Different main jets for the winter?

[neilbardsley]

Does anybody go up a size (or 2) in main jets when driving in the winter. The air is a lot denser than in the summer so our engines could be running lean?

[mycar]

I've read that a change in altitude, (sea level to mountainous areas) can require a change in jet sizes... something that's perhaps more common in the States or on the European mainland... but not seasonal changes here in the UK.

How much more dense is the air at the moment ?

[neilbardsley]

I found this on the www (https://mechanics.stackexchange.com/que ... horsepower)

"Given constant pressure (which seems accurate), the temperature of the air is inversely proportional to the number of air molecules. So colder air means more molecules, and more air molecules means more energy released in each combustion cycle. A drop from 30 C to 0 C is roughly a 10% drop measured in Kelvin, which suggests 10% more energy for each combustion cycle --> 10% more horsepower!

2. Decreased humidity

Water molecules in air can't be particularly helpful for combustion, as those molecules need to heat up but don't combust. 100% humidity seems to correspond to 2.8% water to air ratio - a potentially substantial amount.

However, theories can't be relied upon without taking everything else into account, e.g. an engine may not even adjust for the increased air density, and thus run lean."

[neilbardsley]

Just bgt this so I will report back

https://www.ebay.co.uk/itm/Innovate-Mot ... rk:12:pf:0

[gridgway]

So if 30 degrees is 10%, the engine won't have been setup at 30 degrees. So freezing is only going to be 5% off I reckon.

Graham

[fetuhoe]

According to DIN 70020 the influence of air temperature is:



The formula in the DIN standard also contains a pressure term but if only temperature is considered this can be neglected.

This formula also assume 'dry' air.

Humidity influences the engine by reducing the rate of combustion as well as reducing the amount of oxygen in specific charge volume.

All in all the impact of pressure, temperature and humidity probably results in a set of complex interactions which must be difficult to manage on an engine fitted with carbs, a dizzy with mechanical advance and without any input sensors.

[mycar]

So then my theory that a carbureted engine runs better on a foggy morning is probably wrong then ?

[fetuhoe]

It all depends on the combination of temperature, pressure and humidity - all things being equal humidity is not generally positive but its negative impact may be less than the positive benefit of either lower temperature or increased atmospheric pressure.

[gridgway]

My version of that is to expect better lap times in dry cold March and November than mid-summer!
Graham

[mycar]

Maybe a carbureted engine just sounds better on a foggy morning... especially in a forest.

[gridgway]

So if it is proportional to the square root of the difference it's around half of 5% rather than half of 10%

squrt[(273+30)/273]=1.053