Intercooling System

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The Complete Guide to Intercooling

If you run a turbo or blown car, you need an intercooler for best performance.

By Julian Edgar

When a turbo or supercharger compresses air, the air is heated up. While this hot air can be fed straight into the intake of the engine (and often is), there are two disadvantages in taking this approach.

Firstly, warm air has less density than cool air - this means that it weighs less. It's important to know that it's the mass of air breathed by the engine that determines power, not the volume. So if the engine is being fed warm, high pressure air, the maximum power possible is significantly lower than if it is inhaling cold, high pressure air. The second problem with an engine breathing warm air is that the likelihood of detonation is increased. Detonation is a process of unstable combustion, where the flame front does not move progressively through the combustion chamber. Instead, the air/fuel mixture explodes into action. When this occurs, damage to the pistons, rings or head can very quickly happen.

If the temperature of the air can be reduced following the turbo or supercharger, the engine will have the potential to safely develop a higher power output. Intercoolers are used to cause this temperature drop.

Temperature Increase:

There are a number of factors that affect the temperature increase that occurs when the air is compressed. Firstly, the higher the boost pressure, the greater will be the temperature increase. As a rule of thumb, if you are using a boost pressure level of more than about 0.5 Bar (~ 7 psi), an intercooler is generally a worthwhile investment.

Secondly, the lower the efficiency of the compressor, the higher the outlet air temp. However, it is difficult to accurately estimate the efficiency of the compressor and even if such a figure is available, it doesn't necessarily apply to all the different airflows that the compressor is capable of producing. In other words, there will be some combinations of airflow and boost pressure where the compressor is working at peak efficiency - and other areas where it isn't. While a well-matched compressor should be at peak efficiency most of the time, in some situations it will be working at less than optimum efficiency. This will change the outlet air temperature, usually for the worse.

Thirdly, the turbo- or supercharged car engine is not working in steady-state conditions. A typical forced induction road car might be on boost for only 5 per cent of the time, and even when it is on boost, it is perhaps for only 20 seconds at a stretch. Any decent forced induction road car will be travelling at well over 160 km/h if given 20 seconds of full boost from a standstill, meaning that longer periods of high boost occur only when hill-climbing, towing or driving at maximum speed. While all of the engine systems should be designed with the maximum full load capability in mind, in reality very few cars will ever experience this. This factor means that the heat-sink ability of the intake system must be considered.

If the inlet air temperature of the engine in cruise condition is 20°C above ambient, then on a 25° day the inlet air temp will be 45°C. After 30 minutes or so of running, all of the different components of the intake system will also have stabilised at around this temperature. If the engine then comes on boost and there is a sudden rise in the temp of the air being introduced to this system, the temperature of the turbo compressor cover (or blower housing), inlet duct, throttle body, plenum chamber, and inlet runners will all increase. These components increase in temp because they are removing heat from the intake air, limiting the magnitude of the initial rise in the actual intake air temperature. As a result, the infrequent short bursts of boost used in a typical road-driven forced-induction car often produce a lower initial intake air temperature than expected. This doesn't mean that intercooling is not worthwhile - it certainly is - but that the theory of the temperature increase doesn't always match reality.

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Intercooler Efficiency

An intercooler will do two things - it will lower the temperature of the intake air and at the same time, cause a slight drop in boost pressure. The latter comes from the restriction to flow caused by the intercooler. Some restriction is unavoidable because the flow through an efficient intercooler core needs to be turbulent if a lot of the air is to come in contact with the heat exchanger surfaces. However, if the pressure drop is too high, power will suffer. A pressure drop of 1-2 psi can be considered acceptable if it is accompanied by good intercooler efficiency.

Intercooler efficiency is a measurement of how effective the intercooler is at reducing the inlet air temperature. If the intercooler reduces the temperature of the air exiting the compressor to ambient, the intercooler will be 100 per cent efficient. It will also be a bloody marvel, because no conventional intercooler can actually achieve this! Typical figures for a good intercooler are around 70 per cent.

Intercooler Types:

Most intercoolers fall into two categories - air/air and air/water. There are also those special designs that cool the intake air to below ambient temperatures, using ice, the air-conditioning system or direct nitrous oxide sprays, but they will not be covered here.

