentioned that humidity adds a higher fraction of autoignition and increases combustion instability so who knows!

https://asmedigitalcollection.asme.org/ ... umidity-on

[BennVenn]

I wonder if your timing is too far advanced, and the humidity plus reduced oxygen is bringing your timing closer to optimal?

[immortality]

That is an interesting point.

[VX L67 Getrag]

Yeah you would probably need to log in both circumstances & see what is being commanded to know how much air is being ingested & what timing is at!