Honda Fit (GE8) DIY Short-Ram / Cold-Air Intake Using the Factory MAF Tube

Pick a conical filter with a 3-inch or 2.5-inch inlet; the engine bay is cramped, so a smaller filter fits more easily. The single dimension that matters most: the salvaged MAF tube measures 2 inch at its cut end, so the coupler that joins the connector pipe to that tube must step down to 2 inch. For a 3-inch filter use a 3-to-2 inch reducer; for a 2.5-inch filter use a 2.5-to-2 inch reducer. Both are uncommon silicone sizes -- search for silicone turbo couplers online, or check hardware-store plumbing sections (one builder found a rubber 3-to-2 inch reducer in the Home Depot plumbing aisle). Also gather a short piece of straight pipe matching the filter inlet, several screw-type hose clamps, and a small breather filter for the crankcase line. Do not substitute a 2.5-to-3 inch coupler on the MAF tube (see the assembly step); the original builder tried that and it caused a problem. If you want the mod to be reversible, buy a spare factory airbox lid, Honda part 17210-RB0-000, which includes its own MAF tube.

Unplug the MAF sensor connector and free its wiring retainer clip. Remove the two 10mm bolts securing the airbox (one is low on the left, the other is nearer the battery on the right). Using a Phillips screwdriver, loosen the clamp ring on the large rubber elbow where the box meets the throttle body, then rock the box off the throttle body. Pull the crankcase breather tube off its port with a firm tug. Twist and work the airbox up and out of the bay; if it will not clear, pull the battery to gain room.

Separate the upper lid of the airbox, which carries the MAF sensor and the MAF tube. Remove the MAF sensor from the lid (leave the small coupling beside it alone). Also pull the short straight rubber elbow from inside the box and set it aside -- you will reuse it between the MAF tube and the throttle body, unless you bought a separate 2.5-to-2.5 inch coupler to replace it.

This is the irreversible step: once you begin, the car will not run until the new intake is assembled, so work when you have time. Cut the plastic box away from the MAF tube a little at a time, staying clear of the tube's bore -- never let the blade bite into the tube itself, especially if you have no spare lid. When the tube is freed, dress the rough edges with a metal file and wire brush, then blow all plastic shavings out with compressed air so nothing gets ingested. You may paint the exposed plastic to tidy the scuffs; little of it shows once installed.

Trim the straight pipe to roughly 2 to 2.5 inch long -- just enough to seat inside both the reducing coupler and the filter with no wasted gap between them. Keeping the assembly as short as possible is what lets it fit the tight space later, so err on the short side unless you are running a very small filter.

Build the tract in this order from the engine outward: factory rubber elbow, MAF tube, reducing coupler, connector pipe, filter. On the rubber elbow, the end with the small lip and four cut dashes (slightly wider inside) goes onto the MAF tube; the half-lip end (slightly narrower) faces the throttle body. Orient the MAF tube with its ridged/circular end toward the engine and the heavily cut end toward the filter. Slip the 2-inch side of the reducing coupler over the cut end of the MAF tube. Critical correction from the discussion: use a coupler that actually steps to 2 inch on the MAF tube. Do NOT fit an oversized 2.5-inch coupler and pack the gap with vinyl tape -- a builder did this and a strip of tape was drawn into the bore, obstructing airflow. Also do not overtighten the clamp on the plastic MAF tube; it can crack. Secure each joint with a hose clamp, tight enough to seal without crushing the plastic.

It is a very tight fit. Tuck the filter into the pocket under the brake-fluid reservoir, pushing it as far to the right as it will go. Loosen the hose clamps so the joints can flex, work the rubber elbow onto the throttle body, and clamp it down. Then retighten every clamp. Reinstall the MAF sensor into its tube and reconnect its electrical plug, and fit the small breather filter onto the crankcase line. Position the filter so it does not face directly into the airflow at the front of the car, which reduces the chance of drawing water in wet conditions.

Because the intake changed, reset the ECU so it re-learns fuel trims. Clear its memory either by unhooking the battery lead for a minute or two, or by reaching the ECU (the black box behind the battery) and unplugging the top wiring harness -- there are three, and the top one carries the power wire -- then reconnect it; this clears codes while keeping radio presets. Let the engine idle a few minutes to relearn idle, then drive normally so it can relearn the MAF, MAP, O2, and IAT values. If a check-engine light appears, first confirm the MAF connector is plugged in and that the tube diameter at the sensor is the factory 2 inch, since a wrong diameter can trigger a P0171 lean code. A code thrown by a momentarily unplugged MAF clears itself within a couple of drive cycles.