Air Water Intercooler (AWIC)

George wondered, "If I put a tap on the AWIC, can I brew a nice cup of tea?"

Hot Stuff

At wide open throttle and full boost the charge air coming from a turbocharger is around 100 deg C. (At the high end if you’re flogging a little turbo). This is good for stewing chicken, but colder charge air has a smaller volume so we can pack more charge into the cylinders and enjoy a reduced possibility of detonation. Intercoolers make horsepower.

I’m Suby mid-engined. The essentials are

• Subaru EJ20G 1990

• 2.0litre

• 199bhp

• 8.5:1 CR

• 10lbs boost using a VF10 ( a tiny angry snail by modern standards)

The Subaru intercooler is five core, aluminium of course and 3.9kg dry. The system holds 1.9 litres of water. The intercooler is sized to remove up to 3.9kw of heat from the charge air and the heat exchanger sized slightly larger at 5.5kw. At full boost then the intercooler handles the equivalent of four electric kettles!!!!

The Subaru water pump is another critical piece of kit. It’s an impeller, low 28w/ high 50w, max volume 15litre/min.

Here’s a quiz - when should the pump switch to high power?

(a) on boost

(b) at high AWIC water temps

(c) at wide open throttle

(d) on a timer if any of the above happen

By now you’re probably thinking that’s a lot of fuss and bother, why complicate an intercooler with a water heat transfer step when you could do the same thing in one step by using an air-air intercooler? After all, in the end all the heat goes to the air.

Heat transfer is driven by the heat differential between the hot and cold side. From the moment you use boost and for as long as you keep your foot in, air-air intercooling has a heat differential between the 100C charge air and 20C ambient air flow. In an AWIC the charge air heat transfers into the water readily enough, however there is a smaller differential at the heat exchanger end between the 30C rising to 70C intercooler water and 20 degree ambient cooling air. Practically then the AWIC system struggles to cool its water at heat exhanger end the same rate the turbo pours heat in at the intercooler end – the water acts as a cushion and needs to cool when the engine is off boost.

That’s the downside. The upside is that water is 14X better than air at thermal transfer. The physics of this is too tricky for me. It’s based on water’s thermal mass (it can ‘store’ 4X more heat than air), thin film analysis, and the ability to optimise intercooler turbulator design. Check out “The theory behind heat transfer - Alfa Laval.” If you want to know more..

Freakily enough, off boost the intake air actually cools the water so there is sense in keeping the AWIC pumps running all the time.

But the fuss is worth it to get better packaging. The physics mean that AWICs are much smaller than air-air intercoolers. This translates to shorter intake pipes and less flow restriction/pressure-drop. Nirvana for boosties. Less turbo lag, and more pressure going into the engine. For most driving we only use boost for about 10 seconds at a burst which is easily handled by the thermal mass of the water.

The Interesting Option

Check out the small size of this AWIC compared with its (alleged) 350bhp performance. Check www.frozenboost - lotsa cool stuff.

Better still for mid-engine owners the heat exchanger can be anywhere you can run a water hose - from the mid to the side or front of the car, and to one or more heat exchangers that sit in the air stream away from hot engines and exhaust pipes.

AWICs have one extra trick up their pipes. Power boats can use all that cold ocean water as a coolant and AWIC temps never get above 15C while landlubbers can add ice and drop the water temperature to below ambient - near zero. For dragsters this means a very cool charge air until the ice melts.

The Dull And Boring Option

It will come as a disappointment then that I used a stock Suby AWIC on my car. It sits a bit high but it was free. It handles 200-220bhp on the Suby and opinion is flow-wise it can handle 250-270bhp. The stock AWIC gets a good write-up on autospeed.com which is about a scientific as the ricer community gets.

The heat exchanger is a Trademe “Chinese takeaway” 405h*215w*54t mm unit designed for a Banshee quad bike, $145.00. It looks well made. It sits in own duct separated from the engine and all air flow through the cars side vents must pass through it.

The pump is a Ford Cobra AWIC pump from Frozen Boost – better flow than a Suby pump. Forget bilge pumps and pressure pumps. They’re noisy, not rated for the job and, to me, look naff. The pump’s fitted upside down because it wouldn’t prime with the intake pipe at the bottom. This way it always has a head of water although there’s a risk that a leak could run through the motor. It’s on rubber mounts

Hoses are 19mm reinforced PVC hose from Burnsco. PVC gets dodgy at 60C so we’ll see…

And finally the pump runs all the time at low speed. When I switch to full boost the pump runs at high speed.

The system ran well enough at Manfeild - who could tell with all that rain. The AWIC was warm to touch.

However I have plans for the future. I’m eyeing the AWIC pictured above so that I can I can up the boost on the tiny but response VF10. That will also clean up the engine bay and get the weight of water lower down in the chassis.

I’m temped to fit an extra water tank up front and maybe fit a fan on the heat exchanger – we’ll see how it all works in practice.

The pump switch is dumb so it’d be nice to include a thermal controller with a few sensors for ambient and water temp. and switch to full speed at (Boost>0) OR (Throttle >80%) OR (water temps>30C), to stay on for about 5 minutes once these conditions had passed.

Update

I've added an ECU from a different EJ20G (an RS) and the car is now good for about 220bhp and 12lbs boost. The stock AWIC has gone!! And a Frozen Boost unit is now in place. The pix below shows the layout.

The HKS SSQV blow off valve is vented to atmosphere. A plumb-back would be quieter and prevent over-riching the mixture on down shifts but, phisshhhh phishhhhhhhhh, it seems to intimidate boy racers so it's staying the way it is.

The Frozen Boost unit has lumpy welds but is solid and shows no sign of leaking water or boost.

The AWIC packages well. Features added include a heat exchanger fan, and a water filler cap at the highest point in the system.

The heat exchanger remains cool to the touch after a street run. Nevertheless, the system is always under development. This is the third intercooler and second heat exchanger. The latter has been relocated twice and now I plan to move it to the front of the car in front of the main radiator to get between cooling and weight distribution for track use.

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