Long trip ahead, little prep time - best aero mod?

[NachtRitter]

Crud...

Filled up with #1 diesel on the morning of my 2nd leg... what a major difference in FE! Granted, my avg speed was a little higher (68mph avg) and I had some incredible winds (up to 50mph gusts), but it seemed like I was never able to get the FE from previous days even at same speeds. The FE was ~56mpg over 1,030 miles; elevation change is a 6000ft drop from start point to end point, over half of which was in the last 130 miles.

I'd hoped that I'd be able to make it the entire distance on one tank (which might have been possible if I'd averaged over 65mpg; and I thought that'd be possible since I was heading 'downhill')... but no such luck.

[NachtRitter]

Here are some pics of the aero work I was able to do before leaving on this trip.

Lawn edging air dam and partial wheel covers:


Air dam is 2 strips of lawn edging screwed and taped together. A single strip of lawn edging was 5", which was not deep enough to be useful. The 2nd strip (as you might be able to tell in the image) is another ~2.5" down from the bottom of the 1st strip.

For the wheel covers, I just taped over every other slot. Would have done them all but it was relatively time consuming.

A few more images of the air dam:





I didn't get a head-on shot, but the edge of the air dam is within an inch of the edge of the tire.

The rear belly pan (the one that came loose during the first day):




This was a quickie job as well, but I was relatively happy with how secure it was at the rear. Unfortunately, it was the front end that came loose, and the rear of it stayed well attached... I had jammed the coroplast between the struts so the front stayed in place (that is, it didn't drag on the ground), but it turned into a giant air scoop which allowed the air to exit at the rear... making the whole thing act like a large parachute... I could see the effect on the scangauge. So I finally stopped at a rest stop, detached the whole thing, and threw the coroplast into the garbage... I hated to do that since it was such a nice big piece, but there was no way I could have carried it with me in the car with all the stuff I had packed already.

[aerohead]

On road testing is really problematic.There are just so many variables to deal with and is reason for so many wind tunnels today.
My last trip had a spread of 28-47 mpg.
Where I was 'certain' I would set a new record I didn't.Where I had no expectations of seeing record mileage I realized my highest ever.
If we see any 'trends' emerge from our baseline data I figure we're doing good.
Appreciate the diligence and diciplined record-keeping.Damned nice MPGs!

[NachtRitter]

Thanks Phil,

I definitely agree that there is no way to draw any solid conclusions from the drive, at least not the kind of conclusions one could get from using a wind tunnel. But I believe I've learned a few things, many simply because I had instant feedback on the Scangauge (invaluable) and could 'experiment' while driving.

The obvious learnings (I knew these but confirmed them on this trip):
- An airbrake under the car is bad for FE
- Increased speed is bad for FE
- Diesel #1 is bad is bad for FE
- Driving downhill is good for FE

Then some less obvious ones:
- AC is bad for FE, but not nearly as bad as I thought it would be. The difference in instant FE on the scangauge was only slightly lower when AC was on vs off.
- Being behind a big rig, even when it's back far enough to see the rig's rear view mirrors, helps FE. Obviously within 2 car lengths would have been better, but I didn't feel comfortable that close. Of course, strong winds gusting perpendicular to the direction of travel negate any advantages.
- Warm / hot ambient air temps are bad for FE. More than I expected. This is a tougher one to nail down though... whenever the ambient air was warm, I was also running the AC. But it seemed that warmer intake air (I think the SG gets the measurement from a sensor that is after the turbo and intercooler on the Jetta) translated to lower FE fairly consistently. Turning off AC didn't change FE noticeably (the change was within the noise of the SG instant FE) but the intake temp started to drop and then the FE started to increase. Not by much, but a little. Turn on the AC, no immediate change in FE, but then intake temp would go up and FE would go down a little. Of course, outside conditions where changing constantly (air temp, road surface, other vehicles, slope, etc etc), so it's not something I'd hang my hat on. But the Diesel seems happiest when the engine is hot and the intake air is cold.

[aerohead]

Diesels appear to like heat very much.
A fella in Germany did a special Diesel engine which was tested in AUDIs and VWs.
It had no conventional cooling system but rather was oil-cooled and used a heater core for a 'radiator' and only when 'necessary.'
It wasn't supposed to be adiabatic or in defiance of physics,but was extracting a lot of work from the fuel and swirl in the combustion chambers helped isolate hot spots from critical metallurgy.
Very tasty mpg.