MINI Cooper S rear diffuser w/ underpanel

[khafra]

Originally Posted by InsightfulRay

I took that sentiment quoted above to heart and applied a set of vortex generator tabs to the roof and rear seat side windows of my Mini. I applied a total of 14. Eight on the trailing edge of the roof and three on each of the two rear seat side windows.

I'll post results when I accumulate a few tanks of driving with them. So far, I've noticed the rear glass and hatchback appear to be staying cleaner than before. That's a welcome result.

I'm interested to hear how the comparison goes. From what I've heard so far, a clean separation at the back of the car is better, and "energizing the boundary layer," like vortex generators do, essentially gives up a laminar boundary layer in order to make the boundary layer more reluctant to separate from the surface.

So, it'd be applicable mainly where the angle between the roof and the back window is steep enough that you get separation, but shallow enough that some vortexes would make the air cling better. Not so much applicable when there's nothing behind them to cling to.

When Autospeed did an informal test of trailing-edge VGs on an Insight, they made fuel consumption worse: Browser Warning -- and there's an old thread here taking all that into account, and looking for hard data: http://ecomodder.com/forum/showthrea...nefits-97.html

That thread points out that tank to tank testing may not help, although some A/B drag tests would be cool.

[InsightfulRay]

Originally Posted by khafra

I'm interested to hear how the comparison goes. From what I've heard so far, a clean separation at the back of the car is better, and "energizing the boundary layer," like vortex generators do, essentially gives up a laminar boundary layer in order to make the boundary layer more reluctant to separate from the surface.

So, it'd be applicable mainly where the angle between the roof and the back window is steep enough that you get separation, but shallow enough that some vortexes would make the air cling better. Not so much applicable when there's nothing behind them to cling to.

When Autospeed did an informal test of trailing-edge VGs on an Insight, they made fuel consumption worse: Browser Warning -- and there's an old thread here taking all that into account, and looking for hard data: http://ecomodder.com/forum/showthrea...nefits-97.html

That thread points out that tank to tank testing may not help, although some A/B drag tests would be cool.

Thanks for your post. My last car was a 2006 Insight so I can appreciate the temptation to add VG's to that in hopes of benefits. I suspect the negative result they reported with the Insight is more about the small surface area of the rear of a GenI Insight (hence less surface area for any low pressure wake to act upon) plus the extreme aero tweaking that went into the Insight at the factory. They likely squeezed about as much as possible out of the Insight's CD short of the full boat-tail treatment our fellow ecomodder in Wisconsin has done.

Not a big fan of A/B drag tests. Personally, I don't trust short distance A-B-A testing which is why I'm doing a controlled tank to tank test. Really, "tank to tank" is a misnomer. I'm accumulating several tanks worth of fuel consumption data. My driving route, speed, air temperatures and number of warm-ups per tank are close enough that I have reasonable confidence that a significant improvement in MPG will be detectable. I keep exacting fuel consumption records so I have a solid benchmark for my Mini's fuel consumption. Thus far my first full tank refill (525 miles worth of driving) after installing the AirTabs has increased my fuel economy by 1.5 mpg. This is about what I was expecting. It works out to about a 3% improvement over no VG's. We'll see how I fare on the next tank. The improvement might completely disappear.

[khafra]

Originally Posted by InsightfulRay

Thanks for your post. My last car was a 2006 Insight so I can appreciate the temptation to add VG's to that in hopes of benefits. I suspect the negative result they reported with the Insight is more about the small surface area of the rear of a GenI Insight (hence less surface area for any low pressure wake to act upon) plus the extreme aero tweaking that went into the Insight at the factory. They likely squeezed about as much as possible out of the Insight's CD short of the full boat-tail treatment our fellow ecomodder in Wisconsin has done.

Not a big fan of A/B drag tests. Personally, I don't trust short distance A-B-A testing which is why I'm doing a controlled tank to tank test. Really, "tank to tank" is a misnomer. I'm accumulating several tanks worth of fuel consumption data. My driving route, speed, air temperatures and number of warm-ups per tank are close enough that I have reasonable confidence that a significant improvement in MPG will be detectable. I keep exacting fuel consumption records so I have a solid benchmark for my Mini's fuel consumption. Thus far my first full tank refill (525 miles worth of driving) after installing the AirTabs has increased my fuel economy by 1.5 mpg. This is about what I was expecting. It works out to about a 3% improvement over no VG's. We'll see how I fare on the next tank. The improvement might completely disappear.