Air/Air Intercoolers:

Air/air intercoolers are the most common type, both in factory forced induction cars and aftermarket. They are technically simple, rugged and reliable. An air/air intercooler consists of a tube and fin radiator. The induction air passes through thin rectangular cross-section tubes that are stacked on top of the other. Often inside the tubes are fins that are designed to create turbulence and so improve heat exchange. Between the tubes are more fins, usually bent in a zig-zag formation. Invariably, air/air intercoolers are constructed from aluminium. The induction air flows through the many tubes. The air is then exposed to a very large surface area of conductive aluminium that absorbs and transfers the heat through the thickness of metal. Outside air - driven through the core by the forward motion of the car - takes this heat away, transferring it from the intake air to the atmosphere.

Described above is what is normally called the intercooler 'core' - the part of the intercooler that actually effects the heat transfer. However, there also needs to be an efficient way of carrying the intake air to each of the tiny tubes that pass through the core. End-tanks are used for this, being welded at each end of the core. While some cores are 'double-pass' (the inlet and outlet tanks are at one end separated by a divider, while at the other end the air does a U-turn), most cores are single-pass, with the inlet at one end of the core and the outlet at the other.

Good intercooler manufacturers have two specifications available - the pressure drop at a rated airflow (with the airflow often expressed as engine power), and the cooling effect (normally expressed as a temperature drop at that rated flow). However, many intercooler manufacturers have no data available on either of these factors! To some extent this doesn't matter greatly - the design of the intercooler is normally limited by factors other than heat transfer ability and pressure drop. Because an air/air intercooler uses ambient air as the cooling medium, an air/air intercooler cannot be too efficient - simply, the bigger the intercooler, the better. In fact, the maximum size of an air/air intercooler is normally dictated by the amount of space available at the front of the car and the size of your wallet, rather than any other factors!

It's easy to see how cost is a vital factor - those forced induction cars produced by major car companies as homologation specials (either for rallying or circuit racing) have quite huge intercoolers that dwarf the ones fitted by the same companies to their humdrum cars. Nissan used an air/air core no less than 60 x 30 x 6cm on their R32 Nissan Skyline GT-R and the Mitsubishi Lancer Evolution vehicles also use huge intercoolers. The "bigger is better" philosophy can be clearly seen at work in these cars.

Many factory-fitted intercoolers are undersized. Air/air cores no larger than a paperback book can be found in turbo cars with a nominal maximum output of 150kW. Cars equipped with this type of intercooler can be held at peak power for only a very short time before the increasing inlet air temperature causes the ECU to retard timing or decrease boost. A car fitted with this type of tiny factory intercooler is almost impossible to dyno test - the intake air temp rises so fast that rarely can more than one consecutive dyno run be made before the intake air temp is so high that the engine detonates... On the other hand, the aforesaid Skyline GT-R has a measured intake temp of 45C on a 35C day at 1 Bar boost and a sustained full-throttle 250 km/h!

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Intercooler Mounting

When either increasing the size of a factory intercooler or installing a new one for a custom forced aspirated car, care needs to be given to the location that is chosen. The first point to consider is the amount of ambient heat that is present. An intercooler core absorbs heat just as well as it sheds it. This means that an underbonnet intercooler core can easily become an intake air pre-heater if care isn't taken with its location. Turbo cars run especially high underbonnet temps and so a bonnet vent designed for intercooler cooling while the car is under way can easily become a "chimney" ducting out hot air while the car is stationery - hot air that passes straight through the intercooler core. In fact, the behaviour of the intercooler while the vehicle is stopped is very important if you're in the habit of caning the car in traffic light Grands Prix!

By far the best location for an intercooler is in front of the engine radiator. The car manufacturer will have aerodynamically tested the vehicle to ensure that large volumes of air pass through the engine cooling radiator, and so an intercooler placed in front of that is sure to receive a great amount of cooling air. Note that the intercooler should be in front of any air conditioning condenser as well!

The air/air core should be ducted with the cold air if at all possible. Many people simply place the intercooler at the front of the car, hoping that the air being forced through the front grille will all pass through the intercooler. However, if there is an easier path for the air to take, that's the way it will go. Sheet metal guides can be used to channel the air coming in the grille through the intercooler, and foam rubber strips can be used to seal the escape routes that the air might otherwise take.

