Intake Air Temperature.
The engine intake air temperature has a dramatic effect on the way an internal combustion engine performs. High intake air temperatures will result in decreased power because there's less oxygen per cubic feet of air, also there is a higher chance of detonation occurring. So, why should the intake air temperature be any higher than the outside air temperature of the day?
Firstly, let's look at the case of a normally aspirated engine. Many engines have air intakes that are positioned under the bonnet. This means that a lot of the air that is being drawn into the engine bay has already passed through the radiator - so it's fairly hot! How hot? Surprisingly up to 60 or 70 degrees C on a hot day. Of course, there will also be airflow into the engine bay from around the engine, and past openings like the headlights. But in many cases, on a warm day the air being drawn into an under bonnet engine intake can be as high as 60 degrees! As a rule of thumb, engine power drops by 1 per cent for every 4 degrees C that the intake air temp rises, obviously in the quest for high performance this isn't good.....Just how many high performance air filters have you seen bolted in the engine bay with no heat shields around them? It may look impressive but the scientific fact is they just don’t work like that, high performance engines require cold air to make good power.
For turbocharged or supercharged engines the situation regarding high intake air temps is worse... a lot worse in fact. When a supercharger or turbocharger compresses the air, the air rises in temp considerably. How hot it gets depends on the efficiency of the compressor and how much boost is being used. But to take a common example, with a boost pressure of 1 Bar (~15 psi), the temperature rise is likely to be around 90 degrees C! So on a day with an outside air temperature of 20 C you can be looking at an intake air temp of around 100 -110 C. Also, remember, the turbo could probably drawing in air that's warmer than the day due to high under bonnet temps. All of this is why forced induction cars work best with intercoolers - an intercooler is usually an air to air type heat exchanger designed to bring this temp back down to an acceptable level.
If you ask the driver of a turbocharged car when they think that the highest intake air temps occur they will probably answer "At full boost!” And, if the car is under full load for a minute or so, or on the dyno, that will probably be the truth. But, for a normal road-driven car, that's simply not the case. Instead, the highest intake air temps will occur when the car is sat at the traffic lights, or moving in very slow congested, stop/start traffic. The intercooler won't be working, under bonnet temps will be rising - and the intake air temp will be going sky high.
So, the next time you are sat the traffic lights and contemplating nailing it away from the line, remember the intake air temp has been slowly rising all the time that you have been stationary. When you floor the throttle, there’s a good chance that the engine could detonate (or the ECU pulls the ignition timing way back, stopping detonation and also reducing power). This scenario is particularly common with turbo cars that use engine-mounted intercoolers... hot air rises remember, so the intercooler will be extremely hot
Another common problem is heat soak. Drive a car on a hot day until it is up to operating temp and then park it. Get back in after half an hour or so and it's not uncommon to see intake air temps of 70 or 80 degrees C for the first few minutes, remaining high for a couple of miles of driving. Forced induction cars with water/air intercooling systems will stay high in intake air temp for 10 or 15 minutes, as all that thermal mass of the water needs to be cooled down.
In all cars then, the lower the intake air temperature, the better the overall performance will be.
So we have established that the intake air temperature is important. But how do we know just what’s going on with all these temperatures? We could rig up some test equipment and drive around to check out the readings, but this type of equipment can be bulky and rather expensive, especially if you want to check multiple readings. Also, once you start driving around you will notice that these temperatures are constantly changing depending on your driving mood, traffic conditions and the outside air temperature. What is really needed is a small and versatile in dash display unit that can be installed permanently to measure several engine parameters. An intake air/boost temperature gauge - especially on a car with forced induction - is a very important accessory to have. In fact, it can be an engine lifesaver if you push your motor hard as you will be able to spot the danger signs early. Also if you are into modifying your car’s air intake with a cold flow type you can see if all the work has been worth it. Read on.
Exhaust Gas Temperature.
As the engine works harder, so the temperature of the exhaust gases increase. In a naturally aspirated car, the EGT at full load might be 700 - 750 degrees C if the fuel/air mixture is correct. If the air/fuel ratio is overly lean, this temp will dramatically increase to perhaps 850 or 900 degrees. In a turbo car *, pre-turbo temps of 900 degrees are common at high loads, with post-turbo temps being 800 degrees or more. So as you can see, turbo cars run EGTs much closer to danger. Some turbos are rated at 900 degrees continuous, but - in the real world - if you measure anything over 800 degrees C after the turbo you're starting to push it a bit. However, as with all gauges, the worth of an EGT gauge is in spotting sudden changes away from the norm - they could be indicative of a faulty injector or some other problem with the engine fuel, boost or ignition timing.
* Turbo charged vehicles will read a temperature drop across the exhaust turbine as heat is converted into energy to drive the turbine, this is quite normal and is usually around 100 or so degrees C.
