Friday, June 20, 2008

Engine Inlet Temperature

Looking through the visitor stats for this site, it seems that lots of people are interested in upgrading the intercoolers on the Ghibli. I have to admit that this is something that I looked into myself, since the stock air/air coolers seem a bit small compared with the huge aftermarket coolers that can be purchased for the likes of VW Golfs and Mitsubishi Evos.

So, I pulled up one of the data logs from my PLX Devices R-500, since I have it tapped into the air temperature sensor on the throttle body. This sensor measures the temperature of the charge air entering the engine, after it has been cooled by the intercooler. I have to admit that I expected the temperature to be in the range of 70 to 90 degrees since after going for an enthusiastic drive, the throttle body and plenum chamber are too hot to keep my finger on.

The picture above is a screen grab of the PLX Logger software charting the data in 2D. It's a bit rough... I've been having trouble getting reliable RPM data from the engine speed tap and so this needs to be multiplied by 5 to give the correct engine speed (I think I'm going to tap into one of the ignition timing wires instead). The MAP values are also a little bit out. However, the chart gives a good enough idea of what's happening and the temperature values are correct.

What you see is that at idle, when the car is stationary and no cooling air is flowing across the intercoolers, the inlet temperature is up around 50 degrees Centigrade. It's important to realise that the ambient temperature when these reading were taken was around 5 degrees Centigrade, so pretty cool. As soon as the car starts moving, the inlet temperature rapidly starts to decrease to a steady state around 30 degrees above the ambient temperature. When the car is driven hard at around 380 seconds, there are significant peaks in the inlet temperature. The temperature is quick to fall back to a steady state less than 40 degrees Centigrade.

I still need to capture some log data for summer temperature, but I suspect that steady state with an ambient temperature around 30 degrees Centigrade will be about 70 degrees Centigrade. This is less than I expected and my conclusion from this is that, unless the car is going to be raced on a track where the engine is kept at full load for a long time, then there's no huge benefit to changing out the stock intercoolers if the car is being run with 1.20 barg inlet pressure. Bear in mind that if the car is run on the track, there will be a good flow of air all the time over the intercoolers helping to keep them cool. The highest inlet temperatures are occurring when the car is stationary or moving slowly and increasing the intercooler size is not going to help this.

What I'm therefore thinking of doing is adding a cooling fan behind each intercooler, connected to the water radiator cooling fans. These fans generally only activate when the car is moving slowly or stationary so would be a good control for the intercooler fans. This should then keep the intercoolers at a relatively low temperature when driving around town for example, or when the car is cooling down after a hard drive.

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