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Project Honda Insight, Part 3 - Building an Airbox

Building a high-flowing airbox to fit in the tiny space

by Julian Edgar

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At a glance...

  • Custom-designed and fabricated airbox
  • Uses cylindrical airbox design and cylindrical airfilter
  • Very high flow in a compact enclosure
  • Easy fabrication
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This series is based around a 2001 model hybrid Honda Insight.

The Insight remains one of the most aerodynamic and lightest cars ever made, with a Cd of 0.25 and a total mass of about 850kg from its 2-seater aluminium body.

The intent of the project is to turbocharge the engine, add water/air intercooling and programmable engine management, and then provide new high voltage batteries and a new electric motor control system.

The aim is to build a car with the best performance/economy compromise of any in the world.

The series so far:

Project Honda Insight, Part 1 – Introduction

Project Honda Insight, Part 2 – Fitting an Alternator

This issue: building a high-flow and compact airbox

A new airbox was designed and fitted to the Honda.

Click for larger image

So why not use the standard Insight airbox (pictured) and just connect it to the turbo intake? There were a few issues with taking that approach:

1. The inlet and outlet ducts of the standard airfilter box didn’t suit the air path needed for the turbo. The standard airbox connects to a front-mounted throttle body, whereas the turbo was to be positioned behind the engine.

2. Here in Australia, the standard Honda filter element is rare and expensive – so if a more common, cheaper filter could be used, that would be advantageous.

3. Intake pressure drop needed to be kept to an absolute minimum if fuel economy was to remain outstanding. Despite the standard airbox having a very low (measured) intake restriction, an increase in power (and so airflow) needed to be catered for.

I looked long and hard at using a filter box from another car but none I could find with a large filter would fit into the space - and also have the inlet and outlet ducts in the correct places. The odd thing was that the Honda engine bay actually has quite a lot of room above the gearbox for a large airbox, but the way that space is organised is unconventional.

I therefore decided to build my own airbox to suit just this application.

One airfilter shape that would have fitted in the available space was a long and thin rectangle. But this would have needed in turn a long, thin box that separated into two halves (so that you could change the filter) – and such a design would be pretty difficult to construct. And anyway, a long and thin filter gave a fairly small filter area.

So after lots of thinking about different options, the following design was developed:

  • Used was a cylindrical paper filter element, 290mm long x 118mm in diameter, normally fitted as standard to a Saab 9000. For my money, standard paper filter elements have the best mix of dust catching, pressure drop and cost characteristics of any filter type. A cylindrical filter also has a very large area (imagine it unrolled) – the Saab filter looks to be sized for well over 200kW.

  • The filter element is housed in a cylindrical airbox, made from 150mm diameter, thin-wall truck steel exhaust pipe. Taking this approach meant that most of the airbox was made before I even started.

Building it

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The starting point was the 300mm length of 150mm diameter, thin-wall truck exhaust pipe. I tried finding an offcut but to no avail – in the end I ordered 1 metre of the tube from a local exhaust shop. The 150mm diameter tube gives about 15mm air space all around the cylindrical filter.

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A depression was formed in one of the walls of the tube to give the required clearance between the airbox and the Honda’s starter motor power terminal. This depression was formed in the tube in the following way:

1. Discs that were a tight fit in the ends of the tube were cut from chipboard.

2. The tube was fully filled with sand, kept in place by the chipboard discs and some temporary tape.

3. A 50mm steel tow ball and a hydraulic press were used to form the depression in the tube wall.

Note that both the sand and the chipboard endplates are needed if the tube is not to be crushed. Despite the depression being relatively shallow, a surprisingly large amount of clearance was formed through making this dent.

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At the outlet end of the airbox, a ring was cut from thin steel plate and then welded into place. This ring was then drilled and nutserts (arrowed) installed. (The nuts could also have been brazed into place prior to the ring being installed.)

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A 60mm hole was cut in the removable lid of the airbox. A cone adaptor was formed in steel exhaust tube to adapt the 60mm airbox outlet to the 50mm tube that runs to the turbo inlet duct. The adapter was formed using a cone-shaped steel mandrel and the hydraulic press. An inner extension was used to locate and seal against the internal hole of the airfilter.

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A long exit tube was welded to the lid. The airbox lid is held in place with three screws that pass into the nutserts installed in the inner ring. A rubber gasket sits between the lid and the ring. The exit tube then connects via a silicon coupling to a cast alloy pipe that runs to the mouth of the turbo.

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At the bottom of the airbox, a steel endplate was welded into place. To locate the filter at this end, three small steel spigots were brazed into place prior to the endplate being attached.

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The filter will insert fully into the airbox only when the far end of the filter is nestling within the locating spigots.

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The intake to the airbox comprises a 63mm tube cut at an angle and then welded to the wall of the airbox tube. Prior to welding, an oval-shaped opening to suit the tube was cut in the airbox wall.

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A plastic flared extension is connected to the intake via a short length of rubber hose. This flare is the cut off end of a plastic subwoofer vent tube. This intake to the airbox is positioned behind the left-hand headlight.

No cold air, ram-pressure intake?

Rather than using a forward-facing duct that can breathe only cold air, the airbox intake is located behind the left-hand headlight. There were two reasons for not using a cold-air, ram pressure intake:

1. I wanted to use a 63mm (2.5 inch) diameter pipe for the airbox intake, and creating a duct of this cross-sectional area that could reach across to the right-hand side of the engine bay (an area where access to cold, higher pressure air could be gained) was nearly impossible. With the intercooler-to-throttle pipe in place, there simply wasn’t room for another transverse pipe.

2. I think it very likely that at times I will not want a cold air intake. Measurements of fuel economy on this car (and, as it happens, also on the Honda F1 turbo cars of the 1980s) showed that warmer air gave better fuel economy. Working out how warm air can be provided to the engine, even on cold days, has been an ever-present concern (and has also influenced the water/air intercooler system design – to be covered later). That said, I also don’t want a hot air intake – the ‘behind headlight’ location represents a compromise.

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Three mounting brackets were cut from 30 x 4mm steel bar and were welded directly to the airbox wall. Thick steel was used for these mounts because, as the airbox mounts on the gearbox, the assembly moves with the engine – and this 3-cylinder engine has a fair amount of vibration.

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Flared end tubes to allow the blow-off valve and PCV hoses to connect to the intake were brazed to the exit duct of the airbox. The airbox was mounted at an angle that allows it to fit under the steeply sloping bonnet. Mounting the airbox at an angle was fundamental to fitting such a large airfilter under the bonnet line.

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The airbox was bead blasted, painted with a zinc coating and then powdercoated in wrinkle finish black.


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Despite being placed partly under the throttle feed duct, the new airbox dominates the revised under-bonnet scenery – it is huge. It should also provide extremely low flow restriction and, as a side benefit, the large area of the filter should mean it needs to be changed only rarely.

Next: intercooling requirements

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