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Old 01-25-2020, 12:52 AM   #11 (permalink)
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Gaptooth (retired) - '00 Honda Insight
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Subscribed! I love your boat tail.

The Honda Insight could achieve 100mpg (US) from the factory with a little extra air in the tires. You could look for some of the design elements used in that car perhaps? I'm happy to take some pictures or describe those I'm aware of.

I changed the engine in mine from the thrifty 1.0L to a power-oriented 2.4, and the best economy I've seen since then has been around 65mpg cruising, under the same conditions the old engine was showing 100mpg - cruising at no greater than 50mph.

It seems the new engine isn't (much) less efficient at producing power, but that it has larger parasitic losses. For instance, it uses a lossy alternator instead of a highly efficient DC-DC converter, it has a bigger water pump, bigger oil pump, the serpentine belt looks to be a little over twice as large, another cylinder's worth of frictional parts, a second camshaft, etc. Because of these, it doesn't seem to scale down as well - there's a larger fixed amount of fuel consumed just running the engine to do the same amount of work.

Where the old engine could produce (let's say) 20hp, the new one uses around 33% more fuel. However, the new engine can produce ~65hp at close to the same efficiency with my current gearing. Meaning, with pulse and glide (accelerating for periods with 65hp then shutting the engine off and coasting for a bit, rinse and repeat) I expect I could get very near that same 100mpg. It's possible I could get close to 100mpg with the new engine if I had MUCH taller gearing (all of those lossy parts would be spinning proportionally slower) but it simply wasn't possible to fit gears large enough in the gearbox.

All of this is to say, you may be able to squeeze a little more out of your engine with taller gearing and a set of underdrive pulleys, to slow down those lossy accessories. It's not a bad place to look for gains after you've already picked the low hanging fruit from rolling resistance and aerodynamic drag.

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Old 03-02-2020, 04:48 AM   #12 (permalink)
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Thanks for the help and info!

Small update:

we've completed the front of the car as well, a nice airdam with the front of the undersection closed as well. We've also finalised our camera mirrors, the car looks so much better without those big black mirrors. Tueseday it will get some TLC on the technical side and then we can finally do some proper testing. Keep you updated!

*not visible, but we made the rear panel of the boat tail out of aluminium as well (previously white plastic, see photo on first page) and made the trailer rear lights sunken, so no more blocky structures at the back.
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Old 03-04-2020, 03:43 PM   #13 (permalink)
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There we go, pic of the updated rear as well!

Besides the camera mirrors, all the adjustments seem to be within the limits of the law and re-inspection by local authorities is not needed. That means we can probably get it trough MOT easily =)
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Old 03-05-2020, 01:16 AM   #14 (permalink)
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Turan turan - '09 Volkswagen Touran Freestyle

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Is that tow loop accessible with the air dam?
Not that i would question 106's reliability but still

Have to say, very clean&smooth execution on those ecomods

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Old 03-05-2020, 10:06 PM   #15 (permalink)
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Nice. Subscribed.
60 mpg hwy highest, 50+mpg lifetime
TDi=fast frugal fun

Originally Posted by freebeard View Post
The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 03-08-2020, 12:55 PM   #16 (permalink)
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Blackfly - '98 Geo Metro
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Very fun project, and nice workmanship!

How will you be measuring fuel use to determine the difference?

Have you already done a "before" test to get the baseline? Be sure to note weather conditions - ambient temperature has a significant effect on efficiency on its own.
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Old 03-08-2020, 01:52 PM   #17 (permalink)
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Paint them red and put something like a 107 or 108 sticker on the back. People will think it is a different model
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Old 03-18-2020, 04:36 PM   #18 (permalink)
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further extension

Originally Posted by Dizono View Post
We've thought about that, but a computional fluid dynamics model suggests that a further extension has only minimal gains. So Wunibald Kamm was right

It also shows that further improvements mainly come from the front and the transistion from windscreen to roof. That last one might be a tricky one, but the right shape of 'wind screen wiper shield' might help to shape the flow in such a way that it creates a more efficient flow over the car.
Kamm's best car,the K-5,at Cd 0.23,had an aft-body length equal to 1.63-X the actual body height (no ground clearance included).He did recommend an inflatable boat-tail stinger which would extend the aft-body out to 2.03- X,chopped off like all 'K' forms,which would deliver Cd 0.1909.Extending the tail all the way to its termination point,at 2.72-X yielded Cd 0.1742.
Sure do like what you're doing.
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Old 03-28-2020, 07:09 AM   #19 (permalink)
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I'm currently having computer issues and have resorted to using my cell phone.

1. The underside of the boatail does NOT look finished and closed off, is this correct?

2. Any plans for a belly pan?

3. I would also like to see the underside of the front chin spoiler, is there a partial pan there?

Overall a fantastic project, rivets and all.
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Old 03-30-2020, 08:17 AM   #20 (permalink)
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Thanks, the entire underside of the boattail is closed, it's just a bit recessed to ensure a proper connection to the underside of the car. The front bumper is closed at the underside as well, but there is not (yet) an entire belly pan.

The car is in storage at the moment due to corona... :/

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