Radiator Caps

To understand why you’ll need to know a little bit about MX-5 cooling systems. As the coolant absorbs heat from the engine it expands and adds pressure to the cooling system. Radiator caps are essentially pressure-controlled valves screwed to the top of your radiator. When the pressure exceeds the limit of the radiator cap (around 0.9Bar/13psi for the OEM cap), the hot coolant will be vented to the expansion tank and eventually the floor.

While this is good for stopping your cooling system blowing itself up, it has a negative effect on the overall cooling efficiency. When the OEM rad cap opens, your car may not actually be overheating, rather it is prematurely letting coolant out.

When you fit a high-pressure radiator cap it allows the whole cooling system to be under more pressure, which raises the boiling point above 100C, allowing the coolant, in this case, water, to contain more heat. The reason having more heat helps is because heat transfers faster the greater the difference in temperature.

Say you have two cars, Car A, which has an OEM cap, and Car B, which has a high-pressure cap. The coolant in Car A is going through the radiator at 95C, this is roughly the highest temperature the coolant can get before the OEM cap opens. The coolant in Car B is going through at 120C. 

The outside air travelling through both radiators is 20C. The coolant is Car B will lose heat at a much faster rate than the coolant in Car A because the difference between the coolant temperature and the air temperature is greater, leading to a larger amount of heat transferred away from the engine and into the air.

Another thing that helps, in the long run, is the water staying in the cooling system and not being sprayed all over the floor. When you’ve finished hooning your car around and go for an out lap, you still have enough coolant to continue effectively cooling your car.

Of course, the next thing you have to worry about is not putting so much pressure on the cooling system that it blows itself up. This is where a set of Silicon hoses and an upgraded radiator come in.