The plumbing leading to and from the intercooler should produce only a minimal pressure drop. Factory turbo cars often use intake ducts that smoothly increase in size from the diameter of the turbo compressor outlet (often only 50mm or so) to the inlet diameter of the throttle body (perhaps 80mm) and if this can be done, it's an approach which should be followed. Intercooler plumbing should have gentle curves and be as short as possible. Don't forget when you are planning the plumbing that the engine (and so also the blower or turbo!) moves around, while the body-mounted intercooler core does not. This means that some rubber or silicone hose connections must to be incorporated in the plumbing to absorb the movement.

The return duct from the intercooler should be insulated to avoid it picking up heat from within the engine bay. Lagging the pipe with fibreglass or ceramic fibre matting works effectively without being too bulky. The pipework can be finished off with a wrapping of aluminium adhesive tape of the type sometimes used to seal roofs. Also note when planning the intercooler pipework that the compressor cover of a turbo can be easily rotated to allow the outlet to come out at a different angle. This can reduce the number and tightness of the bends required.

Some people believe that if they fit a very big intercooler with large ducts, the volume of charge air within it will unduly slow throttle response. Their concern is unjustified however - throttle response problems (for example, turbo lag) are largely the result of other factors within the forced induction system, not the volume of air within it.

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Sourcing the Core

There are a number of ways of getting together a very good air/air intercooler. Those companies specialising in the production of intercoolers (Spearco in the US is one of the largest) have a huge variety of cores and end-tanks available. However, as an aluminium item of fairly intricate construction, they are not cheap. For a really big air/air intercooler complete with end tanks, expect to pay about as much as you would for a turbo.

An alternative in Australia are the Japanese importing wreckers. While few factory turbo cars have really large intercoolers (and even less factory supercharged cars have them!), there are at least a couple of large ones available. As mentioned previously, the Nissan Skyline GT-R and Mitsubishi Evolution model Lancers all have very good intercoolers. The Nissan Pulsar GTiR also has a large intercooler (pictured), while the Mazda RX7 single turbo Series 4 has an engine-mounted intercooler that has a good flow, despite its appearance. Welding two of the RX7 intercoolers in series has also been shown to work very well.

You can also produce your own intercooler by modifying heat exchanger cores designed for other duties. However, having personally done so, I can advise that it is a great deal of work! One source of efficient heat exchangers are old airconditioners. Domestic and industrial refrigerative airconditioners use copper tube and aluminium fin heat exchangers for both their evaporators and condensers. When the airconditioner is discarded (perhaps because of a faulty compressor) these components are sold off at scrap value - less than the price of a few spark plugs! If you are patient and handy, you can cut off each end of the core and make plates that fit over the multiple copper tubes. Making end tanks that attach to these baseplates is then straightforward. The resulting copper-cored air/air intercooler is efficient and very, very cheap.

Another alternative it is to visit truck wreckers. Diesel turbo truck intercoolers are absolutely huge. They can also often be picked up very cheaply from insurance repair jobs, where the core has been twisted slightly, or one end tank damaged perhaps. If you chose with an eye to modification, the core will be able to be shortened without new end tanks being required - which substantially reduces the amount of work! This way you need only make new blanking plates for the ends of the shortened tanks. However, be aware that reducing the number of tubes of a truck intercooler in this manner can also reduce its flow by an unacceptable amount.

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Water/Air Intercooling - Everyone always talks about air/air intercoolers. So what are the pros and cons of water/air designs?

Water/air intercooling is used less frequently than the air/air approach. However, it has several benefits, especially in cramped engine bays. A water/air intercooler uses a compact heat exchanger located under the bonnet and normally placed in-line with the compressor-to-throttle body path. The heat is transferred to water which is then pumped through a dedicated front-mounted radiator cooled by the airflow generated by the car's movement. A water/air intercooler system consists of these major parts: the heat exchanger, radiator, pump, control system, and plumbing.

Technically, a water/air intercooler has some distinct cooling advantages on road cars. Water has a much higher specific heat value than air. The 'specific heat value' figure shows how much energy a substance can absorb for each degree temp it rises by. A substance good at absorbing energy has a high specific heat value, while one that gets hot quickly has a low specific heat. Something with a high specific heat value can obviously absorb (and then later get rid of) lots of energy - good for cooling down the air.

