Hi Everyone, heavy wagon / SUV 'project' Volvo V70 XC

[sikbrik]

I see where you guys are headed with the *no improvement* comment I made on the roof rails. To be fair, I should have stated no perceptible improvement which mirrors MetroMPGs corrections. If I had crossbars and mounts, I'm sure there would have been a more noticeable change but even without that, I'm sure there was *some* change.

However, just to satisfy my curiosity, how do "we know" that removing side rails on every make and model actually offers an improvement? The imperceptible difference that I didn't notice could have just as easily been negative, no? I mean, that weird 90s NASA study shows great improvements in MPG by ADDING things to the front and top of cars and vans that appear to mimic what my roof rails might do. They seemed to indicate that some shapes tend to do better if the air is directed over the top of the car and around the sides of the car - if the airflow is split properly - which many of the mods I've seen on this site help with, and so isn't it entirely possible that having a well designed roof side rail on a specific shape of car could actually "help"?

Sorry for the run on sentence and the devils-advocate question but it seems to me that many european cars come with siderails (only siderails, no crossbars) and so thought and windtunnel time might be put into their design and placement so as to use them to the carmakers advantage. Maybe? At the very least, we can say that an aftermarket rail or afterthought of a rack placed on an econobox will surely have more effect when removed than a rail on a car originally designed with a rail in mind?

But I do get the point...it might have helped and if I can find 100 more things to modify that slightly, I might see 1 mpg instead of 0. Point taken. Just not easy to swallow as I was hoping to not have to make 500 ore more small improvements. . (And burning through 500 tanks to prove them out!)

[MetroMPG]

sikbrik - most people here would agree it's good to ask questions like that, with an open mind. I'm tempted to reply to your question with a question:

How many of the highest-efficiency vehicles or prototypes can you name that have roof rack side rails?

It's generally understood that roof racks (and side rails on their own) increase both frontal area and drag coefficient. (Consider also that we more often drive in cross-winds than we drive in no-wind/headwind/tailwind situations, where side rails will have a bigger impact than might be thought at first.)

OK, sure - most high-efficiency vehicles are "econoboxes", not designed with racks/rails in mind.

So what if we look at some higher-efficiency non-econoboxes? EG: Toyota engineers deleted the racks/rails from its RAV4 EV, specifically to improve efficiency (http://ecomodder.com/forum/showthrea...-ev-23146.html) . As did GM with its full-size hybrid SUV's. Just 2 examples off the top of my head - there are probably more.

I wouldn't be surprised if someone could come up with an unusual design where side rails somehow helped, but I think it's pretty unlikely that design is on the road.

[sikbrik]

Very logical reply with great examples and info. Thanks for indulging my curiosity!

[brucepick]

Good to see you're working towards improved mpg in this car. We drove only RWD Volvos for about 10 years and I did much of the maintenance. They were great cars!

You mentioned an undertray. Don't underestimate it or let it drop off the radar. I found a lot of benefit from the one I built for my Civic.

Grill block is some of the best low hanging fruit. I strongly recommend a digital temp readout though, if using a grill block. Especially in your case with a turbo. You don't want the rad fan coming on; the power to spin it will cost you in mpg. ScanGage will show engine temp; I imagine UltraGauge would also.

Remember - 2 mpg sounds like "only" 2 mpg. But in your case it's about 10% of your total yearly fuel bill so =$$$$. 4 mpg improvement is even more savings for you.

Good luck with it!

[sikbrik]

Working with ARD in Portland, OR to provide an 'eco' tune for my ECM, along with a few other engine efficiency mods. The tune should effectively lean out the AFR just enough to make an economy difference without melting my engine. I may push this further later down the road once I get my aquamist water injection setup completely. Going to run just the new tuning for a while with everything else the same to get some baseline comparison of its effectiveness. Then the aero and other mods will proceed.

FWIW, the Torque app and my onboard trip computer seemed to agree that I averaged between 30.3 and 30.7 mpg on a pure highway trip this weekend. 250+ miles at 65mpg, just under 2300rpm, cruise control all the way, no boost to speak of. Should probably get a real scangauge one of these days. We'll see how my tank averages out with the trip computer and the app to see how accurate that 30mpg actually was. I was pleased, though, even if it's off by 2mpg since my only 'mods' were tire pressure and using Cruise Control and I couldn't break 27 mpg before.

[sikbrik]

Well, I got my ecm back from ARD tuning and there's definitely more power and it runs smoother but that's not what I worked with them for.

This is a tough one to A-B-A test so I'm not going to try to sell this one as fact and I have no long-term testing at all done yet but I did drive it around town and my scantool and onboard computer are indicating 1mpg average better (and I know I'm down at least 2psi in the tires) but the big change was a short highway jaunt I just took. I used to see a max MAX of 30.2 mpg in sustained cruise-controlled 60mph on this stretch of road. Today both read 4.5 mpg higher both ways on the trip. I'll call it a win.