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

[cliffyballs]

Hey everyone,

First time post and its gonna be a tough one, but coach said go big or go home...if i have to go home i want to use as little gas as possible!.

I would like to build a boat tail for a 2003 Jetta GLS 2.0L gas engine. I know little to nothing about boat tails other than what they look like.

My plan is to use a trunk mounted bike rack and modify it accordingly.

I want to use plexiglass at least on the top so i can still see out of the back.

I will eventually use trailer lights for brakes and blinkers.



So my question/s:

How long do i need to make it?
What things do i need to keep in mind?
What angles do i need to maintain?
What am i forgetting that will cause worse MPGs?
What do i think i am doing right that is actually wrong?

Any help would be great!

[Joenavy85]

Full boat tail on a Matrix

this should be able to help you out with all of your questions

[cliffyballs]

The problem is that all the boat tails are on on hatch backs not many if any that i can find are done on sedans. But i think that post answered most of my questions,

What angle? 15d

How long? 4-6ft

[aerohead]

Originally Posted by cliffyballs

Hey everyone,

First time post and its gonna be a tough one, but coach said go big or go home...if i have to go home i want to use as little gas as possible!.

I would like to build a boat tail for a 2003 Jetta GLS 2.0L gas engine. I know little to nothing about boat tails other than what they look like.

My plan is to use a trunk mounted bike rack and modify it accordingly.

I want to use plexiglass at least on the top so i can still see out of the back.

I will eventually use trailer lights for brakes and blinkers.



So my question/s:

How long do i need to make it?
What things do i need to keep in mind?
What angles do i need to maintain?
What am i forgetting that will cause worse MPGs?
What do i think i am doing right that is actually wrong?

Any help would be great!

cliffy,I'll try a go at your questions:
* The length will lock in the drag reduction,so you you might want to target a specific mpg improvement,and from there we'll know how far to go.If you look at the 1st-gen 1999 Prius,at Cd 0.29,and the latest 2011 3rd-gen Prius,you'll notice that by adding only 7.3" length,moving the rear body up to near the boat-tail curvature,along with a few tweaks,the drag was cut to Cd 0.25.Since 'cars' have achieved Cd 0.10,you can see that there is substantial room for improvement.
* As far as 'angles',it's more about degree of curvature.There is an Aerodynamic Streamlining Template here at the aero forum which you could use as a guide for designing your tail.All you need is a decent photograph of the Jetta taken from the side,taken at enough distance from the car to accurately show the current roofline.Also,it's a good idea to get two friends to sit in the front seats when you take the photo,so it will be 'settled.'
* Since the tail needs really good airflow to it in order to perform to top level,I would recommend rear wheel skirts to smooth that lower flow.

[cliffyballs]

Right now the way it sits, i would say im getting 26-28 mixed driving. Soon i will be commuting 80+ miles a day and would love to get near 40MPG. I think that should be reachable for a 2.0L manual Jetta...

[aerohead]

Originally Posted by cliffyballs

Right now the way it sits, i would say im getting 26-28 mixed driving. Soon i will be commuting 80+ miles a day and would love to get near 40MPG. I think that should be reachable for a 2.0L manual Jetta...

It would be really good to find out what the car will do if you were to top off the tank( so full that you can actually see the gas in the filler neck just sitting there motionless),get directly up to a constant highway speed,and then just hold it at that speed long enough to get an mpg reading.
Later on,these kinds of numbers are exactly what you may use to calculate your progress.
Mixed-driving mpg figures may make it virtually impossible to reduce your data to a meaningfull level.The only variable you can have is the shape of the car!

[Pendragon]

I realize this is an aero question, but you might want to also look at some drive train mods such as cleaning up the air intake. Most OEM intakes strangle the engine and then there is the matter of the exhaust. You can usually get some benefit of a cat back reduction in back pressure by replacing the muffler and/or some of the rest of the exhaust without getting too loud. That and replace the lubricants with good quality synthetic lubes and don't forget to give it a good tuneup and clean out the injectors and intake valves with an additive such as 44K.

[aerohead]

Originally Posted by Pendragon

I realize this is an aero question, but you might want to also look at some drive train mods such as cleaning up the air intake. Most OEM intakes strangle the engine and then there is the matter of the exhaust. You can usually get some benefit of a cat back reduction in back pressure by replacing the muffler and/or some of the rest of the exhaust without getting too loud. That and replace the lubricants with good quality synthetic lubes and don't forget to give it a good tuneup and clean out the injectors and intake valves with an additive such as 44K.

The only point I would make,is that should you do any of these mods you must completely redo your baseline mpg with these new mods in place or you'll never be able to determine what did what when aero-modding later on.

[Nevyn]

Wasn't there somebody with a Chevy celebrity that tailed it? Argh, can't think of who...

*off to use search function*

EDIT
Found him! Wagonman76 did one on a sedan:

1989 Chevy Celebrity 4 door - kammback ideas

[skyl4rk]

Nissan Versa Base 1.6