All right, more testing yesterday. This time, I took readings at 10 locations on the factory spoiler, rear glass and roof, up to just behind the B-pillar, at an inboard position (not centerline) and outboard.
One important difference between yesterday and Wednesday's testing: wind was out of the south both days, so there was crosswind in my earlier East-West testing. I used a different route yesterday, an exactly North-South county road (everything here is conveniently on a one-mile grid) and, with winds again out of the south, that means lower yaw, maybe even zero yaw. All testing was done at 80 km/h indicated with a pitot tube giving reference static pressure and readings averaged over 45 seconds or so each run.
No add-on spoiler:
Add-on lip spoiler:
Difference:
(all values given in Pa)
Observations:
-On the roof, pressure is lower toward the middle (faster flow speed).
-Over the window, this flips; pressure is lower toward the edge. This may indicate vortex formation there.
-Adding the lip spoiler produced positive pressure difference from ambient just in front of it, on the factory spoiler, and ambient pressure on part of the window.
-Adding the lip spoiler increased pressure a significant distance up the roof.
In the past I've not thought much of small spoilers such as this, believing that they wouldn't do much of anything. But based on these two tests so far, I think I was wrong!