HyperFlow  -Design Guidelines and principles for Motocross Race Bike Cooling Systems and Radiators
In most vehicles, small increases in engine power can usually be accommodated by the original cooling system, since there is usually some safety factor designed in and because the vehicle will not always be driven under the worst conditions or highest temperatures. MX cooling systems in current generation high power density engines have no head room, and run on the ragged edge of effective cooling.
Standard factory MX cooling systems are a compromise between cost, mass, size, strength and ability to reject sufficient heat under most circumstances. A combination of high ambient temperatures, a good rider that is using a high percentage of the available power, and a tight track will push most standard systems to the limit.
There are actually many popular misconceptions regarding the heat transfer performance of an engine cooling radiator,  these misunderstandings may  reduce cooling performance rather than improve it.