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.

http://i772.photobucket.com/albums/y...tendedwing.jpg

http://i772.photobucket.com/albums/y...taillights.jpg

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.

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.

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.

It appears to me you could put more taper in it, top and sides, for more benefit.

http://ecomodder.com/forum/attachmen...1&d=1416460216

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.

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 :thumbup:
PS,'like the 56-mpg too!

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.

http://i772.photobucket.com/albums/y...ateinwhite.jpg

http://i772.photobucket.com/albums/y...ateinblack.jpg

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.

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.

I'll find some flat ground to retry. Trying to get a perfectly level picture on unlevel ground is difficult to say the least.

The sides taper in at 13 degrees using the widest part of the car and the widest part of the tail.

The material was a square sheet but didn't worry about keeping it straight since it was getting trimmed in the back. I think I'm going to try tightening it up though to see how much more downward angle I get and also if it brings the sides in more.

If you post a picture without the template on it, I or someone else here can rotate it for you in Photoshop.

Material is just sheet metal squares. This is a low budget (free) project car. I have less than 1k in everything and most of that was tires and fixing an oil leak.

http://i772.photobucket.com/albums/y...thtemplate.jpg

Wow, I am surprised how much the angle changed from photo to photo! Looks like you're fine after all. :)

I leveled out the very end a little. Couldn't be more than 1 inch though. Just took that little lip with the rise out of the end.

A flat photo does make a world of difference.

I did some testing a while back with a now defunct software program and I could never get the Cd figures anywhere near the ballpark of where they should be ( user error of course )

One of the tests that I did was to compare A full boattail, closed off kammback, open kammback, and a kammback with a box cavity.

Despite showing Cd counts much higher than they should have been, there was a consistent .1 count in the Cd between the open box cavity and fully open kammback and the enclosed design.

Figures were most likely around 10 counts off ( ! ) but showed a Cd reading of . 36 versus .37 for the closed version.

Based on that, I am going to say that an open design like you have will be more beneficial than one that is sealed.

Fully closed off :
http://i877.photobucket.com/albums/a...b17/closed.jpg
Box cavity :
http://i877.photobucket.com/albums/a.../boxcavity.jpg
Fully open :
http://i877.photobucket.com/albums/a...7/opentail.jpg

Well I'm unsure of my results at this point. I did the extended boattail, underbelly tray finished and then an alternator delete.

Drove 200 miles at 75-80 MPG. Winds were 45-50 MPH, temperature 45 degrees and speed was 75-80.

I only managed 43 MPG.

With that in mind, I'm going to test the alternator delete to that effect first. If I reconnect it, and mileage isn't better, I'm going to look at the boat tail design with a true a-b-a test.

Honestly, I would recommend testing the aerodynamic modifications first. It would be good to have a number for the alternator delete, but I cannot imagine a scenario when it would not help.

unsure
 

Could you give us a sense of the driving conditions under which you achieved your baseline mpg?
*Winds sound like last Sunday.All the 18-wheelers down here pulled off the interstate and parked 'til it blew out.The big-dogs never attempt to test (or race) if the winds are above 8-12-mph.Direction could totally kill your mpg if you had any headwind vector (80 mph + 50 mph = 130 mph airspeed!).
*The 45-degrees will increase rolling resistance and prolong the time and distance to get them up to whatever temp they're going to see.
*The 45-degrees will affect the warmup and equilibrium viscosity of the gear oil in the transaxle,increasing driveline losses.
*The 45-degrees might also have increased the air density compared to baseline,and drag varies directly with density.
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If you're pulling that kind of mpg under those kinds of conditions I'd be thinking that your already in 60+ mpg territory under better conditions.I'm impressed!:thumbup:

Thanks Aerohead for the input. I think the weather is crap now to compare anything unless it's same day and same direction of travel.

Yesterday I was annoyed at the MPG and didn't have time to test A-B-A for 200 miles. So I decided to try a top speed test.

Previously the car mainted 94 MPG on level ground and around 99 max. Yesterday It held 100 to 101 on level ground and 106 with a nice hill to get a running start. So I'm pretty sure aerodynamics must be a little better.

The bad part is I think I blew a headgasket....

a little
 

I found Hucho's original top speed research from 1976 and it reported a 11.59% top speed increase for a 30% drag reduction for a car with radial tires.
If,say.the top speed went from 94 mph,to 101 mph,then from Hucho's relationship it would suggest a 19.26% drag reduction necessary to get that speed.:thumbup:
Sad news about the head gasket.It's a days job if you have the parts on hand.Watch your back.I did my Honda a year ago and I'm still sore from it.