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Old 06-24-2022, 10:14 AM   #31 (permalink)
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A little off topic, but this is something I want to do with my project car. The idea is to make a rear diffuser, and route the engine exhaust into the front opening. For that car, fuel economy isn't really a concern. But less lift and/or drag would make it faster on the racetrack.
It's not *entirely* off topic. The basic gist of improving fuel economy is more advantageously rearranging the forces acting on the car, in particular the aero forces.

If you are going for a racing car, a common tactic is to angle the radiator to lean forward and then use a shaped hood bonnet. That gives cleaner flow at the nose, improved radiator performance, and downforce on the front wheels. It's also convenient, since everything is right there are the front, which means you can do professional ducting on the airflow. I almost did that to Champrius before I discovered I got better fuel economy increasing the hood insulation.

If you want to combine the radiator with the rear diffuser, I'd put the outlet into the wake ejected above the diffuser plane. A thin linear rectangle 15% of the area of your inlet on top of the diffuser would be perfect. That ejects the air directly into the wake, harnesses the maximum low pressure for suction, and provides tiny but real amounts of thrust and downforce on the real wheels.

The biggest issue I would see is your airflow ducting. Optimally you'd want a squished horizontal oval shape of consistent cross sectional area to go directly from the radiator to rear end of the vehicle. But that would most likely inconveniently intersect the middle of the cabin, like in freebeard's pic.

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Old 06-24-2022, 12:12 PM   #32 (permalink)
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Quote:
A little off topic, but this is something I want to do with my project car. The idea is to make a rear diffuser, and route the engine exhaust into the front opening.


A vertical Coanda nozzle fed by engine cooling air and/or exhaust. On the left.

It's easy when the engine is at the right end of the car.

Maybe I should revisit this. in 2014 I couldn't merge the boat tail and car body.
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Old 06-24-2022, 08:20 PM   #33 (permalink)
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Quote:
Originally Posted by freebeard View Post


A vertical Coanda nozzle fed by engine cooling air and/or exhaust. On the left.

It's easy when the engine is at the right end of the car.

Maybe I should revisit this. in 2014 I couldn't merge the boat tail and car body.
Hey freebeard! Is that your Superbeetle?

I didn't understand what you were trying to do the first time you showed it. If you are trying to make an integrated boat tail / radiator exhaust, then the optimal design is to start with that right pic. Then figure out what your radiator inlet and outlet areas are going to be. Find where the cross section of tail is equal to the outlet area and then truncate it there. Run a duct with that cross sectional area from the radiator to that new opening. Put on a ribbed rim with a flame and call it the Beetlemobile.

By the way, you want the tail to be symmetric from above but asymmetric from the side. The Beetle has most air go over the whole upper body, leaving the underside with less airflow. If the radiator intake is from underneath the vehicle, that further lowers where the exit plane should be. With certainty the optimal height is somewhere between a) at most the halfway point of the car body excluding wheels (i.e. the undertray and top of car) and b) at least the halfway point of the nose (same undertray plane and where hood meets windshield).

Do you have access to pressure measuring equipment? If so, then we could precisely determine where the exit plane should be. If you don't mind doing a mockup first, you could also tuft test. Start at the midpoint between a) and b) and then look at the tufts. Then tweak the exit plane up or down until you get the smoothest most attached flow.

You could also get some real gains by rerouting the radiator flow. I don't know how the Superbeetle is set up, but if those lines under the rear window are the exhaust, it's likely that air has to take a sharp turn to get up into the radiator inlet, then another sharp turn to get to the outlet. The optimal airflow is to gently deflect from underneath (or the sides), then take the straightest path to the outlet, with as gradual an angle change as possible. So if the radiator is at the bottom of the boot (say), then have the duct run along the bottom of the inside of the tail. The less we force air to zig zag around, the less drag we get.

As for fastening, my newest love are Rivnuts. If the body metal is still solid, you could easily Rivnut the tail onto the body with a good seam. If you have a tow hitch (or can put one on), then just engineer things so that most of the tail weight lies on the hitch. That will further reduce the stress on the tail / body fasteners. Otherwise, put the main weight on the back bumper / something else reasonably solid.

Hope this helps!
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The goal is 70 mpg this time around.

Stats from Champrius v3.2:
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Old 06-25-2022, 05:09 AM   #34 (permalink)
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Quote:
Hey freebeard! Is that your Superbeetle?

I didn't understand what you were trying to do the first time you showed it.
No. The picture dates to 2014, two years before I got the SUPERBeetle. I did a number of design, but in the software I was using, I could't merge my additions.

The Coanda design follows the Tropfenwagen, The one in the upper right follows Breer's work at Chrysler in the 1930s. Neither considers engine cooling.
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You could also get some real gains by rerouting the radiator flow.
Here's the thing. The whole engine is radiator. The Beetle engine compartment is a plenum. Cooling air is drawn by a squirrel-cage fan. All cooling air exits two ducts about 3x6".

Here're some other designs from the same time


And the boat tail mockup from 2014:


Wait a minute the Superbeetle is in the picture I sold the Baja Bug last week ($500).
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Old 06-25-2022, 07:58 AM   #35 (permalink)
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Cool stuff. My first car was a '74 Beetle. Unbeknownst to me, it had a spun rod bearing. So I had to rebuild the engine before I could drive it. I was a high school student, with no prior experience building engines. So that was quite a project!

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