MINI Cooper S rear diffuser w/ underpanel

[khafra]

That's cool--I've seen propane injection talked about, but all I've heard from people who weren't selling it is that it's dangerous and harmful to engine longevity. I'd love to hear about your experiences with it.

[InsightfulRay]

Originally Posted by khafra

That's cool--I've seen propane injection talked about, but all I've heard from people who weren't selling it is that it's dangerous and harmful to engine longevity. I'd love to hear about your experiences with it.

My experience was very positive. The system was set to add a small amount of propane to the intake ONLY under conditions of high turbo boost. So when you were in a hurry, you had a little extra power because the fuel mixture was richer since it had the traditional ECU controlled amount of diesel fuel plus the metered amount of propane. It was a "Bully Dog" brand system. Had a small, trunk mounted propane tank that was slightly smaller than a traditional BBQ tank and it was DOT approved.

[Arragonis]

I haven't followed every post but is there some A-B-A testing of tabs anywhere with a measurable difference on a normal car ? I'm thinking something like coastdown tests with these attached vs not.

I know carefully designed ones deployed by carmakers have an effect, these other ones seem to have no effect at all.

BTW I'm not intending to have a go at the OP or anyone else for using them, we all want better FE and efficiency, I just think hard evidence on whether they work and, if they do, how to employ them would be useful.

I have a boxy hatchback and I will probably buy a boxy hatchback quite soon so I have an interest in knowing...

[InsightfulRay]

Originally Posted by Arragonis

I haven't followed every post but is there some A-B-A testing of tabs anywhere with a measurable difference on a normal car ? I'm thinking something like coastdown tests with these attached vs not.

Haven't seen anything like the A-B-A testing your mention other than a windtunnel test on the airtab manufacturer's site. One must be skeptical of course, but it was quite detailed anyhow. Perhaps that would be of interest to you?.

I do know that the airtabs I purchased are designed for vehicles that have a rear surface that is at an angle of 30 degrees or more from the roof. This is why I tried them on my hatchback. I suspect that a "normal car" such as a standard sedan or fastback coupe would not see any benefit.

[InsightfulRay]

Originally Posted by khafra

Yes, if the miles driven and gas consumed for three tanks prior to tab installation is all you have, that'll help--but we might not be able to get a good confidence level in the results. The more post-tab data, the better; as that will help to narrow that part down.

Y'know, I really should be recording all my fill-ups, since I'll probably try aero mods sooner or later and the TDI doesn't need refills very often.

Here's the poop on the 3 tanks prior to installation of the airtabs. Sorry for the sloppy format. Wasn't sure how to attach a tab-delimited file from a spreadsheet. First column is date of fill, 2nd is miles driven since fill-up, 3rd is gallons, and last is ridiculous decimal precision

3/25/2011 375 8.3 45.180722891566
4/7/2011 412 9.1 45.274725274725
4/21/2011 503 11.2 44.910714285714
1290 28.6 45.104895104895

[khafra]

Nice! That's way more consistency than I get tank-to-tank; I think I'm going to have to record what the mpg computer says about similar trips (home to work, etc.) instead, and hope that if the mpg computer is inaccurate it is at least consistently inaccurate. If we can get some post-tab numbers I might be able to eke out a confidence interval from this

[InsightfulRay]

Originally Posted by khafra

Nice! That's way more consistency than I get tank-to-tank; I think I'm going to have to record what the mpg computer says about similar trips (home to work, etc.) instead, and hope that if the mpg computer is inaccurate it is at least consistently inaccurate. If we can get some post-tab numbers I might be able to eke out a confidence interval from this

Here you go. These are all after installation of the Airtabs. Again, apologies for the sloppy format:

6/1/2011 424 9.1 46.593406593407
6/7/2011 488 10.1 48.316831683168
6/7/2011 476.5 10.3 46.26213592233

[khafra]

Thanks, InsightfulRay! Now, don't quote me on this to any statisticians; I asked one I know whether I can in good conscience give a confidence interval for an experiment with just three data points in each condition of the variable, and he laughed at me.

But according to the standard deviation calculator at Standard Deviation Calculator - Calculate mean, variance of the numbers

The pre-tab mpg has a mean of 45.09 and a stdev of 0.18
The post-tab mpg has a mean of 47.05 and a stdev of 1.10

So, the airtabs dramatically increased the variance in your mpg
(I can't think of a mechanism by which that would occur, unless they increased your susceptibility to crosswinds and one of those tanks was during a week of heavy winds).

Because of that big variance in the post-tab mpg, the distribution for the post-tab values is smeared out over a big range. If you could do three more fillups and post those numbers, we could probably get a much higher confidence.

*However*, according to Student's T-Test (which I picked because it's the easiest to use, not because it's the most appropriate here), there's only an 8.9% chance that nothing changed between these two distributions of numbers.

That's not good enough for journal publication, but if nothing changed besides the airtabs, that's pretty damned convincing to me.

[InsightfulRay]

Originally Posted by khafra

Thanks, InsightfulRay! Now, don't quote me on this to any statisticians; I asked one I know whether I can in good conscience give a confidence interval for an experiment with just three data points in each condition of the variable, and he laughed at me.

So, the airtabs dramatically increased the variance in your mpg
(I can't think of a mechanism by which that would occur, unless they increased your susceptibility to crosswinds and one of those tanks was during a week of heavy winds).

Because of that big variance in the post-tab mpg, the distribution for the post-tab values is smeared out over a big range. If you could do three more fillups and post those numbers, we could probably get a much higher confidence.

*However*, according to Student's T-Test (which I picked because it's the easiest to use, not because it's the most appropriate here), there's only an 8.9% chance that nothing changed between these two distributions of numbers.

That's not good enough for journal publication, but if nothing changed besides the airtabs, that's pretty damned convincing to me.

Wow, this is cool stuff even if I don't understand it.

I can attest that nothing else was changed but the airtabs so we can put that potential contamination to rest. I'll continue to forward consumption data as I accumulate it. I did another tank just recently but it was a great deal more city driving than I usually do hence lower MPG (44.5 to be exact). It would definitely skew things so I won't send you that one. I anticipate a fill-up later this coming week and the driving conditions are fairly typical of the 3 "post airtab" tanks I sent to you. I'm on an indicated 50.9 mpg as of now with about 200 miles on this tank.

So I see statistics can be fun — especially if you have someone else doing your analysis!

[InsightfulRay]

Originally Posted by InsightfulRay

So I see statistics can be fun — especially if you have someone else doing your analysis!

So, Khafra, here's my latest tank to add to your statistics.

date 6/24/2011
miles driven 299.3
gallons to fillup 6.1
Calculated MPG 49.07

This is more representative of the driving conditions I experienced during the previous tank stats I shared with you. Again, this is with AirTabs installed.