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Old 06-07-2013, 09:43 AM   #5 (permalink)
Cyborg ECU
California98Civic's Avatar
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 5,536

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 59.47 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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diffuser, undertray, and prototyping for testing

The rear box cavity will need a diffuser and the diffuser will need at least a partial under tray (belly pan). So I am thinking of designs.

The first four pages of this diffuser build (and one later post with photos of the prototype) are useful: I want to imitate his use of togglebolts in the existing subframe, aluminum c-channels(?), and folded coroplast on the diffuser.

Here is an image of a recent Civic hybrid's undertray. I like the fact that they leave space for things to heat to escape and for service access. I'm also thinking of oil drip from my ancient engine:

And here is my first effort at "drawing" a prototype:

dark green = vertical line, curved inward toward the trunk lid
orange = vertical section follows tumblehome, curves to spoiler & trunklid
light green = flat spoiler

The goal is to allow flow to "skip" the dead air around the tail lights, create cleaner separation and a smaller wake, and therefore as I understand it produce less drag. I know any benefit will be smaller than a boat tail. I don't want a boattail because of the loss of window and trunk functionality and the increased fabrication difficulty.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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