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Old 11-19-2014, 04:53 PM   #1 (permalink)
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Open or closed partial boat tail

Like the title says, How much am I losing MPG wise by not having the rear of the boat tail closed off? I haven't finished the taillight frame so I can enclose if it will help.




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Old 11-19-2014, 05:03 PM   #2 (permalink)
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I'm not sure about the exact numbers for having an open boat tail, but I think that it is still better than not having anything back there. You might have more lift if you leave it open though, since air will (possibly) push against the top of the tail.

Anyways, if you close it, you will see your MPG go up vs having it open.
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Old 11-19-2014, 06:20 PM   #3 (permalink)
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how much

Unfortunately,there's little,to no data for direct comparisons.
Boat tails are closed.Box-cavities are open.And box-cavities are typically straight panels with no plan taper.
AT Dynamics' 'Trailer Tail' does have plan taper and is open in the back.They have demonstrated a 5.54% mpg gain at 65-mph with a semi-trailer rig.
Sepp,one of our members in Belgium reported a small mpg penalty for his Nissan pickup aeroshell,when the back was open.
Perhaps you could do a cardboard and duct tape rear bulkhead and quick tests to see.It would be important data.
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Old 11-19-2014, 06:38 PM   #4 (permalink)
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I will try and do some testing. Since I don't have a MPG gauge of any sorts, the accuracy would be very hard to see unless extremely different.
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Old 11-19-2014, 06:41 PM   #5 (permalink)
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It appears to me you could put more taper in it, top and sides, for more benefit.
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Old 11-20-2014, 01:11 AM   #6 (permalink)
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That definitely needs more taper!

Also, I would try sealing it to the body before trying to compare open and closed.

Why don't you have an MPGuino?

Also, please take a picture of the full car, perpendicular to its side, so we can compare it to the template.
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Old 11-20-2014, 06:07 PM   #7 (permalink)
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tail

I'm estimating:
*about 57-inches length extension from hatch gap at top.
*8-degree down-slope (no separation and good rearward visibility).
*around 3-degrees diffuser up-slope.(okay)
*Festiva appears to be raked quite a bit (nose down,tail up).This would keep the entire roof top in attack,right up to the gap.Kinda like Probe-IV when dropped for highway.
*Some type of former might be helpful for inside the very back to reinforce the sheetmetal,prevent deformation.
*Yes,at least a plan-view photo would be nice,for a look at plan-taper of tail,and rear 'transom' area.
I like it
PS,'like the 56-mpg too!
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Last edited by aerohead; 11-20-2014 at 06:09 PM.. Reason: add PS
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Old 11-21-2014, 12:08 PM   #8 (permalink)
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Some more pics. The bottom diffuser needs some work but I can play with that. Somehow I ended up with a downward slope when I got the car off ramps. The bulge in the picture is from the exhaust can so not much I can do with that.

I will measure the rear of car to determine side inward slope. It looks like 6-8 degrees when looking from front of car. Looks like it expands out from back of car. Weird. Maybe I will try getting a picture from off the roof or something.



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Old 11-21-2014, 12:43 PM   #9 (permalink)
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Your tire is angled quite a bit up in that photo, so if you tipped the photo to correctly match the template, the boat tail would stick out above the template. I'm not trying to put you down; it's just that these angles are kind of sensitive and you have to be very careful when setting it up!

When I was building my boat tail ("Anal Probe" project on EM), others suggested a 10 degree taper on the sides. It would be worth some searching on your own to verify, but 7 degrees likely leaves some fruit on the tree.
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Old 11-21-2014, 12:47 PM   #10 (permalink)
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What material is that?

It looks like you used a rectangular piece wrapped in a circle. The result is a cylinder. A conic sector* wrapped in a circle makes a cone.

I'd stiffen the back edge.

*not the right term. Think semicircular strip.

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