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Project Honda Insight, Part 4 Intercooling Requirements

Sorting out an unusual intercooling system

by Julian Edgar

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At a glance...

  • Requirement for an intercooler with variable efficiency
  • Water/air system
  • Solution: heat exchanger, pump and radiator
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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

Project Honda Insight, Part 3 – Building an Airbox

This issue: the requirements for a water/air intercooling system.

The intercooling system of the Honda is one of the most complex parts of the Insight’s modification - in requirements, design and execution.

Firstly, in this article, the requirements and a brief overview of the solution. Next issue: making it all happen.


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An intercooler is used to cool the air that has been heated by the turbo through its compressing of the intake charge. This heated air has less density, and therefore contains less oxygen – so reducing the amount of power that can be produced. Cooling the air brings back this power potential.

Hot intake air is also more likely to cause engine-destroying detonation. Detonation is where the fuel explodes rather than burning on a progressively expanding flame front. Cooling the intake reduces the likelihood of detonation, allowing more boost and ignition timing advance for the compression ratio being used.

The aim of the intercooling system is usually to produce the lowest possible intake air temp. So if the boosted air exiting the turbo has a temperature of 80 degrees C, reducing this to (say) 50 degrees C is good – and it’s even better if it can be reduced to 35 degrees C!

In a car using a conventional air/air intercooler core, the reduction in intake air temp depends on two factors - the ambient air temp, and the intercooler’s efficiency. The higher the core’s efficiency and the lower the ambient temp, the lower will be the intake charge temp.

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In a water/air intercooled car, the same ‘efficiency’ parameters apply – outside air temp and intercooler system efficiency. However, in a water/air system, there are more factors affecting the intercooler system’s efficiency:

  • underbonnet heat exchanger efficiency

  • front-mounted radiator efficiency

  • the efficiency of the water flow system (pump, lines)

Importantly, the efficiency of a water/air system is easily adjustable, simply by altering the pump speed.

The Honda

So why is all this relevant?

Well, in the Honda I want to be able to regulate post-turbo intake air temperature – rather than just have it as low as possible.

This is the case because of one reason: best fuel economy occurs when the intake air is warm.

Decrease intake air temp below a certain point and fuel economy suffers – presumably, because the port-injected fuel is not atomising as well. Some references suggest that best fuel economy might occur at intake temps as high as 60 or even 70 degrees C – whether that is the case for the Insight, I don’t yet know.

A major reason for selecting a water/air intercooling system on the Insight is to be able to provide, at times, a warmer air intake that would be achievable with just an air/air intercooler.

However, for all the normal reasons for why an intercooler is required on a turbo car, I want also to be able to provide as cool an intake air temp as possible – when it is needed. That is, on high boost.

Achieving these two objectives – low intercooler efficiency off boost (and probably also at low boost levels), and high intercooler efficiency at high boost levels - can be partly achieved by varying the speed of the pump used in the water/air intercooling system.

But there’s another thing to think about - thermal mass.

In a water/air intercooling system, it’s normal to have a fair amount of water in the underbonnet heat exchanger. In fact, many times the underbonnet heat exchanger also comprises the water reservoir of the system. This water gives the system high thermal mass – on a short-term boost event (like accelerating away from traffic lights), the water that is in the core can immediately absorb lots of heat… even before it is passed through the front-mount radiator to cool it.

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In one car I owned with a self-built water/air intercooler system (pictured), measured intake air temps in normal urban driving were much the same with the electric water pump turned on or off.

That seems unbelievable – so what was occurring?

What was happening was that the boost event dumped heat into the water, and this heat then gradually fed back into the intake charge when the off-boost air was cooler than the water in the core.

To put this a different way, there was enough thermal mass in the heat exchanger to absorb the short-term temp spike.

Of course, if the pump were switched off, a long boost event – like climbing a long, steep hill - would cause intake temps to rocket as the stationary water got hotter and hotter. In that case, the pump was needed.

In the case of the Insight, I don’t want a system with a lot of thermal mass in the heat exchanger – if I want the intake air to be warm, then I don’t want the intercooler system to be working… even with the pump switched off.

So where does this leave us?

1) The need is for an intercooling system with an efficiency that can be varied at will – easiest with a water/air system where pump speed is controlled.

2) An underbonnet heat exchanger with low thermal mass means that controlling pump speed will have a more significant impact on intake air temps than if the system had a high thermal mass.

Intercooler bypass?

So why not just use an intercooler bypass, where the intake airflow can also travel through a different, direct turbo-to-throttle-body path? Place a variable flow valve in the bypass and you can have whatever mix of intercooled and non-intercooled air you want.

