If it looks like a duct, and sounds like a duct…

Update by Scott Lear to the Honda Civic SI project car
Jun 23, 2006

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The front brakes on our Civic are getting too hot on track. Fortunately, the Honda Challenge rules allow for ducting to provide sweet cooling air to those toasty discs and calipers. There are a couple of rules that apply to our situation:

e) Brakes may be ducted from existing holes in the vehicle’s bodywork provided they extend in a forward direction (from brake forward). Auxiliary lights not listed as “required” items in this reulebook may be removed to facilitate brake cooling ducts.

7.10 BODY
A) 11) Two (2) openings may be cut in the front valence to allow up to a three (3) inch diameter duct leading to the front brakes only. Factory fog lights may be removed and holes used for brake ducts or left open.rnThe first part of the rule doesn’t really help us: our Civic Si has no fog lights, and even if it did, the holes on the outer corner of the grilles are right in front of the frame structure that holds the radiator in place, meaning our tubing would have to be fairly small to negotiate past this tight clearance. The second rule suits us perfectly; we’re allowed to cut two openings that allow for a three-inch diamater duct. It’s important to note that the rules don’t state that the opening must be three inches or less, only that the opening must allow up to a three-inch diamater duct. This opens the door to some nice purpose-made ducting pieces.rnOur brake ducting materials arrived from Racer Parts Wholesale. We’re not surprised to be pleased with the quality, the ducting inserts are made of robust black plastic and the 3-inch duct hose is 10 feet long and really well made. We had priced some aluminum ducting materials at the Home Depot and nearly went that route, but these RPW components are designed for the rigors of racing and will hold up better in the long run, and the price was just a few bucks higher.

For the most efficient duct path, we had to cut holes outboard of the lower grille, which is where the bumper starts to curve backwards. Since our RPW duct inlets are fairly flat, this would mean a bit of a gap on the outside edge; it’s not going to look factory, but it falls well within the 20/20 rule of thumb (at 20 miles an hour 20 feet away you’re not going to notice ‘em).rnFirst off we used our all-conquering Dremel tool to cut the duct inlets down to size. A reinforced fiberglass cut-off wheel sliced through the plastic with little difficulty.

Next, we measured the size of the duct inlet’s insert footprint and made a cardboard template to trace on the bumper. We did our best to cut inside the line, but we learned on the first duct that because of the curvature of the bumper we had a slightly larger hole than expected; we were still within the limits and the flange covered the hole with sufficient overlap, but our right-side hole was smaller still and the duct fit more snugly before riveting. Be sure to start small and go bigger.

To cut out a hole in the bumper, we switched our Dremel to a drill bit and cranked it up to 5. At maximum speed, the side of the drill bit slices through the plastic bumper with ease, although it requires a couple of steady hands since the bit will want to walk the dremel in the direction it’s spinning. You get lots of neat warm plastic confetti, so be sure to use eye protection (as you always should when using a tool that spins something at 30,000 rpm).rnNext we started drilling 1/8-inch holes for aluminum rivets. Because we’re dealing with plastic, we put small rivet washers on the inside to prevent pull-through. The bumper’s contours prevented us from doing the spacing exactly even, but they’re in there pretty darn good and that’s all that we really care about.rnWe test-fitted our ducting hose and cut off a piece that was at least a foot longer than we’d actually need; we’ll do the routing soon.

Our first real snag was the low friction between the ducting hose and the duct inlet plastic. No matter how hard we tightened the hose clamp, we the hose would slide off the inlet with a slight tug. We tried removing some of the inner wire skeleton from the end fo the hose and folding the soft material inside to make a tighter seal, and while it helped some it wasn’t a real fix. We needed some kind of flange for the inlet plastic; our rivet gun provided the solution.rnBy inserting four very short aluminum rivets tail-end out relative to the duct inlet, we created a ridge against which the hose clamp could rest. In a happy coincidence, the ridges ended up being very snug against the inner diameter of the duct, so much so that the only way to slide the duct over the inlet was to screw it on using the inner wire’s track as the thread. With the ducts screwed on and hose-clamped down in place, we’re able to lift the bumper off the ground by the duct alone.

All that remains is to route the ducting as close to the brake as we dare. But the lure of delicious fajitas drew us away from the project at this stage, look for a final installment soon.

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