Boat tail on a normal car? (3-box sedan)

[Weather Spotter]

the Pics you put up are to small to see, try hosting them on photo bucked then linking them.

But the idea looks about right. The angle will change as you build it

I like the hatch removal idea, then the tail area just adds to your cargo space. If you hinge the glass piece (like I did) then you have a hatch back car

[cliffyballs]

Side shot of possible design:

http://i1217.photobucket.com/albums/...s/Taillook.jpg

Streamline Template:

http://i1217.photobucket.com/albums/...Streamline.jpg

I did an RPM to speed comparison:

75MPH = 3800RPM
70MPH = 3500RPM
65MPH = 3000RPM
60MPH = 2800RPM
55MPH = 2600RPM

This was all in 5th gear

Then a 10mph coasting slow down test

70-60 11.3 second average
60-50 13.2 second average
50-40 16 second average

Taft test on the trunk coming next

[3-Wheeler]

Originally Posted by Pendragon

.... Most OEM intakes strangle the engine and then there is the matter of the exhaust......

Regarding the comment about the intake being a hindrance to fuel economy.

If you are truely driving for the best fuel mileage, with the engine running just past idle, then the intake would not be much of a factor.

On the other hand if you driving 75 mph on the highway and into a head wind, well there's not much that you can do to get good mileage at that rate. (hp= v^3 function)

Jim.

[cliffyballs]

I was wondering if a rippled design would work. I was thinking of overlapping material down the length of the boat tail. So i guess from a side view it would look like stairs?? Any thoughts?

Here is the link to the amature taft testing video...i could only get up to about 50mph the actions starts at about 40 seconds

http://s1217.photobucket.com/albums/...-31-38_772.mp4

[Pendragon]

Originally Posted by 3-Wheeler

Regarding the comment about the intake being a hindrance to fuel economy.

If you are truely driving for the best fuel mileage, with the engine running just past idle, then the intake would not be much of a factor.

On the other hand if you driving 75 mph on the highway and into a head wind, well there's not much that you can do to get good mileage at that rate. (hp= v^3 function)

Jim.

Not to make too fine a point of it, if for no other reason than, as it turns out, the OP's car has already had the work done by his brother-in-law (and a cat back exhaust, too), it is sometimes amazing how restrictive the stock intake systems are on many vehicles. You would think that the manufacturers would place more emphasis on this area of design because of the potential for fuel economy gains, but it does not often appear to be the case.

My earlier estimation was that a gearing change should get the RPM for a 70 MPH cruise speed down to 2,500 to 2,600 RPM as compared to the reported 3,500 RPM at present. With the planned aero mods the OP may be able to run somewhat fewer RPM, but that is still enough for there to be some benefit to freeing up the intake and exhaust.

On the other hand, it is true that this is not a max horsepower situation.

Cheers

[euromodder]

Originally Posted by cliffyballs

I was wondering if a rippled design would work. I was thinking of overlapping material down the length of the boat tail. So i guess from a side view it would look like stairs?? Any thoughts?

You'd still get most of the benefits if the steps are quite small - allowing the air to easily re-attach.
The bigger you make them, the more you'll lose.

Grumman used overlapping plates on the fuselage of the Wildcat and Hellcat.

[Weather Spotter]

From the video it looks like there is a lot of side turbulence on your car.

For the stepped design, If the overlaps are like siding (small, say under an inch) the air will reattach and you are fine. If they get bigger the air will not flow smoothly over the tail and you gains go away.

[cliffyballs]

That side turblance i think is coming from my mirrors, because rear of the car actually is tapered somewhat. I am planning on making new mirros to practice my fiberglass skills (i do not want to delete them completely).

Which leads me to a design question:

I am considering starting the boat tail on the rear of the car where the Jetta's curvature ends. The first picture is of the Jetta without the trunk hatch (photoshopped) and the white area is what the boat tail would cover from the back.

http://i1217.photobucket.com/albums/...ls/removal.jpg

The other two pictures are of structure and support ideas, crudly photoshopped and paint.

http://i1217.photobucket.com/albums/...withwindow.jpg

http://i1217.photobucket.com/albums/...idesupport.jpg

Thanks everyone!!

[Weather Spotter]

Both options look good to me. The bigger one will net you more gains but the smaller one will look less odd.

[Pendragon]

You might want to ask people who have actually used the longer boat tails whether they had problems with sensitivity to crosswinds, especially ones quartering from the rear. Just looking at the diagram it appears that the center of pressure would be moved much further to the rear than the present stock configuration, but I really don't know how much effect that might have in the real world.