Air has a specific heat value of 1.01 (at a constant pressure), while the figure for water is 4.18. In other words, for each increase in temp by one degree, the same mass of water can absorb some four times more energy than air. Or, there can be vastly less flow of water than air to get the same job done. Incidentally, note that pure water is best - its specific heat value is actually degraded by 6 per cent when 23 per cent anti-freeze is added! Other commonly available fluids don't even come close to water's specific heat value.

The high specific heat value of water has a real advantage in its heat sinking affect. An air/water heat exchanger designed so that it has a reasonable volume of water within it can absorb a great deal of heat during a boost spike. Even before the water pump has a chance to transfer in cool water, the heat exchanger has absorbed considerable heat from the intake airstream. It's this characteristic that makes a water/air intercooling system as efficient in normal urban driving with the pump stopped as it is with it running! To explain, the water in the heat exchanger absorbs the heat from the boosted air, feeding it back into the airstream once the car is off boost and the intake air is cooler. I am not suggesting that you don't worry about fitting a water pump, but it is a reminder that in normal driving the intercooler works in a quite different way to how it needs to perform during sustained full throttle. However, the downside of this is once the water in the system has got hot (for example, after you've been driving and then parked for a while), it takes some time for the water to cool down once you again drive off.

The Heat Exchanger:

Off the shelf water/air heat exchangers are much rarer than air/air types. Water/air intercooling has been used in cars produced by Lotus, Subaru and Toyota. A few aftermarket manufacturers also produce them. If you want to make your own, the easiest way to go about it is to jacket an air/air core. Pick an air/air intercooler that uses a fairly compact core that still flows well. If it uses cast alloy end tanks (as opposed to pressed sheet aluminium) then so much the better. (Plastic end tank types need not apply!) The core is then enclosed in 3mm aluminium sheet, TIG welded into place. Water attachment points can be made by welding alloy blocks to the sheet metal, with these blocks then drilled and tapped to take barbed hose fittings. Pressure-test the water jacket to make sure that it actually does seal, and make sure that the water flow from one hose fitting to the other can't bypass the core. Small baffles can be used to ensure that the water does fully circulate before exiting.

Another type of water/air heat exchanger can be made using a copper tube stack. These small heat exchangers are normally used to cool boat engine oil, exchanging the heat with engine coolant or river or seawater. While the complete unit uses a cast iron enclosure and so is too heavy and large for car applications, the core piece itself can be enclosed to make a very efficient heat exchanger. Comprising a whole series of small-bore copper tubes joining two endplates, the core is cylindrical in shape and relatively easy to package. The induction air flows through the tubes while a water-tight sheet metal jacket can be soldered around the cylinder. The resulting heat exchanger is a little like a steam engine boiler, with induction air instead of fire passing down the boiler tubes! The one here is shown installed on a car undergoing fuel pump testing.

As with air/air designs, the more efficient that you can make the heat exchanger, the better is the potential system performance. If you plan to use an off-the-shelf heat exchanger that has specifications available for it, you will be interested to know that the 150kW turbo Subaru Liberty (Legacy) RS uses a factory-fitted water/air exchanger that has a 4kW capacity. This heat exchanger also works quite effectively when power is increased to about 210kW. Remember in your design considerations that you want a reasonable store of water in the actual heat exchanger (2 or 3 litres at least) to help absorb the temperature spikes.

Radiator and Pump

The front-mounted radiator for the water/air intercooler should be completely separate to the engine cooling radiator. Some turbo trucks use the engine coolant to cool the water/air ntercooler, but their efficiency is much reduced by taking this approach. Suitable radiators that can be used include large oil coolers, car air conditioning condenser cores, and scrap domestic air conditioning condensers. If you use a car airconditioning condenser there is likely to be available a small dedicated electric fan that attaches to the core easily. This fan can be triggered to aid cooling when the vehicle is stationary. The radiator should at least match (and preferably) exceed the cooling capacity of the heat exchanger, but again finding proper specifications is often difficult. The Subaru Liberty (Legacy) RS with the 4kW heat exchanger uses quite a small radiator, only 45 x 35 x 3cm.

