Cooling Systems
Keeping Your Mini Cool: The Workings of the Coolant System 
Tech Tip: Overheating Issues
Cars that overheat "at speed" may be sending the water through the radiator too quickly, not allowing sufficient time for the water to be cooled before returning to the engine. Typical cars are those with high numerical gearing, 3.75-4.35, turning high rpms. Fixing this problem is a simple matter of changing pulley sizes. The bigger the pulley, the slower the flow. CAUTION: Too large a pulley may cause overheating at idle!