An air-to-water intercooler is only part of a system. Coolant has to be moved from the core to somewhere it can lose heat, and then returned. That means a pump, a circuit of hoses and a heat exchanger or reservoir, all working as one loop.
Each part can be sound on its own and the system still perform badly if the parts do not match. A powerful pump with a restrictive circuit, or a large tank with poor flow through it, both undercut the whole design.
The pump's job is to keep coolant moving through the core fast enough that the core always sees cool liquid rather than liquid that has already absorbed heat. If flow is too slow, the coolant in the core warms up and cooling falls away.
Flow also has to be steady across the whole core, not concentrated in one path. Uneven flow means part of the core works and part does not, which is the liquid version of poor air distribution in an air-to-air unit.
The heat picked up by the coolant has to be rejected somewhere. Some systems use a small radiator in the airflow; others rely on a reservoir that simply holds a large volume of liquid to absorb heat over a short event.
Which is used depends on how long the system has to work. A radiator can reject heat continuously, so it suits longer use. A reservoir mostly delays the rise in temperature, which suits brief, intense runs where there is no time to reject heat anyway.
The reservoir holds the coolant and gives the system its thermal buffer. A larger volume takes longer to warm up, which helps during a short run. A well-made aluminum water tank is often the heart of this part of the system, sized to the event and mounted where it can be filled and checked easily.
Some builds add ice to the reservoir before a run, using the reservoir as an ice box to start the coolant cold. That lowers the starting temperature and buys more cooling during the event, though it is a tactic for competition rather than daily use.
Air trapped in a water system is a common reason for poor performance. Air pockets reduce flow, stop the pump working properly and can leave parts of the core dry. A system that is not bled correctly will never cool as it should.
Good design makes bleeding easier: high points where air can gather, a sensible fill point and a layout that lets air rise to a place it can escape. This is a detail that is much easier to build in than to fix later.
A water system needs more attention than an air-to-air core. Coolant level, pump operation and the condition of the hoses all have to be checked, and any leak means a loss of cooling as well as a mess in the engine bay.
Keeping the surrounding hardware tidy helps. Even small parts such as a catch can are part of keeping the bay and the fluid systems under control. A water system rewards owners who understand what it is doing and check it accordingly.
