The Holes!

[compdrag]

Originally Posted by seifrob

Congrats to you. Be happy with your results, reagrdless what did it.

Thanks

[elhigh]

Run a few tanks with the holes - being as consistent as possible, don't do any testing or jackrabbit starting, just drive, if possible, like a robot every day - and then drive a few more tanks with the holes closed up. Tape them from the inside, of course - it looks to me like your finish on the back is pretty good, and taping it over from the outside will just look awful. Plus taping from the inside, pressure will tend to force the tape on, instead of off.

Then fab up a quick-and-dirty pan to cover the back end and smooth things out approaching the rear bumper. Get a few tanks in that way.

I'm curious to see how the different approaches shake out against each other. I don't think it's the best approach but I'll give you this: it looks well done.

[compdrag]

Originally Posted by elhigh

Run a few tanks with the holes - being as consistent as possible, don't do any testing or jackrabbit starting, just drive, if possible, like a robot every day - and then drive a few more tanks with the holes closed up. Tape them from the inside, of course - it looks to me like your finish on the back is pretty good, and taping it over from the outside will just look awful. Plus taping from the inside, pressure will tend to force the tape on, instead of off.

Then fab up a quick-and-dirty pan to cover the back end and smooth things out approaching the rear bumper. Get a few tanks in that way.

I'm curious to see how the different approaches shake out against each other. I don't think it's the best approach but I'll give you this: it looks well done.

I know that without the holes I get 28 MPG driving like a robot. I haven't changed the way I have been driving since the holes and that first tank got me a little better numbers. I just hope this next tank does the same. Only thing is I keep doing stuff, and that doesn't help my defense any lol. Thanks again

[aerohead]

I found what I was thinking of,from a 1986 SAE book on recent progress in fuel economy.
The paper is from Subaru's R&D of the Cd 0.29 Subaru XT.The particular citation is on page 20 of the book,with Fig. 19- Influence of flat-bottom.
The image shows three different under body configurations.
*The lowest drag is for that of a sub-trunk which fills the void between the gas tank and the rear bumper,with the bottom of the bumper even with floor of the trunk.
*The second lowest drag is for that of the same configuration,excepting that the bottom of the bumper has been raised above the elevation of the sub-trunk floor.This additional void is all turbulence,and adds 2% to the drag.
*The worst-case scenario is with an even higher bumper bottom,and removal of the sub-trunk,which adds 12% to the drag.
-------------------------------------------------------------------------
When air loses a surface to flow over,it immediately turns into the void,triggering turbulence.The sub-trunk prevents the deflection and helps shape the flow smoother.
With the XT,the concept is further promoted by including the bottom of the bumper as a means to shape the flow.
--------------------------------------------------------------------------
Dead air pools inside the perimeter of the bumper,with the active flow passing underneath,just as air flowing over a pool of water.
--------------------------------------------------------------------------
By cutting holes in the 'dam' your inviting flow separation,turbulence,and higher drag.
------------------------------------------------------------------------
It looks like you can't change the drag more than 2%,so the most your fuel economy could change is around 1% which I submit to you,cannot be measured outside of a wind tunnel.Your car,with zero modifications will have a seasonal mpg variability of 12% anyway.Any minute change,as with the holes,will be lost in the signal-to-noise ratio.
I have the graphic with me and hope to get it scanned before the store closes.
Here we go

[Xist]

Does anyone remember the member who asked about opening the sunroof and back window on his SUV and having the air flow through the cabin. Like Ecky said, we close our grills because airflow is dirty inside the car. Also, Aerohead telling me I created an aerodynamic problem with my grill block by now bell-mouthing my vent.

So, if you are to do this properly and expect positive results, create a system: Do a proper belly pan with NACA ducts leading to the bumper openings, presenting a smooth surface for air the entire way.

