[JAGD]

Ok we were driving our 535 6-speed a couple of weeks ago (I love this car) on the Oklahoma turnpike. Cruise set at 80 mph and very little wind. Mileage was 30.1 mpg. It started to rain, so after a potty stop I reset the computer and after 30 minutes we were getting 26.9 mpg with no wind and speed set the same. I know that there is increased air resistance or density involved as well as less traction. However, I have never owned a turbo charged car before and was wondering if some of the 10% loss of mileage was due to the turbo(s) being less efficient in rainy conditions? Any thoughts?

[kman_bmw]

The rainwater on the road can create additional resistance, though not normally noticeable to the driver, unless it is a very heavy rain. You can notice this especially when you drive through standing water with only the left or right wheels going through the water, the vehicle will pull dramatically due to the resistance of the wheels going through the water. You will also notice it when driving in extremely heavy rain, takes much more pedal to go the same speed, ergo, worse gas mileage. Finally, and though probably not a measurable factor, the rain will cause the tire to want to stick (adheasion) to the road as it rolls causing more resistance. You can test this with a matchbox car. Roll it on a normal table and then put a little water on the table and watch the difference in how far it rolls. Here's an article about it also.

http://ezinearticles.com/?Gas-Mileag...omy&id=1596541

[Needsdecaf]

Nothing to do with the engine's efficiency whatsoever. Other factors...increased rolling resistance likely. Or different terrain.

[BMWrules7]

Nothing "whatsoever?"

Cooler air is denser. Humid air is denser, but due to the water content and not oxygen.

Thus, turbo charger performance can be affected negatively by humid air.

For example, 65 percent humidity at 95 degree F. can reduce oxygen content to the same as 1000 ft. elevation.

Originally Posted by Needsdecaf

Nothing to do with the engine's efficiency whatsoever. Other factors...increased rolling resistance likely. Or different terrain.

[Needsdecaf]

Originally Posted by BMWrules7

Nothing "whatsoever?"

Cooler air is denser. Humid air is denser, but due to the water content and not oxygen.

Thus, turbo charger performance can be affected negatively by humid air.

For example, 65 percent humidity at 95 degree F. can reduce oxygen content to the same as 1000 ft. elevation.

Intake air temperature has a far greater impact on timing, fuel / air ratio, injector duty cycle, etc. Thus, unless it has heated up considerably (usually the opposite of what happens when it rains) then there would be no appreciable affect due to the humidity in the rain.

Having owned a turbocharged car of one fm or another since 2004 and having had one that had extensive datalogging to monitor performance, I never once saw any parameters negatively affected just due to rain.

Also, humid air is not more dense, it is less dense.

http://en.wikipedia.org/wiki/Density_of_air#Water_vapor

[BMWrules7]

Ok, I should have said "appears denser" due to the water content, but in reality there is less oxygen---which is what I said albeit inartfully.

[HighlandPete]

Originally Posted by JAGD

Ok we were driving our 535 6-speed a couple of weeks ago (I love this car) on the Oklahoma turnpike. Cruise set at 80 mph and very little wind. Mileage was 30.1 mpg. It started to rain, so after a potty stop I reset the computer and after 30 minutes we were getting 26.9 mpg with no wind and speed set the same. I know that there is increased air resistance or density involved as well as less traction. However, I have never owned a turbo charged car before and was wondering if some of the 10% loss of mileage was due to the turbo(s) being less efficient in rainy conditions? Any thoughts?

I do believe it's more the increased parasitic losses we encounter in the rain, wipers, lights, possibly more load on the climate control to dehumidify the air, etc.

I've observed this on all cars I've ever owned and driven. The better our fuel economy the more it shows when we add aggressive conditions.

There is another possibility which makes the mpg drop appear more pronounced. The particular car could be right on its 'sweet spot' at the easy run, 80mph cruise. Add to the road load and the drop off is not linear, a sort of tipping the balance. We find it in diesel engines, with variable EGR ratios, etc. Certain loads and speeds tip the economy, outside of what is expected. Can even get a 'blip' of improved economy at an increased road speed, before it drops off again.

HighlandPete