Quote:
Originally Posted by JulianEdgar
The percentage change can be worked out from the bar graph. Cd = 0.215 with mods up to box cavity, Cd = 0.210 after box cavity. A 2.3 per cent reduction.
It's a very small change in drag. What I measured was also a very small change in drag, so I can't see how you can say that I "got quite different results than Ali". Given the different size wakes, I'd say they're in the same ballpark.
Without further evidence, I'd say that box cavities - especially on cars without large wakes - probably aren't worth pursuing... certainly not as first steps in drag reduction, anyway.
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Whether 2.3% is worth it depends on what you find attractive, or interesting, or are able to build. When I road tested a box cavity I got results that were not detectable against the noise, but I did not yet have a difuser. Your 2.3% extrapolation ignores both the argument of the presentation and its findings: Ali argued the box cavity was interdependent with other mods and he specifically says the 2° difuser was a benefit only when coupled with the box cavity. They are interdependent. With the cavity and the 2° difuser the measured drag is 0.204, which is more like a 13% reduction from 0.215. The intedependencies as part of the mod.
EDIT: a quick afterword just to add that Ali tested half a dozen different box cavity designs because his results were affected not only by the other mods but by small changes in the design of the cavity. All these considerations increase the difficulty of the mod, but what's cool about it is how rematively low profile and stealthy it could be. A box cavity of Ali's type can be disguised as part of a mounting structure for a bike rack, possibly.