Cool--to me, it's all about measuring the variance in the control set and the experimental set, and seeing how many standard deviations apart they are. It'd be great to see the raw numbers from your last five pre-VG tanks and the first post-VG ones, as well as the placement of your VGs.

[curv872]

Originally Posted by InsightfulRay

Thus far my first full tank refill (525 miles worth of driving) after installing the AirTabs has increased my fuel economy by 1.5 mpg. This is about what I was expecting. It works out to about a 3% improvement over no VG's. We'll see how I fare on the next tank. The improvement might completely disappear.

Very interested in your continuing results. Do you have pictures of your setup?

[euromodder]

Originally Posted by InsightfulRay

Thus far my first full tank refill (525 miles worth of driving) after installing the AirTabs has increased my fuel economy by 1.5 mpg. This is about what I was expecting. It works out to about a 3% improvement over no VG's. We'll see how I fare on the next tank. The improvement might completely disappear.

At least on ecomodder, you'd be the first to prove Airtabs / VGs actually work to improve FE

[InsightfulRay]

Originally Posted by khafra

Cool--to me, it's all about measuring the variance in the control set and the experimental set, and seeing how many standard deviations apart they are. It'd be great to see the raw numbers from your last five pre-VG tanks and the first post-VG ones, as well as the placement of your VGs.

Will do over the weekend. On work lunchbreak right now.

[InsightfulRay]

Originally Posted by curv872

Very interested in your continuing results. Do you have pictures of your setup?

Here's one I had with me.

[California98Civic]

Originally Posted by InsightfulRay

I'm doing a controlled tank to tank test. Really, "tank to tank" is a misnomer. I'm accumulating several tanks worth of fuel consumption data. My driving route, speed, air temperatures and number of warm-ups per tank are close enough that I have reasonable confidence that a significant improvement in MPG will be detectable. I keep exacting fuel consumption records so I have a solid benchmark for my Mini's fuel consumption.

If you could elaborate on the data points and driving techniques, I would appreciate it. I have wondered about this as a n alternate testing strategy because going to my 18 mile "course" can be thought of as a wasted trip from an eco perspective. And the time! I'd rather use my very regular commute, if I can, reliably.

[InsightfulRay]

Originally Posted by California98Civic

If you could elaborate on the data points and driving techniques, I would appreciate it. I have wondered about this as a n alternate testing strategy because going to my 18 mile "course" can be thought of as a wasted trip from an eco perspective. And the time! I'd rather use my very regular commute, if I can, reliably.

Well, I have two tanks under my belt after installing the airtabs. So far the average for my last 3 tanks BEFORE installing the airtabs was 48.9 mpg. My two tank average AFTER installing the airtabs is now 50.8 mpg.

That works out to an approximately 3.8% improvement. By "tank", I mean a fill. Typically I take on about 10-11 gallons whenever I fill up. Now, of course, we all know how unreliable a one tank calculation is since I'm subject like everyone else to air pockets in my tank, how many "click off's" past the first auto shut-off I dispense, etc. So, even though I try to control as many factors as possible when I fill up, I feel more confident with a larger data set like several tanks worth before I break out the champagne thinking I've improved my fuel economy.

At this point, I'm somewhat more confident in saying that the airtabs do improve FE. I've previously reported in this thread how they are definitely keeping my rear window and hatch cleaner. However, I'd like to get 5 tanks worth of experience before I'm really convinced. It is suggestive that the 3.8% improvement I'm showing so far is about what the company quotes on their website.

Someone may suggest I take them off and continue the experiment by gathering an additional data set sans Airtabs. I'm not planning that because peeling off each airtab, even though it can be done (I had to trash a couple because I made mistakes during the installation), does ruin your chances of getting it back on. The adhesive tends to peel the backing off of the airtab body, so it's kind of a "once it's on, it's on" kind of a deal.

As far as the driving techniques, I do mild hypermiling. Tires are at 45 psi, gentle acceleration, glide whenever I can, anticipate traffic flow to avoid using my brakes, combine errands so as to avoid short trips, stay at 55 mph in far right lane on freeway, avoid the A/C. I don't draft trucks or turn off the ignition (FAS) though.

[euromodder]

It seems to be that way on the pic, but does the rear spoiler really protrude above the sloping rear roof line ?

If so, you wouldn't get the drawback of creating additional frontal area by adding the Airtabs on the roof.