Such an approach also has the potential to improve off-boost fuel economy through reduced flow restriction (a direct path will have less restriction than one travelling through an intercooler).

But there were two reasons for not taking this approach on the Insight.

1) There is very little room to add another duct, complete with a variable flow valve (eg an electronic throttle body).

2) I think that the Honda will be on boost for most of the time. The Honda uses a small, low-power engine and high gearing – in most driving conditions it has been designed to use wide throttle angles (so gaining better Brake Specific Fuel Consumption). I’d therefore expect that the intercooler will be needed in most driving – and that’s especially the case since much of my driving is at 100 and 110 km/h on country roads.

Other requirements

Another requirement of the intercooler is that it have a very low pressure drop. Any pressure drops in the system (indicative of flow restrictions) will reduce overall engine efficiency, so harming fuel consumption and of course power.

The practical outcome of this is that the underbonnet heat exchanger, and the intake plumbing, needs to be sized as if the engine were developing far more power than it actually is.

And of course there are practical requirements too. The system needs to be able to fit in the car, and it needs to be affordable!


So what is the complexion of the system designed to meet these requirements?

Heat exchanger

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The chosen heat exchanger is a commercial unit constructed from TIG-welded aluminium. Dubbed a “universal liquid/water to air intercooler”, it cost AUD$269 through eBay in Australia. (The design is available much more cheaply in the US.)

The overall size of the unit is 230 x 230 x 90mm, with 63mm (2.5 inch) air inlets and outlets. The water fittings are half-inch NPT, with ¾ inch (19mm) brass barbed hose fittings supplied. The inlet and outlet are at 90 degrees to each other and the design uses cast alloy end tanks.

For the Insight, the benefits of this design were that:

1) It could fit into the available space – that is, where the 12V battery normally lives under the bonnet. The 90-degree dogleg inlet/outlet arrangement was particularly helpful in arranging the plumbing.

2) It uses a core design where only 400ml of water (most in the end tanks) is contained within the heat exchanger body, reducing thermal mass (as is desired in this application). However, that said, at 3kg, the aluminium still has substantial thermal mass.

3) This type of core is used on engines developing hundreds of kilowatts – it is very much oversized for the Insight application, and so the pressure drop through it should be very low.

Front mount radiator

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The front mount radiator takes an unusual approach.

While radiators designed for water/air intercooler use are available, they seem very expensive for what you get. An alternative is to use an engine oil cooler, but probably because they need to withstand high pressures, these too are expensive in large sizes.

Automotive and domestic air conditioning condensers, that I have used previously in water/air intercooler builds, have plumbing diameters that in is this case would be too restrictive.

Instead, I chose to buy the smallest new air/air intercooler core I could find. It cost AUD$99 on eBay. This little design uses 40mm (1.5 inch) plumbing connections, has a core size that is 300 x 155 x 64mm and weighs about 1.8kg.

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I used my lathe to turn-up some aluminium hose fittings that I then TIG welded in place of the standard intercooler hose fittings.

This resulted in a compact, excellent flowing radiator that contains 1 litre of water – this amount of water adding a decent thermal reserve (but one not positioned in the intake airflow).


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The pump that I used is a Johnson Controls CO30P5-1 impeller design. I have had this high quality pump in storage for a long time, waiting for my next water/air intercooler system - it appears it is no longer made. It uses ball bearings and a stainless steel shaft.

The pump has a spec’d flow of 23.3 litres a minute at a ‘head’ of 15kPa (the equivalent of 4.9 feet of vertical lift). It draws 2 amps at 12V. It has ¾ inch (19mm) barbed hose fittings. Testing has shown that it responds very well to variations in supply voltage, still flowing a small amount of water (albeit at no head) at only 4V.

Note that the pump is not self-priming – that is, it cannot draw it up water from a lower level.

Header tank

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A header tank was made that has two functions – it allows water to be added, and it is positioned sufficiently high that no air locks develop in the system.

Because of the tight available space, nothing suitable could be found off the shelf and so the tank was made. I used as the starting point the canister from an old aluminium fire extinguisher. An aluminium radiator filler fitting (eBay) was TIG welded to the top, a mounted bracket added, and a ¾ inch NPT aluminium fitting welded to the wall near the base.

It has an internal volume of 750ml and it is T’eed into the system.


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Hose runs were made from normal ¾ inch heater hose and, where a tighter bend was required, pre-formed moulded rubber hoses designed as aftermarket replacements for specific cars were used.


The system as described will fit under the Insight’s bonnet (and behind the Insight’s bumper), have the ability to vary intake air temp at will, and should have excellent flow and - when required - excellent cooling ability.

Next issue: making it all happen.

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