Reduced to its common reasons for being essential in an internal combustion engine, cooling system efficiency can have a material influence on parts life and crankshaft torque. These include the reduction of thermal stresses and strains, pre-ignition and detonation (particularly the latter), breakdown of lubrication system effectiveness, distorted cylinder bores, potential damage to pistons and rings, and damaged cylinder head gaskets--among other results from an inefficient cooling system. Interestingly, and traceable to some of the previously-listed consequences of an improperly-operating cooling system, it's known that heat rejected into an engine's cooling system--in a gasoline-fueled engine--has less to do with load and more to do with engine speed. If for no other reason, this condition places particular importance on your choice in coolant pump when building, or rebuilding, a racing engine.
The bottom line becomes establishing a balance among coolant flow rate, coolant residence
In much the same fashion as determining the dimensions of a racing intake manifold (runner lengths and section areas) or set of headers (primary pipe length, pipe section areas, and collector sizing), it's important to consider the speed at which the coolant pump will be operating and its efficiency at this, or range of, rpm. It never hurts to discuss this issue with your choice of coolant pump manufacturer. Many include pump evaluations on a coolant pump dyno, and can tell you at what speed their design is the most efficient, particularly if you provide information on the range of engine rpm you plan to use. Although virtually any of the problems associated with improper cooling should be avoided, detonation likely heads the list. Let's consider a couple of thoughts about that. You're probably aware that there's a fundamental difference between detonation and pre-ignition. Pre-ignition is exactly that; combustion that begins from a source other than a timed and controlled ignition spark. It could originate from a hot-spot, like something over-heated in the combustion space like excessively heated spark plug tips, glowing bits built-up carbon or other unwanted sources of "ignition" can be culprits. Detonation, on the other hand, is a form of spontaneous combustion, brought about by inordinately high temperatures or pressures in the combustion space. Even trace detonation, not evidenced by audible knock, can be damaging if left unchecked. Tell-tale bits of aluminum on plug porcelain and a loss in power characterize detonation, along with really high cylinder pressures that often lead to mechanical damage of parts exposed to these conditions.
Of note is the fact that such excessive pressures in the combustion space are not the only source of damage. High temperatures play a role as well. Temperature gradients that over-stress certain areas in the combustion space can also develop. For example, it's not uncommon to find such gradients develop in cylinder head material lying between the intake and exhaust valve areas. Based upon exhaust valves operating at much higher temperatures than intakes, cracks, and valve seat distortion can result in a damaging temperature gradient between the two. Plus, lubricant breakdown, in the areas of piston rings and ring lands, can also result when they are operating in temperatures above 200 degrees Centigrade.
In addition, believe it or not, an engine's volumetric efficiency can be affected by decreased cooling efficiency, and this sometimes gets lost in the shuffle of typical problems associated with improper cooling. It works like this: Engine heat (particularly as it affects intake manifold temperature), indirectly influences inlet air (and air/fuel charge) temperature. The higher the temperature, the less dense the air. The less dense the air, the less the amount of air will pass into the cylinders during any given inlet cycle. The net effect of this is a reduction in volumetric efficiency, and this points directly to a loss in torque. So it's clear that proper cooling is necessary for much more than preventing an over-heated engine. But there's more to the cooling system story, so let's now focus on coolant pumps.
It's probably fair to compare a conventional coolant pump to the power absorber of a water brake dynamometer. Both move fluid as a function of shaft speed, impeller and impeller cavity design and type of working fluid. Based on pump shaft speed (rpm), pumps can impose parasitic losses to net engine torque, particularly if pumps are not selected based on anticipated engine speed. On engine dynos, we have personally observed power gains of 35-40 hp on racing engines producing 500 hp at 7,000 rpm when pump drive belts broke during a test.
Further, it's important to consider ways of keeping a pump's impeller cavity as close to its capacity volume as possible, throughout a given pump's operating range. By preventing this condition, you run the risk of allowing impeller cavitation, a loss in coolant circulation and reduced cooling efficiency. In a V-type engine, establishing and maintaining an acceptable cylinder bank-to-balance of coolant flow also plays an important role in the pump selection process.
Once again, consultation with your choice of pump manufacturers is a wise decision before making a purchase. And if you can find a cooling pump manufacturer with experience in both pump functions and types of coolants (such as Evans Cooling Systems), finding a solution is made a bit easier. Even if you decide to experiment with an existing pump, adjustments to drive pulley ratios can bring pump shaft speed into a range that addresses the potential cavitation problem (often around the 3,000-5,000 shaft rpm speed).
From a practical standpoint, consultation with a reputable engine builder is yet another route to take when matching coolant pumps. We recall a conversation with Keith Dorton about this subject and, if memory serves, one example he shared referenced his use of Adams & Drake pumps. For engines operating in the 7,500-8,000 range, he said a flow rate on the order of 50 gpm (gallons/minute) seems to work best, especially at tracks he called "bullrings" where cars are pretty well stuck together and engines are more reliant upon cooling system efficiency than cool environmental air. All good suggestions. You will also find that there are differences in heat transfer rates when comparing coolant pumps built from aluminum to those of cast-iron design. "Just one more little thing," (as Smokey would comment) that you don't want to overlook when posing questions to your coolant pump provider.
As a final note, don't forget that as coolant flow rates are adjusted to optimize temperature control, you need to provide a means to keep the coolant sufficiently long in the radiator that it can do its job as well. Once again, the bottom line becomes establishing a balance among coolant flow rate, coolant residence time in the radiator cores, total ignition spark timing, and type of coolant.
Next month, we'll expand and conclude this little discussion on cooling systems and related topics by working through fundamentals of the three basic ways to route coolant through an engine: conventional "forward" flow, reverse flow, and "modified" reverse flow. Each has specific characteristics and benefits, not the least of which relates to companion adjustments that can be made to an engine as a result of which type of coolant routing is used.
Maybe to whet your appetite, would you think that the amount of ignition spark timing possible, short of parts damage, is typically different between two engines, one of which utilizes a "forward" flow route and another using a reverse flow route? How about the possibility that the basic volumetric efficiency (potential torque output) favors the reverse flow approach? On second thought, maybe that's too much whet...