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Old 08-12-2013, 01:40 PM   #1 (permalink)
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New in Kansas

I've been experimenting with aerodynamics and fuel economy off and on since I was a kid, mostly with motorcycles. A number of folks who've posted here have gone far beyond what I've done, but I've been very successful with some simple mods and practices.

My first attempt was with my '63 Dodge Dart wagon (170 slant six, 3 speed stick). I opened the air cleaner, recurved the distributor, and replaced the muffler-on-back with a 2 inch glasspack and tailpipe that dumped in front of the left rear wheel. Jetting remained stock. Mileage went from 21 to 28mpg. The stock tailpipe was so small and convoluted, I think it might not have passed a marble.

My first attempt with a motorcycle was to replace the Windjammer fairing on my R75/5 BMW with a Meyer Plastics Daytona fairing. Both had similar frontal area, but the latter was bullet-shaped. I remember gaining 5mpg, the result being in the low '50s.

An '80 Harley Electra Glide followed. I didn't do any aero stuff, but I had to retard the timing when premium fuel disappeared in '81, and that cost me 4mpg. I replaced the stock "ham can" air cleaner (which had notoriously bad airflow patterns, and made the carb seem lean) with an older stock open-back oval air cleaner, and gained the 4mpg back with no jetting changes. Lesson learned: for leanest jetting, carbs need to draw from calm, stable air, which makes "pod" filters seem less than desirable. Resulting mileage: 48-50. Without going into detail, another lesson was that stock Harley parts were better than any of the aftermarket stuff I tried.

I later had a '74 BMW R90/6. Changes made: dual-plugged cylinder heads, airbox backside drilled with 1" holesaw, front muffler baffles drilled with same, BMW "S" bars and fairing with tall windscreen, 2.90 gearing (longest-legged available). Jetting remained stock. Mileage went up from 44 avg to 58, with cruising speeds ranging from 60 to 80. I twice recorded 66.

In '97, I had a '79 Kawasaki KZ1000ST, which was a heavy shaft-drive bike that got @36 mpg and turned 3800rpm at 60. I recurved the ignition (full advance at 3500rpm) and assembled an exhaust (one stock muffler adapted to a MAC 4-into-1 head pipe). I had several intake combos to try (the bike came with a parts bike and a lot of spare parts); after trying pod filters (they verified the Harley “lesson learned”) and a '74 Z-1 airbox (NOT good), I settled on a '78 KZ1000 airbox. I would've tried a stock KZ1000ST airbox if I'd had all the pieces; it looked good. Result: mileage up from 36 to 48. With a Rifle touring fairing, it would break 50.

Most recently, I played with a Yamaha SR500, with very disappointing results. The previous owner had installed a rather restrictive sportster muffler and very rich jetting, yet it got around 52mpg. I bought a nice Supertrapp muffler and an aftermarket carb for it, and mileage dropped to 38. I put the stock carb back on; experimentation resulted in much leaner jetting than the previous owner had used, but mileage was only in the mid-40s. My best guess: if a cam has enough overlap, exhaust restriction can be beneficial for mileage.

In the mid-'90s, I experimented with different touring fairings on the same motorcycle, and I wrote about it for the Moto Guzzi National Owner's Club a couple of decades ago. I'll copy that to another post later.

I now have a couple of Yamaha XS650 twins, an '80G and a '78E. The G, stone stock, gets a fairly reliable 53mpg in mixed local use. I like it just the way it is, but I may do some aero mods in the next year. I'm sure I can break 60mpg with simple stuff. The E gets about 38; I know it needs tuning, and I've rarely ridden it. It was built before 1 January 1978, making it exempt from any EPA requirements, so I may try some intake and exhaust changes after I have the rest of it sorted. A consideration: All Yamaha ever did with the XS650 to meet new and increasing EPA requirements was to tighten up intake and exhaust breathing, yet it's those later models that get the best mileage. My best guess regarding the SR500 haunts me.

I have other vehicles to play with. My wife's '06 Focus gets 32-35 on the highway, and I wonder if replacing the expensive ($450!), hard-to-access, and impossible to non-destructively-inspect air filter will help. We have a 97 Nissan pickup, 2.4L/5spd, with 220,000+ that gets 24-28mpg depending on the time of year; I'm sure I can get it up over 30 with aero mods.

Finally, I have my Dad's '77 F250 with a 400 and C6 auto. It gets 12 to 12.5mpg. I already have the parts (stick tranny, Edelbrock sp2p manifold, Autolite 4100 carb) to get it to a reliable 15 (I think), I just have to install them someday. I further think a set of stock Australian 2bbl heads will bump that to 16, and that's without any gearing or aero mods. It'll be years before I find out for sure, but I honestly think I can get 16-18 mpg at 60 pulling an Airstream with this truck, and that’s without EFI or headers.

That's probably as long-winded an intro as has ever been posted here.

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Old 08-12-2013, 02:59 PM   #2 (permalink)
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Join Date: Dec 2007
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CM400E - '81 Honda CM400E
90 day: 51.49 mpg (US)

Daox's Grey Prius - '04 Toyota Prius
Team Toyota
90 day: 49.53 mpg (US)

Daox's Insight - '00 Honda Insight
90 day: 64.33 mpg (US)

Swarthy - '14 Mitsubishi Mirage DE
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That is quite the vehicle history haha. Welcome to the site!

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