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I don't agree with aerohead's assessment. The lip spoiler at the rear edge of the factory spoiler increased pressure by 30 Pa on the rear part of the roof! These pressures are the average of E-W runs; the data for no lip spoiler and rear lip spoiler are:
Roof:
No lip spoiler: -140 (E)/ -200 (W)/ -170 (avg)
Lip spoiler: -110 (E)/ -170 (W)/ -140 (avg)
Upper window:
No lip spoiler: -50 (E)/ -80 (W)/ -65 (avg)
Lip spoiler: -70 (E)/ -70 (W)/ -70 (avg)
Lower window:
No lip spoiler: 0 (E)/ -20 (W)/ -10 (avg)
Lip spoiler: 0 (E)/ -10 (W)/ -5 (avg)
Hatchback base:
No lip spoiler: -20 (E)/ -50 (W)/ -35 (avg)
Lip spoiler: -30 (E)/ -50 (W)/ -40 (avg)
I'm hesitant to ascribe significance to a difference in average pressure of 5 Pa. However, on the roof where the reading with the lip spoiler was consistently +30 Pa--a much larger difference--I think it's safe to say it is reducing lift. So is the spoiler placed at the top of the window, but at the expense of decreased pressure over the window behind it; the question is, how much further forward does the pressure increase over baseline?
I forgot to bring the spoiler with me so I didn't get to test it today, but I have some time tomorrow evening. I'm going to measure pressures further forward on the roof, and outboard of the centerline to see if I can get a better picture.
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