November 14, 2007

Moving the radiator to the rear of the car

Everything runs hotter in Florida. So we spend a lot of time thinking of ways to make our racecars run cooler.
On the Electric Imp, our priority is keeping the batteries cool. Heat over 60 degrees Celsius will damage them.

Our next priority is keeping the inverters cool and motors cool. The inverters need to stay below 75 degrees Celsius. The motors need to stay below 175 degrees Celsius.

The motors and inverters are liquid cooled. Cooled water comes from the radiator, flows through the two inverters, then the two motors. The hot water then flows back to the radiator.

The radiator is at the front of the car. Outside air flows through the radiator picking up the heat from the motors and radiators. That warmed air is then used to cool the batteries.

This is a working system. At our race in Savannah in April, the batteries stayed within their limits. They got hottest, up around 55 C during the long 30+ mile race. 5 C is not a lot of margin. The inverters ran at about 50 C, well below their 75 C limit. The motors hovered at 170 C, very close to their limit.

It would make more sense to use the coldest air to cool the batteries and the slightly warmed air to cool the radiator.


We remove the stock Subaru radiator from the front of the car.

air box with radiator removed

We fit a sturdy air filter (designed for a home air conditioning unit) to keep rocks and dust from the car.

a filter replaces the radiator

We order a custom aluminum racing radiator from Howe Racing Enterprises. We spend some time on the phone with them discussing our needs. The radiators cooling fin area is 10" by 26" by 2". That is 520 cubic inches. The original radiator was 28" by 13" By 1/2" or 182 cubic inches so we should have over double the cooling surface area.

The radiator arrives when promised and looks well made.

fitting the Howe Racing radiator

We fabricate brackets to attach to the radiator tabs.

attach radiator tab

We mount the water pump down low to get keep it full of water.

water pump mounted

The pipes run from the radiator forward with the return to the top opposite corner of the radiator.

radiator mounted with hoses in

The water goes into the first inverter.

to inverter right, from motor left


Out from the second inverter, into the rear motor, out to the front motor, back to the radiator. It is simple.

out from the front inverter, headed to the rear motor

We make the radiator cap the highest point and accessible for quick checking and filling.

radiator fill

For proper air flow, there must be high pressure in front and low pressure in the rear and no other places for the air to go. We get out the Coroplast and Velcro to make this happen.

Some sections become smooth works of art.

radiator to rear exit

Seen from the outside.

smooth air flow

Some less so...

radiator to rear exit pump side

And some are just the best we can do under the circumstance.

radiator to back of battery box

We stick the cover on and we are ready to test.



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April 18, 2007


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