Advice for improving airflow with live rear (axle)

[Sam Powell]

Hi Guys, Still talking my old Dart here. I am kind of at loose ends on this project of improving the bottom of the car. I have finished the lower front, and now need to decide where to mover from here. I must keep in mind that this large cast iron engine produces a lot of heat that must be flushed out and down, and escape at the bottom of the car. I must leave the area under the engine and right behind it open. Right behind the engine is the torsion bar cross member. Further back by about 18" back is the trans cross member. I was going to close up the space between these two on each side which would probably smooth out the flow some. But the center must be left open for heat dissipation. There is a front sub frame which starts right at the Transmission cross member. I must leave this available for a jacking point, so cannot cover it on the bottom, but can fill between it and the side of the car. There is a very large pinch weld which extends down at the lower edge all along each side of the car. I can fasten things to this, but would like to use clips of some kind if possible, and not drill holes in this, primarily for rust consideration. I would rather not create open steel down where all the water and mud get thrown. So on to the topic of the thread:

As I stare at the bottom of the car, it seems maybe I should now start at the back and work my way forward, just to get the overlaps going in the right direction. So the live rear axle seems to be maybe the biggest obstruction under the car, and the one that might be the hardest to solve. Anything I do behind it must be coordinated with how I deal with the rear. It must be left free to move around. Is there any literature, or web information about how to handle this challenge? Any ideas floating around out there? Thanks for thinking about this. Am I just stuck with this liability?

Sam

[euromodder]

The front end is where most efficiency is to be gained.
A Partial belly pan at the back can lead to parachuting (catching wind underneath it) and means the wind would be dragging at the belly pan.

You could leave a slit between 2 parts of the belly pan to extract the heat from the engine/
(aft part being mounted somewhat higher than the front part)

At the rear axle, you could do something similar, splitting the pan @ the axle and flexibly mounting both parts to the axle so they can move up and down with it. Remember to give the belly pan parts some play to move fore and aft as the axle moves vertically.
(front pan once again being mounted lower than the rear one so the wind won't catch the rear)

[Sam Powell]

Thanks.If I understand you correctly, you are envisioning attaching something to the rear, and not the car body. It is very clear that the simplest thing I can do for a start is fill in the space between the back bumper and the rear frame. The rear frame member runs transverse at the end, and then there are brackets bolted to it and a bumper bolted to them. Both of these areas are acting as parachutes now. While covering the space between the two is a small thing, it is maybe a start. There is a space of about a foot between them.

I went to a car show yesterday with the Dart, and went around looking under cars. There is a guy who shows up with a 64 VW beetle regularly, and I was especially interested in the bottom of that car. It is completely flat. It is about as flat as you could imagine the underside of a car being. I had one of those back in the day, and it always yielded 32 MPG regardless of how you drove it. I am sure that flat bottom, among other things of course, helped the efficiency of that design. You have to wonder how well that car would perform with more modern technology in the engine?

Sam

[euromodder]

Originally Posted by Sam Powell

Thanks.If I understand you correctly, you are envisioning attaching something to the rear, and not the car body.

Nope, everything gets fitted to the underside of the car.
Or (partly) on the rear axle.

See drawing ... nobody's going to hire me as a designer I'm afraid

Both of these areas are acting as parachutes now.

Doing away with this parachuting is a good part of the improvement you get with a belly pan

The aim of a belly pan is to improve upon a vey bad flow situation underneath a car.

Fitting a partial belly pan aft, doesn't help much as the airflow that reaches it has already gone bad.
Fitting a partial belly pan under the front does help as it delays the flow going bad.

There is a guy who shows up with a 64 VW beetle regularly, and I was especially interested in the bottom of that car. It is completely flat. It is about as flat as you could imagine the underside of a car being.

It's all about re-inventing the wheel ...

Car manufacturers have long known what they can do to improve aerodynamics.
It's about time they start doing it again ...

[Sam Powell]

Thanks. I will keep staring at it. The more ideas the better. I have a lift, so do not have to lie on my back to look at it, or work on it.

Sam

[aerohead]

If you intentionally leave an open gap to accommodate the vertical movement of the axle,you can construct the pan superstructure 'before' and 'after' it.
And the aft portion can be designed as the diffuser.
With the pan on,the majority of energy will be maintained,and flow will be directed to 'jump' the gap.
Yes there will be a local perturbation,but most of the flow will just blow on by.
Later,you can deal with the axle area as a fine detail if you like.
That opening will provide an extraction point for hot air.

[RedDevil]

What about leaving the bottom as it is and just add an air dam and maybe side skirts?
Instead of making the air flow smoothly under the car, you'd make it move smoothly with the car. You would not enclose it completely so all the heat can still get away.

The front of the Dart is basically a big rectangle and will push a sizeable bubble of air ahead. An air dam would work well with that (in my very humble opinion, posting just after THE expert on these matters).

[UltArc]

Can I come over and use your lift? lol

I will try to find my images of what I did (a kind of similar situation), and post them. I attached it under the rear axle, on the rear sway bar (attached to it at the sides and center).

[Sam Powell]

I have built a pretty decent air dam already. Here is a photo of it. Maybe improving the bottom is a bit of wasted time and energy. I have lowered the car a bit since this photo was taken.


In the driveway.


Close up.

Thanks for the ideas. I really appreciate your taking this seriously and adding your insights. Most of my Mopar friends think I am a bit crazy for caring about this.
Sam

[Sam Powell]

In light of other threads going here, I am thinking I should fill in that bottom opening in the bumper. It was for the intercooler when it had a turbo. Maybe using tighter screen would be of benefit. I was also considering putting material behind the grill such as to funnel the air all through the radiator. About half of it just slams into the radiator core support and headlight buckets. That cannot be good.

Now that the airdam is complete, I am just hunting around for a next step, or another project.

Sam