Open or closed partial boat tail

[paintme205]

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.

[Sven7]

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

[paintme205]

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.

[Sven7]

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

[paintme205]

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.

[Cd]

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 :

Box cavity :

Fully open :

[paintme205]

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.

[Xist]

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.

[aerohead]

Originally Posted by paintme205

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.

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!

[paintme205]

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....