Having just installed the
right side under body panel for the engine [ $40] the only difference I noticed was a bit higher intake air temperatures for my very simple
hot air intake modification ducting added earlier once the panel was installed . The intake draws off the catalytic converter heat shield and the panel must have quietened down some turbulence and outside air circulation i guess.
Didn't notice any change to coolamt temp via the Scangauge from the addition of the previously missing panel. My mileage following installation actually went down on the first long highway trip at speed after the panel was installed, but not because of the panel.
If you're going fast, no doubt every little bit will help somewhat; but to pin down how much benefit accrued to economy as a percentage will be washed out by variables such as tires, temperature, traffic on any particular day, and wind speed, especially in winter with denser air. That's stuff it would take a wind tunnel test to figure at a guess.
My rough estimate/experience is from 100kph to 110 kph there's a real hit to economy; ten percent faster, maybe 20 percent more fuel, to use round numbers. It's a square of the velocity type of thing.
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A major reason I installed the missing underbody panel, was not only for the aerodynamics and completeness, but also because of the deep snow anticipated in coming months that can pack up under the car; melt; freeze; drop down in nasty ice chunks; throw belts; wreck seals!; and worse.
[It may be a moot point for many without deep snow, but I had real bad luck with another small low ground clearance vehicle where similar under engine panels had been removed. [Done at the urging of a good mechanic 'to keep the car running cooler' in summer and save me 'a bunch of problems']. Ya,..True, but: ...
Result in Winter: two new serpentine belts; replace bent crank pulley; replace crank seal damaged by a thrown fouled belt; a rebuilt alternator, due to oil-up and short-out of original; hrs of $hop Time; Loss of Use x2; Aggravation 2x+ .. all from snow packing up where it shouldn't go, forming ice and then melting and falling into pulley belt mechanism because there was no plastic panel to prevent debris from getting up there in the first place! 'nuf said. rant/]
There's a pulley very low down that could present similar grief in the Insight.
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[
complete parts list uk/us pricing see: front fender]
If you do decide to
order a set of panels #12 #17, the one clip [#36] that attaches to the front of each side of the panels to the horizontal aluminum part back of the bumper must be ordered as well; there _is_ a clip [#26] attached with the panel that a 6x16mm 1.25???pitch [best check pitch!] 10mm headed bolt passes though.
fwiw??!: The other four 10mm headed one piece washer-bolts are 6x20mm long with the same pitch cost $2.40/bolt from Honda.
The four 6x20mm 10mm washer-bolts were Insight specific and special order out of Japan = ~4 wks.
[..a metric nut and bolt supplier will have bolts and separate washers to accomplish the same task for much less with no waiting.]