Rear Hatch Glass doubles as Kammaback?

[coltonandrew]

I have the 'Aeromodding the Astro Van' forum open and upon a little research, saw the best gains in aero modding happen in the back. Thankfully, our Van has a rear glass panel that raises on struts.. so this got me thinking: If we can find a way to safely hold the hatch glass open at a proper angle, could this double or act as a Kammaback?

I got to thinking of interior vacuum's and drag etc etc and brought myself to a mute point where I am really unsure of it being beneficial. Id like to make it work, and any advice or wisdom would be great

Thanks!

[Frank Lee]

Breathe enough exhaust through that window and all you'll be able to do is make a mute point.

[coltonandrew]

Originally Posted by Frank Lee

Breathe enough exhaust through that window and all you'll be able to do is make a mute point.

Ahh good point. Spending x amount of money on some plexi glass doesnt seem too worth it either.

We always drive with the windows down as well, so I wouldn't be too worried about carbon monoxide.

Regardless of, it's not a risk I'd like to take.

So then theoretically speaking; would propping the rear hatch open work for mpg gains on the highway or would the potential vacuum deem the idea detrimental to mpg?

[Frank Lee]

I doubt it; could be some nasty vortices off the edges due to lack of sides. But really, it might not make any difference anyway because the top of the window isn't flush enough with the roof.

[Jasen]

i've driven with that hatch open on the astro plenty of times hauling material's, never had any exhaust sucking, throbbing headache, drifting into a bridge abutment issue

[ChazInMT]

Keep in mind that modern autos create much less CO emissions than older carbureted cars did. CO2 is not the same as CO. An air concentration of 380 ppm is constant for CO2 because that's what us puny humans have done to the atmosphere over the last 150 years. A level of 350ppm CO would kill you dead if you lived in a house with that level, it would take 6 hrs or more. At 3,500 ppm, say goodnight Gracie in 30 minutes. CO2 would have to be at 30,000 ppm or higher to kill you by asphyxiation, not poisoning. 5,000 ppm CO2 is the legal limit for exposure, so that is survivable for fairly long exposure.

Catalytic converters remove 99% of the CO from the exhaust, so the level is manageable at 100-1,000ppm AT THE EXHAUST.... by the time this is diluted down, it is negligible.

Diesels create 5 to 10 times the amount of CO at 1,000-5,000 ppm CO so they're worse.

Anything not properly tuned or without the catalytic converter is a CO death pump.

As always though, better to be safe than sorry...or dead.

Relating to the OP: You need to manage the air coming off the sides of the vehicle as well as across the top, just taking care of one or the other invariably means creating large pressure differentials within close proximity to each other. These lead to drag which would at least offset any gains that would otherwise be achieved, and may create more drag than the original configuration. The Kamm was really intended for vehicles with a trunk and a notch back to manage air coming off the roof, something with a square back needs an entire extension of the form in order to be effective.

[aerohead]

Originally Posted by coltonandrew

Ahh good point. Spending x amount of money on some plexi glass doesnt seem too worth it either.

We always drive with the windows down as well, so I wouldn't be too worried about carbon monoxide.

Regardless of, it's not a risk I'd like to take.

So then theoretically speaking; would propping the rear hatch open work for mpg gains on the highway or would the potential vacuum deem the idea detrimental to mpg?

The lowest pressure on the ASTRO is around the windshield/A-pillar area.
If you have wind-wing, vent windows,opening them subjects the inside of the van to this low pressure.If the vent windows are open,it will bring air in from wherever it can get it,including the tail.It could be nasty.
The hatch itself would not be able to do the pressure recovery as an entire tail section would.And its own downwash (if flow remained attached to it) might actually aggravate the homogenous wake you already have.
If you could prop the hatch open,but close it in on sides and bottom with some surrounding rigid envelope you'd have you a boat tail.
With some rudimentary powertools you could fab up a crude wooded skeleton with 1/8"mahagony door skin,skin ,drywall screws and house paint to integrate into the opened hatch,or replace it.It would be enough to test.'

[Frank Lee]

I mainly just wanted to work "mute point" into my reply somehow.

[Jasen]

What about something like this



Maybe not as big seeing that the Astro body tapers from mid body to tail and there is a slight arch of the roof line.