An electric pump is the simplest way of circulating the water, with the type of pump chosen influenced by how the pump is to be operated. Some factory systems have the pump running at low speed continuously, switching to high speed at certain combinations of throttle position and engine airflow. If you follow a similar approach, the pump that is chosen must be capable of continuous operation. Another approach is to trigger the pump only when on boost, or to trigger a timing circuit that keeps the pump running for another (say) 30 seconds after the engine is off-boost. The latter type of operation will mean that the pump operating time is drastically reduced over continuous running.

Twelve volt water pumps fall into two basic types - impeller and diaphragm. An impeller pump is of the low pressure, high flow type. In operation it is quiet with low vibration levels. A diaphragm pump can develop much higher pressures but generally with lower flows. A diaphragm pump is noisy and must be rubber-mounted in a car.

Suitable impeller type pumps are used in boats as bilge pumps and for deck washing. They are relatively cheap and have very high flows - 30 litres a minute is common. However, they are not designed for continuous operation and generally don't have service kits available for the repair of any worn out parts. Diaphragm pumps are used to spray agricultural chemicals and to supply the pressurised water for use in boat and caravan showers and sinks. They are available in very durable designs suitable for continuous running and have repair kits available. Flows of up to 20 litres a minute are common and they develop enough pressure (45 psi) to push the water through the front mounted radiator and heat exchanger without any problems.

The factory water/air intercooler system in the Subaru Liberty RS uses an impeller-type pump rated at 15 litres a minute (all flow figures are open-flow). It is automatically switched from low to high speed as required. This is an ideal pump because it was designed by Subaru to circulate the water in a water/air intercooling system! However, it is a very expensive to buy new, but if one can be sourced secondhand it is ideal.

A cheap and simple impeller pump is the Whale GP99 electric pump. It is so small that the in-line pump can be supported by the hoses that connect to it. It flows 11 litres a minute and has 12mm hose fittings. It is 136 x 36mm in size and is suitable for discontinuous operation. This pump is available from marine and caravan suppliers.

The Flojet 4100-143 4000 is a diaphragm pump suitable for water/air intercooler use. The US-manufactured pump uses a permanent magnet brush-type fan-cooled motor with ball-bearings and is fully rebuildable. The pumping head uses four diaphragms which are flexed by a wobble plate attached to the motor's shaft. The 19 litre/minute pump uses ¾ inch fittings and is 230mm long and 86mm in diameter. It is available from companies supplying agricultural spray equipment.

The Flojet pump needs to be mounted either vertically with the pump head at the bottom, or horizontally with the vent slots in the head facing downwards. This is to stop any fluid draining into the motor if there are any sealing problems in the pump head. At its peak pressure of 280 kPa (40 psi), the pump can draw up to 14 amps; however, in intercooler operation the pressure is vastly less and so the pump draws only about 5.5 amps at 12 volts. The pump is noisy (as all diaphragm pumps are) but mounting it on a rubber gearbox crossmember mount effectively quietens it. Note that these pumps are much louder when mounted to the car's bodywork than they are when sitting on the bench!

Control Systems

As already indicated, there are a number of ways of controlling the pump operation. The simplest is to switch the pump on and off with a boost pressure switch. This means that whenever there is positive manifold pressure, the pump circulates the water from the heat exchanger through the radiator and back to the heat exchanger. If boost is used frequently and for only short periods, this approach works well. However, it is better if a timer circuit is used so that the pump continues to operate for a short period after boost is finished.

A suitable pressure switch is an adjustable Hobbs unit (pictured), available from auto instrument suppliers. However, this switch is relatively expensive and a cheaper unit is easily found. Spa bath suppliers stock a pressure-operated switch that is ideal for forced aspirated car use. The pressure switch is designed to work as part of the air-actuated switching system which is used in a spa bath so that bathers don't have to directly operate high voltage switches. The switch triggers at around 1 psi and costs about half that of a traditional automotive pressure switch. If a switching pressure above 1 psi is required, simply tee a variable bleed into the pressure line leading to the switch. Adjusting the amount of bleed will change the switch-on point.

