Another take on the rainy day/mileage improvement theory
 

Something I was thinking about last night as it poured rain all the way from downstate to home, and I was getting record mileage with the 6000 wagon.

I know that very small changes in the map sensor have a big effect on fuel use. I used a map sensor modifier in the Celeb, and when turning it down just a little, it would cut out when touching the gas. But I did find one spot about 100-150 mv lower than stock where it seemed to give a hint more power under light load. So I left it there. I dont know if it contributes to my mpg gains or not. At the time I didnt notice any better or worse mileage but I was also not hypermiling yet either and I was getting a measly 32 mpg downstate and back consistently. Right now the car is under 2 feet of snow so it wont be experimented with till probably May.

The map sensor is manifold absolute pressure. I was wondering if it might actually measure the differential between manifold vacuum and atmospheric pressure. If so, then I was wondering if changes in atmospheric pressure could throw the map sensor off a bit, possibly in our favor. I believe a barometer measures atmospheric pressure and rain is way on one end of the scale.

Any thoughts?

Wagonman,
Yes you are correct. MAP sensors do measure the difference between the manifold vacuum and atmospheric pressure.
The info sent to the ECU helps determine the fuel volume supplied BUT there are also other factors like engine temp , intake temp , etc.
In addition the ECU in most cases has a range built in so it compensates for altitude , weather , various quality of fuel , engine wear etc so the drivers don't have to.

Yes also to the barometer observation They measure only air pressure and the writing on the gauge tells of rain , sunny , change etc.

Cheers , Pete.

Nearly forgot...one factor in rainy day mileage is less resistance from the tyres which are "lubricated" by the water on the road. This is also why stopping distances are greater in the we than in the dry.

Pete.

Huh? Think about that for a bit. First, when it's wet, the tires have to do work pushing the water out of the way, thus higher rolling resistance and lower fuel economy (as confirmed by the mpg gauge in my Insight). Second, the stopping distances are greater only because the lubricating effect of the water lowers the amount of braking force that can be applied before you transition from rolling friction to sliding. That is, if you brake with the same force, and it's not enough to cause you to go into a skid, the stopping distance will be the same.

Rain is a pretty generalized term. I've seen rain pour down so hard even with the wipers on full I couldn't see either the center line or the shoulder line. More frequently here in the Dakota's (part of what was once known as the Great American Desert), rain is the gentlest little mist - just barely enough to wet the road. What does your MPG gauge say in the later situation? (That is a serious question. Please don't read anything into it.)

We don't really get that sort of rain around here, at least not frequently enough for me to remember ever having driven in it. Of course we don't get that much rain, period (roughly 1/3 as much as the Dakotas, on average), and what we do get tends to be Pacific storms. The misty little raindrops never manage to make it over the mountains :-)

I don't know about everyone else, but my mileage goes down quite a bit in the rain. For one I'm using a lot more power to run the wipers, headlights, air conditioner and defrost blower. Secondly, driving through water puddles really slow the car down, so after the puddle slows me down, I have to accelerate back up to speed after passing the puddle.

Rain totally kills my milage, but for purely social reasons. Living in the most densely populated area of the most densely populated state that happens to seemingly have the most dense (mentally) population: 3 raindrops = panic. Traffic slows down to the point where I do 25 on a 6 lane highway and that's before the accidents start piling up. Poor drainage means closed roads and lanes which increases congestion. People with their wipers going so fast they have no idea it stopped raining, people who drive excessively slow but still slam on their brakes at every opportunity, and people who swerve across lanes to avoid what might possibly have been a puddle but not really.

And then you throw in a slight breeze and possibly a slight chill in the air....

water vapor!!!
 

One thing that can help mileage when it's raining is that the water vapor in the air helps to cool/dense up the air going into the engine, which can then bump up timing, etc. The guys who were selling those quack "mpg improvement kits" that were basically a water jug with a vacuum line running to it were trying to use that idea, but were introducing the vapor in the wrong place and time.

Back before the computer days a person could easily advance their timing by 4 to 6 degrees on cool, wet nights, and not notice any spark knock, all the while getting much better gas mileage!

...and as anyone would presume: your mileage may vary!!!

But doesn't the extra water vapor in the air reduce the amount of oxygen available for combustion?--meaning a richer mixture?
Ray Mac