RB26 Air Intake Temperature and Heat Soak

The RB26 has an air intake temperature sensor located in the intake plenum. If you research about it, you'll generally find people providing feedback that the sensor responds slowly and is impacted by heat soak. I plugged my laptop into my car's stock ECU via an ECUTalk cable and utilised their software to log the temperature sensor as I drove to see how it responds.

Drive 1

To give a bit of context about the drive:

• I started logging at approx. 5:30PM and ended at approx. 6:30PM.
• The drive was about 1 hour.
• I was driving during peak hour weekday traffic. This resulted in a lot of idling in traffic as well as low speeds.
• Ambient temperature was about 20°C.

Entire log.

After about 20mins of driving (including a few minutes of warm up idling), the temperature sensor went from 19°C to 50°C. It then essentially stayed between 50°C and 59°C for the majority of the drive, dipping down into the high 40s when I had some road speed. The ambient temperature was definitely not 59°C and so I believe its a matter of the engine heat transferring itself to the sensor.

Quite a progressive ramp up.

Drive 2

To give a bit of context about the return drive:

• The car sat for approx. 3.5 hours from the end of Drive 1 until 9:08PM, where I then started logging again. The logging process and drive continued until about 9:40PM.
• The drive was about 30 mins.
• Since it was much later, the road had less cars on it and was more free flowing, although there was still traffic around.
• Ambient temperature was about 14°C.

Entire log.

Now what's really interesting is that even though the engine had been off for approx. 3.5 hours, the air temperature sensor was reading 40°C once I started the engine again. That's 51°C to 40°C in 3.5 hours. Not much of a drop! This can really show how heat soak stays in the engine even for such a long period of time after it is shut off.

I then waited about 30-60 seconds and started to drive very lightly. Fast forward 3 minutes later to 9:11PM from engine start up and and the temperature sensor was now reading 25°C. The ambient temperature was about 14°C which I'm sure helped, however all it took was 3 minutes to almost halve its value.

One sneaky thing to note is that even though the air intake temperature dropped from 40°C to 25°C in 3 minutes, it also crept up to 36°C 7 minutes later at 9:18PM. I believe this to be from the engine warming up and some of that engine heat transferring to the temperature sensor.

A very sudden drop, followed by a somewhat progressive ramp up.

You can see that the air intake temperature was 35°C to 43°C for the rest of the drive.

My theory for why the temperature drops with more road speed is that generally the engine would be at a higher RPM allowing more air to pass through the intake system into the engine. I believe this air would essentially run past the temperature sensor and cool it down (to a point). But then once idling around, the amount of air flowing past the sensor is less, and more engine bay heat builds up which then in-turn heats up the sensor. I have a stock airbox installed in the car which has a snorkel on it reaching out of the engine bay and into the front bumper area (so its always getting fresh air). But if you have uncovered pod filters installed instead, I imagine the engine bay heat would also play into it as well and potentially slow down how quick the temperature sensor readings drop.

As you can probably tell, it doesn't seem to be the most reliable or amazing system for monitoring the air intake temperature, but remember the R32 GT-R was designed in the 80s! The sensor works, but I wouldn't 100% trust what its picking up. A metal sensor connected to a metal intake plenum and then bolted to a metal intake manifold (between paper gaskets in its defence) seems to allow a lot of heat transfer. Aftermarket sensor solutions do exist which are more accurate, respond quicker to changes in temperature and are not so influenced by direct heat, however these also need to be combined with a tuneable ECU to be able to utilise the different sensor.