Honda Civic SiR

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Underhood Temperature Readings

One of the cheapest ways to increase engine horsepower is to make the intake charge cooler. A cooler intake charge is more dense and, therefore, contains more oxygen per unit of volume. More oxygen generally means more power and thus having colder air entering the engine is desirable.

There are several ways to decrease the intake charge temperature because there are several places where the intake charge can get heated before entering the combustion chamber. The first is by actually sucking in hot air from the engine bay. The rest of the heating of the intake charge occurs as it travels up the intake, through the throttle body, through the intake manifold and finally into the combustion chamber. By reducing the temperature of any of these pieces you reduce the amount of heat that the intake charge absorbs on it's way to the cylinders.

Obviously a good modification is to make sure that you are sucking in cold air. This can be achieved by creating a cold air box in the engine bay. This should be sealed from the hot underhood air, or by sucking air from outside the engine bay altogether. Sucking air from outside the engine bay is problematic in Auckland as it rains very heavily on occassion. This means that there is a chance (albeit small) of a water getting into the engine via the intake. And, if enough water gets sucked in at once, you may be unlucky enough to hydraulic the engine. This is very bad and normally ends up in breaking some major internal part of the engine (water is not compressible, and as the piston moves up on the firing cycle it can't physically be pushed all the way to the top at which point something has to give, i.e. break). So, a cold air box in the engine bay is the safest solution if you too live in a wet climate.

In my current setup the intake piping is also highly polished to help reduce the heating of the intake through radiation. Both the throttle body and intake manifold are painted with a ceramic white paint to help reduce heat soak through conduction and radiation of heat from the engine. There is also a Hondata Heatshield between the intake manifold and head to greatly reduce heat soak from the very hot head.

Cable Free Indoor/Outdoor Thermometer

To see just what the difference in air temperature is between the general underhood and cold air box I purchased a Cable Free Thermometer from Dick Smith Electronics. This thermometer can be used to monitor the difference in temperature between the cabin, underhood and the cold air box. Typically this is used for measuring indoor and outdoor temperatures, but it will do the job that I want it to do nicely.

Cable Free Thermometer

The Cable Free Thermometer uses "433Mhz tranmission technology" to transmit the temperature readings from the remote sensor units to the main sensor unit. This is much easier than using a cable type indoor/outdoor thermometer because you don't have to worry about wires getting in the way. The remote units can handle up to 70°C, and I've only got close to this temperature once. The main unit also stores the maximum and minimum temperature which is useful for seeing how much the underhood temperature increases as the car sits after a long drive.

The main unit supports up to three remote units and I currently have two. One attached to the tower strut brace and the other affixed close to the end of the intake tract. The differences in temperature between the two positions is quite substantial so the cheap cold air box solution I have appears to be working quite well.

Recorded Temperature Measurements

Below is a table listing some of the temperature measurements that have been recorded after driving home from work, which is about a 25 minute drive (including about 10 minutes of motorway driving). The readings from November and December are after installing some Bonnet Spacers to raise the rear of the bonnet to allow more heat to escape.

Recorded temperature readings
Date Cabin Temperature Cold Air Box Temperature Underhood Temperature
22-Sep-2002 14.7°C 14.4°C 43.2°C
24-Sep-2002 16.1°C 15.1°C 49.7°C
03-Aug-2002 14.6°C 14.6°C 43.8°C
25-Nov-2002 15.6°C 17.4°C 51.4°C
25-Nov-2002 31.8°C 27.2°C 54.8°C
04-Dec-2002 17.4°C 23.4°C 60.1°C
09-Dec-2002 13.6°C 24.3°C 58.3°C


As the results show, the cold air box is definitely doing its job properly. I am not so sure about the bonnet vents though because I don't have any other readings to compare them with. The bonnet spacers seem to have incresed the underhood temperatures so I will be removing these and then relook at the measurements. Although with the bonnet spacers in the maximum temperature reading under the bonnet once the car has been sitting is very rarely much higher than 4-5°C higher than the temperature when the car stopped. The previous readings, prior to adding the bonnet spacers, were taken when the ambient temperature was higher which may have affected the readings. (Cabin temperature is not really a good measure of the outside temperature).