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Old 03-15-2010, 04:00 PM   #41 (permalink)
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Woohoo, sheet metal!

Excellent looking mods. Really nice work on them. I'll be waiting for the tuft testing on that kammback, I'm looking to do one myself.

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Old 03-17-2010, 03:36 PM   #42 (permalink)
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preliminary report

Vision out the plexiglass is acceptable.

I've driven it since monday with about 1/4 of the fasteners in and tape covering the joints for this week.

About 150 miles now and there is a coating of dust on the upper window so I'm pretty sure that confirms that the angle is too steep right? should stay clean if it has attached flow I think.
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Old 03-19-2010, 09:08 AM   #43 (permalink)
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Painter's tape and yarn will tell you more than just dust. 'Hoping to hear what a few tufts say.
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The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
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Old 03-29-2010, 03:42 AM   #44 (permalink)
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Well I haven't done anything with the Kamm extension because I'm due for new tires in the back and they will be taller than what I have now so I'm trying to wait and see how much that effects the angle. I will tape some tufts on later this week but it's supposed to rain the next couple of days so probably Tuesday or Wednesday.

I did work on the exhaust some to fix a mistake I made that has been bothering me lately. I've posted some pics and stuff at the blog linked in previous posts. Basically I removed the 1.75" ID pipe and replaced it with 1" ID, from the manifold to the converter. The engine feels much better now in the RPM range it actually operates in, it was done about 50 miles from the end of the last tank so this tank should show any differences.

I also made a small piece of sheet metal (no pics yet) so I can completely block the lower grill for the morning commutes, maybe more if it doesn't overheat. It's held on by some neo magnets from old hard drives so it can be removed easily if cooling becomes an issue. I also pulled the timing back some in anticipation of higher operating temps and potential pinging.

Also added some old pics of exhaust to my album in the gallery here.

Last edited by robertwb70; 03-29-2010 at 04:00 AM.. Reason: additional info
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Old 03-31-2010, 01:11 AM   #45 (permalink)
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I got the tufts on today and the ones on the upper rear window "wag" at all speeds, I'm not 100% sure how to interpret that but I think they should lay straight and still if the flow is good shouldn't they?
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Old 03-31-2010, 01:56 PM   #46 (permalink)
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Wagging is better than flailing wildly in every direction. That would indicate turbulence which you don't want. Smooth and flat is ideal but as I recall, Metro had some wagging in the video of his cardboard boattail and I believe he still considered it a success.
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Old 03-31-2010, 07:39 PM   #47 (permalink)
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wagging

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Originally Posted by robertwb70 View Post
I got the tufts on today and the ones on the upper rear window "wag" at all speeds, I'm not 100% sure how to interpret that but I think they should lay straight and still if the flow is good shouldn't they?
I've been looking at wind tunnel images of Mercede's Boxfish car and some solar racers.
The smoke at the back of these structures is absolutely trying to scrape the paint off!
I suspect that the wagging represents a more 'feeble' flow,not not quite energized enough to be slammed straight back and 'rigid' from the stronger boundary layer.
Sounds like your right on the margin.
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Old 03-31-2010, 07:48 PM   #48 (permalink)
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That's what I think too, so maybe I'll raise it up about 1" on the trailing edge, especially if it doesn't change after I get the new tires. I don't seem to be seeing the gains I was expecting based on the results of the 1/2 length cardboard version I tried.

In the planning stages of a complete front end makeover to lower the stagnation point and add an adjustable radiator airflow inlet. Also starting to think about front wheel skirts but that may be a while.
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Old 04-11-2010, 04:27 AM   #49 (permalink)
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I propped the hatch open about 1" to reduce the angle a couple of degrees and I taped the gap so as not to suck too many exhaust fumes into the cabin. I'll drive it tomorrow and see what happens, maybe I'll drive it all week like that even...we'll see.

I'm thinking it should be fairly obvious when it starts working, the cardboard prototype sure was- I saw 2.5-7 MPG improvements in my commute back from school with it. I'm starting to think of a final angle of 12 degrees just to be on the safe side...

I put a couple of pics in my album here titled Kamm extension.
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Old 04-13-2010, 11:08 PM   #50 (permalink)
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Less wiggling at what appears to be about 13 to 13.5 degrees right now, I think I'll shoot for 12 once new tires are on.

Fill up today 46.12 MPG, a new best tank. Car started acting like it was running out of gas right before I fueled up, not sure whats going on because it only took a little over 7 gals so I know it wasn't empty.

I'm thinking it might have something to do with some Sta-bil I added at about the half tank point but time will tell.

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front spoiler airdam, geo metro automatic, geo metro wheel skirt, kamm kammback extension, side mirror delete

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