Honestly, though you probably already know this, a boxy wagon is pretty much aerodynamically hopeless. There is no substitute for a boat tail on a wagon but VGs, in theory, are better than a small extension (small as in 6-8"). VGs create an aerodynamic simulation of a boat tail by energizing air flow, which delays the inevitable forming of turbulence and a resulting low-pressure area. Of course, unless you have them framing your roof and side panels they won't do much good. Ideally to compliment your roof and side panel VGs a smooth body pan and floor-mounted VGs would probably help a great deal.
Of course, you could use both the extension and the VGs, which would be even better than just one, as it would provide a little more space laminar flow and probably aid in the simulated flow attachment.
Thanks. This confirms my suspicions re. rear end aero improvements.
True, rear-drive Volvos are nicknamed bricks for good reasons. But I seem to be making progress on it. I've built a front air dam reaching to 7" from ground level, and belly pan from air dam reaching back about 26" to approx the front axles line. With tires at about 40 psi, I get a pretty decent glide out of it now.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.
The whole time I've been reading about these VGs....and I've been wondering: Why hasn't anyone put these on the bottom of the car?? Attach them to the belly pan...even the bottom of the airdam. From what I understand, it should help the airflow smooth out and "skip" over parts of the rough underbelly of the car.
Raccoonjoe, I remember someone talking about doing it once, or actually doing it, but I don't remember where I saw it. Without a full body pan, I don't think VGs will help much, unless you have a very smooth underbody on your car because the vortex they create is just going to wind up running into something under the car and being destroyed.
Remember that when you put VGs on, you need to orient them according to the air flow, not the body lines of the car. So you might need to do some tests to see what the actual air flow direction is. That is why the outboard VGs on the Evo are "straight" instead of "angled".
Out of total curiousity, I did order a set of primed Vortekz generators even though I had no plans of installing them on either my Volvo or my Wife's VW Golf. One of my co-workers was curious as well, so I decided to make the $26.37 investment(freight inclusive).
They arrived only 2 days after I ordered them. The included instructions/templates are identical to the info provided on the website. The delta fins themselves appear to be well made...free of any mold lines or casting burrs. The plastic is very flexible, so a flex agent is required if you decide to have them painted. The 3M adhesive pads are custom-cut to the fin base shape and fit perfectly. Overall, I was rather impressed with the quality of materials.
Ok, so now you may be asking yourself " So, SL8Brick...what did you do with them since you didn't install them on your own car?" Well, it was my 18yr old nephew's birthday a few weeks ago. He drives a 1986 Chevy Camaro IROC thats seen better days and he's at that age where "boy racer" toys are still enticing. I sent the "Vortekz" fins to a local body shop and had them painted Chevy 'Flame Red', the same color as his Camaro. I finally installed the fins on his car last weekend. The included installation templates worked as advertised. Perhaps the most difficult part of the install was establishing a 'center line' on the roof. The T-tops on the Camaro made this quite easy, but I could see how it could become more labor intensive on a large, smooth roof with no point of reference. I had all 10 fins installed at the specified angles within an hour. They look unique on the Camaro and my nephew was pleased....whether or not they'll actually function on a fastback is beyond me.
__________________ 1993 Volvo 240 Wagon - 323k miles (awaiting recommissioning) 1999 Audi A6 Avant Quattro - 149k miles(the NEW daily driver)