Forum » EFI Tuning Fundamentals » Air density affected by humidity ?

Air density affected by humidity ?

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals

= Resolved threads

Page 1

I have a question regarding air density. If I'm using a standalone with VE table and I have a MAP and air temperature sensor. Lets say I tuned the VE table correctly at temperature 20 Celsius with 20% humidity. If the car is run at the same temperature 20 Celsius but the humidity now is 80% , I think the VE table will need re tuning as there is no compensation table for humidity nor there is a sensor that reads humidity, Am I correct or I'm missing something ? Please advise. Also if what I advised is correct what is the expected difference in VE table if humidity does change from 20% to 80% as oxygen is a lot less in high humidity. I know each engine will differ but an approximate percentage you would expect. Thank you.

Good question, one I hadn't really put thought into.

From what I could find on-line, there is ~2.5% variation in oxygen partial pressure (amount) between 0 and 100% humidity, assuming I interpreted it correctly - "humidity reduces ambient oxygen concentrations (dry air is typically 20.9% oxygen, but at 100% relative humidity the air is 20.4% oxygen)".

That would suggest it would be slightly rich with greater humidity, but as it is roughly proportional your 60% change would be expected to be closer to 1.5%. That said, the increased humidity will act similar to a water injection system, if to less effect, which would normally be beneficial.

Something that is somewhat surprising to many is that humid air is LESS dense as the water molecules are lighter than the nitrogen, oxygen and other trace gas molecules they displace.

Again, not something I had considered, so may be in error - anyone else?

It is great that you researched it. If 1.5 % only is the difference then it is not a big issue but if this 1.5% is at that temperature and when temperature increases this 1.5% can increase then there can be a bigger difference , I know there is compensation for temperature by modern ecu automatically but will it keep this 1.5% constant or it can be increased by a factor if for example the temperature is higher by 20 celsius. Just an idea , not sure if it is right.

I didn't check that, the humidity may indeed vary with temperature for the same mass of water. It's very late/early here, will try and remember to check that tomorrow - brain rather fuzzy now.