Northeast cool mornings and Turbos WOW MPG?

[gorskined]

I have been really busy lately haven't had the old girl out since the hot days of summer.
until yesterday, i drove her all day .I have to say i don't know if just the season( cold air) and not having driven her in a few months or its all in my head because my daily driver is a diesel Jetta but wow felt like i had an lots of extra HP. The car just wanted to go and it was happy on the upper edge next to the red .

spent a tank of fuel and averaged 8mpg . but it was very spirited driving my afr under full boost is 10.5 +- 2 . off boost high rpms mid 13S would that be normal

is there any hp statistics on the turbo cars hot days vs cool days ?

Regards Ned

[930cabman]

From what I know, cool air is more dense, but also requires more fuel to air. When she is running well it may be difficult to not have your foot into it. First snow here in Buffalo today, all my toys are tucked in.

[Dr J]

By the ideal gas law, for a given pressure (atmospheric plus boost), the density * Temperature is a constant. So if temp decreases 10%, density increases 10%.

Temp here is relative to absolute zero (degree Rankine or degree Kelvin for our metric friends). Degree Rankine is degree F + 460* and degree Kelvin is degree C + 273*

So going from 80*F to 40*F is not a 50% increase in density but an 8% increase.

HP is proportional to the mass of air (hence proportional to density) so you would have a theoretical 8% increase in HP.

As to fueling:
Density is related to the number of molecules of air coming in so fuel needs to increase by the same proportion. This means more fuel(8% more in our example) but it does not change the desired afr. If your afr changes with temperatures, it is a reflection of the capabilities/tuning/design of your fuel control system.

Many would consider an AFR of 10 to be on the rich side, meaning that your car would be even faster if leaner, (and get better gas mileage) but you don’t want to lean to the point of detonation.

It certainly is fun having the extra HP. I find myself routinely trading MPG for that nice acceleration. This is definitely a YMMV! Enjoy the benefits of the cold temps. BTW, it is the same for normally aspirated engines.

[gorskined]

Originally Posted by Dr J

By the ideal gas law, for a given pressure (atmospheric plus boost), the density * Temperature is a constant. So if temp decreases 10%, density increases 10%.

Temp here is relative to absolute zero (degree Rankine or degree Kelvin for our metric friends). Degree Rankine is degree F + 460* and degree Kelvin is degree C + 273*

So going from 80*F to 40*F is not a 50% increase in density but an 8% increase.

HP is proportional to the mass of air (hence proportional to density) so you would have a theoretical 8% increase in HP.

As to fueling:
Density is related to the number of molecules of air coming in so fuel needs to increase by the same proportion. This means more fuel(8% more in our example) but it does not change the desired afr. If your afr changes with temperatures, it is a reflection of the capabilities/tuning/design of your fuel control system.

Many would consider an AFR of 10 to be on the rich side, meaning that your car would be even faster if leaner, (and get better gas mileage) but you don’t want to lean to the point of detonation.

It certainly is fun having the extra HP. I find myself routinely trading MPG for that nice acceleration. This is definitely a YMMV! Enjoy the benefits of the cold temps. BTW, it is the same for normally aspirated engines.

You lost me the fist few lines but the conclusion made perfect sense , great explanation .
i would love to see low to mid 11s under boost, im convinced i have an air leak under full boost.
going to start looking into that as soon as the first salt hits the roads here in Connecticut . by the looks of things i may have a week or two left to play.

Regards Ned