Well there you have my dilemma...I disliked living in the cold so much that some 50 years ago I moved from Pennsylvania to Texas.I prefer the heat to the cold but my 2005 XK8 loves the cold. The temps dropped down to the 30's all day yesterday and when I stomped on the throttle I was greeted with at least another 15~20 HP (seat of the pants dyno).So, while I still dislike the cold my XK-8 likes it so much (and I like my XK-8 so much) that I may have to come to some understanding with the kitty.

wj

 

I have a similar effect. More pickup on rainy days winter days with 70ºF -ish degree weather vs hot summer days at 100ºF. Unfortunately, you can't make use of the extra power in such situations.

 

Wj, I've had the same experience. I wonder what causes it. I mean there's not a significant increase in the number of oxygen molecules 50 degrees Fahrenheit colder right?

I should go for a cruise tonight, it's down to 17 degrees. If there wasn't snow I would.

John

 

I seem to get worse mpg out of mine in the cold (Im in Pa now) Id trade ya temps for a month! Maybe my yard would be able to dry out! Lol

 

Note combustion engines love cold ambient air. Simply put, the mixture of air/gas going into the piston chamber gives a better bang when the air is cool.

Google the Otto Cycle (formal name for the suck, squeeze, bang, whoosh - four stroke)

That's why there's intercoolers on superchargers which compress the air going in.

It's really noticeable on small engines. Compare the performance, in the cold and in the heat, of go-karts or ride-on lawnmovers .

 

In our modern vehicles the ecu controls the spark timing. I'm sure that the major increase I feel is because the engine can tolerate a more aggressive spark advance without detonation, which leads to more power. ie...the ecu is retarding the timing as the engine warms up and the warmer the air the less aggressive the timing and thereby less power. This is still a great compromise...thank you modern technology.

wj

 

The greater the air density, the larger the fuel/sir charge is that goes in on the intake stroke.

Cool dry days = greater density, larger fuel/air charge and more power. The exact opposite on hot humid days (Texas summers).

Humidity/moisture actually lowers air density, but if the engine has active knock sensing, the 'wet' can suppress detonation (free water injection) and result in some increase in power.

I was once commanded to rebuild an aircraft engine because he couldn't hit redline in the summer (again, Texas). There was nothing wrong with the engine at all. All he needed to do was wait for fall.

 

Yes, there really is!!

 

Thanks is bladerunner, that would explain it.

Now you'll have me thinking about Avagadro's number, Boyle's law, and who knows what for the 1st time in way too many years. Gotta love science -).

John