Another approach to triggering pump operation is to use a throttle switch. A micro switch (available cheaply from electronics stores) can be used to turn on the pump whenever a throttle position over (say) half is reached. A cam can be cut from aluminium sheet and attached to the end of the throttle shaft. If shaped with care, it will turn on the switch gently and then keep it switched on at throttle positions greater than the switch-on opening throttle angle.

If a two-speed pump operation is required, the pump can be fed current through a dropping resistor to provide the slow speed. When full speed is required, the dropping resistor can be bypassed. Suitable dropping resistors are the ballast resistors used in older ignition systems or the resistor pack used in series with some injectors. The value of the resistor that is used will depend on the pump current and its other operating characteristics. In all cases, the resistor will need to dissipate quite a lot of power and so will need to be of the high wattage, ceramic type. The resistor will get very hot and can be placed on a transistor-type heat sink mounted within the airstream, perhaps behind the grille. When experimenting with resistors and a pump, you should know that placing the multiple resistors in parallel will increase pump speed while wiring the resistors in series will slow the pump.

Another approach is to use a temperature switch, so that the pump doesn't run when the intake air is not actually hot. This situation can occur on boost if the intake air temperature is very low because the day is cold. Overly cold intake air can cause atomisation problems, although this is not normally a problem in a high performance car being driven hard!

However, running the pump when the intake air is perhaps only 5 is pointless and it can be avoided by placing a normally-open temperature switch in series with the boost pressure or throttle position switches. If the switch closes at temperatures above (say) 30 degrees, the pump will operate only when it actually needs to. A range of low cost temperature switches is available from RS Components (stores world-wide). Note that in all pump control systems a relay should be used to operate the pump

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The Water Plumbing:

The most obvious place for the pump to be within the system is immediately after the radiator, so that it is then subjected only to relatively cool water temperatures. However, this can't always be done because some designs of pump are reluctant to suck through the restriction posed by the radiator. Depending on the design of the radiator, its flow restriction may be substantial. During the assembly of the system it is therefore wise to set it all up on the bench. Check water flows with the pump running (at different speeds, if this is the approach to be taken) and with the pump in different positions within the system. The pump position that yields the greatest water flow should be the one adopted - even if that places the pump immediately after the heat exchanger. In practice, the temperature of the water exiting the heat exchanger will not be extremely high if the water volume circulating through the system is adequate.

A header tank should be positioned at the highest point of the system. This should incorporate a filler cap and can actually be part of the heat exchanger if required. Note that a water/air system can be pressurised if required by the use of a radiator-type sealing cap. Be careful that the system design allows air to be bled from any spots where it will become trapped. Air in the system degrades performance and can cause pump problems. A filter placed in front of the pump is a good idea and very cheap water filters can be found in the garden irrigation section of hardware stores. These filters use a fine plastic mesh design and can be easily placed in-line.

Selecting an Intercooling System:

Both air/air and water/air systems have their own benefits and disadvantages. Air/air systems are generally lighter than water/air, especially when the mass of the water (1kg a litre!) is taken into account. An air/air system is less complex and if something does go wrong (the intercooler develops a leak for example), the engine behaviour will normally change noticeably. This is not the case with water/air, where if a water hose springs a leak or the pump ceases to work it will not be immediately obvious. However, an air/air intercooler uses much longer ducting and it can be very difficult to package a bulky air/air core at the front of the car - and get the ducts to it! Finally, an air/air intercooler is normally cheaper than a water/air system.

A water/air intercooler is very suitable where the engine bay is tight. Getting a couple of flexible water hoses to a front radiator is easy and the heat exchanger core can be made quite compact. A water/air system is very suitable for a road car, with the thermal mass of the water meaning that temperature spikes are absorbed with ease. However, note that if driven hard and then parked, the water within the system will normally become quite warm through underbonnet heat soak. This results in high intake air temperatures after the car is re-started as the hot water takes some time to cool down.

Advantages/Disadvantages:

Type of Intercooling:
Air/Air
Advantage:
* Efficient
* Cheap
* Japanese import cores readily available
Disadvantages:
* Longer induction air path
* Packaging of large intercoolers difficult
* Large pipes to and from intercooler required

Water/Air
Advantage:
* Short induction air path
* Easy to package
* Excellent for short power bursts (ie typical road use)
Disadvantages:
* Heavier
* More complex
* More expensive
* Heat exchangers harder to source