[aerohead]

Originally Posted by Xist

Does anyone remember the member who asked about opening the sunroof and back window on his SUV and having the air flow through the cabin. Like Ecky said, we close our grills because airflow is dirty inside the car. Also, Aerohead telling me I created an aerodynamic problem with my grill block by now bell-mouthing my vent.

So, if you are to do this properly and expect positive results, create a system: Do a proper belly pan with NACA ducts leading to the bumper openings, presenting a smooth surface for air the entire way.

*If you open the sunroof and backlight simultaneously,the air will flow from the back of the car forward to the sunroof opening,bringing exhaust gas with it.
*As to the grille,A shallow,hard-edged opening will only pass 61% of the airflow.A deep,sharp-edged opening will pass only 86% of the flow.With a bell-mouthed opening,she'll flow 98%.This is why I chose the 'turbine inlet' trash can lid for the grille opening on Spirit.

[compdrag]

Ok well I went through another tank, and with the temperature getting colder and my 1 mile drives to work, and not being able to get operating temperature I still got above what I had been getting. I came in at 30.3 MPG, tomorrow I'm doing 160 miles to do my swim test and then 160 back so I will get really good highway numbers. Before the holes, and doing a 800 mile trip to Montana these best was 32mpg. I am out to blow that out of the water.

[MPGomatic]

There's something to it. I've been running a coroplast rear bellypan on Slambo to alleviate the parachute effect for a while now. If nothing else, it's quieter at speed.

https://www.youtube.com/watch?v=zFl6n1T1FCw

[Vman455]

Originally Posted by Ecky

Right, I imagine this helps fill the low pressure zone in the wake, but it probably also does it in a very turbulent manner. A kamm or boat tail + belly pan would be ideal, but there are plenty of situationally effective less than ideal solutions.

Originally Posted by aerohead

I found what I was thinking of,from a 1986 SAE book on recent progress in fuel economy.
The paper is from Subaru's R&D of the Cd 0.29 Subaru XT.The particular citation is on page 20 of the book,with Fig. 19- Influence of flat-bottom.
The image shows three different under body configurations.
*The lowest drag is for that of a sub-trunk which fills the void between the gas tank and the rear bumper,with the bottom of the bumper even with floor of the trunk.
*The second lowest drag is for that of the same configuration,excepting that the bottom of the bumper has been raised above the elevation of the sub-trunk floor.This additional void is all turbulence,and adds 2% to the drag.
*The worst-case scenario is with an even higher bumper bottom,and removal of the sub-trunk,which adds 12% to the drag.
-------------------------------------------------------------------------
When air loses a surface to flow over,it immediately turns into the void,triggering turbulence.The sub-trunk prevents the deflection and helps shape the flow smoother.
With the XT,the concept is further promoted by including the bottom of the bumper as a means to shape the flow.
--------------------------------------------------------------------------
Dead air pools inside the perimeter of the bumper,with the active flow passing underneath,just as air flowing over a pool of water.
--------------------------------------------------------------------------
By cutting holes in the 'dam' your inviting flow separation,turbulence,and higher drag.
------------------------------------------------------------------------
It looks like you can't change the drag more than 2%,so the most your fuel economy could change is around 1% which I submit to you,cannot be measured outside of a wind tunnel.Your car,with zero modifications will have a seasonal mpg variability of 12% anyway.Any minute change,as with the holes,will be lost in the signal-to-noise ratio.
I have the graphic with me and hope to get it scanned before the store closes.
Here we go

I found myself thinking about this the other day, and I'm not sure that the conclusion that airflow through the holes would help fill the wake is correct. The Subaru figures indicate a low-pressure area behind the spare tire well which causes turbulence. If holes are added in the bumper, for the air to begin flowing it would need to be higher pressure than the low-pressure wake behind the bumper. Do we know this is the case? Seems to me the first thing to do here would be run a manometer to in front of a stock bumper cover and behind it and see what the pressure differential is, if there is one, and thus if air would actually flow into the car's wake or not.

[Prophecy99]

great thread ! loved reading