It's one of the biggest challenges in engine building, getting the most power from each octane point of fuel. If you think these considerations are something new, you'd be surprised--it's a topic that's been at the forefront of internal combustion engine research since the early days of the automotive engine. Making the most of an engine combination is a lot more involved than just running as much compression ratio as you think you can get away with. When you look at what's happening in an engine, the power output is directly linked to cylinder pressure applied during the power stroke, and there are many factors affecting this directly. The best measure of this force is the BMEP, or Brake Mean Effective Pressure, and if you spend much time on the dyno, you'll notice the BMEP moves in almost lockstep with the torque curve. What puts a ceiling on the BMEP? Usually, it's the engine's detonation limit of the fuel being used.
The exhaust gas crossover...
The exhaust gas crossover is designed to add heat--good for vaporizing fuel when driving in cold weather, but murder on octane requirement in hot weather. Consider that aluminum transfers heat at five times the rate of cast iron, and you might be tempted to block the crossover in your aluminum intake if performance is the ultimate goal.
Detonation
The topic of detonation has generated some interest, since power output can only be increased until it's capped by detonation. We're always looking to make more power, so if we're limited to pump gas, we need to build engines that will go further up the BMEP scale, and make more power before detonation. Just what is detonation? It is accepted that detonation is the result of autoignition of the end gas, which is the part of the air/fuel charge that has not yet been consumed by the normal flame travel. The expanding combustion gases involved in the normal propagation of the flame front raise the pressure and temperature of the remaining gases to a point where the end gas auto-ignites. If the autoignition is violent and encompasses a significant portion of the remaining mixture, you've got full-fledged detonation. With detonation will come a very high-pressure spike in the combustion chamber, producing a pressure wave and temperature rise that damages engine parts in a hurry.
Fortunately, there are a number of factors we can control when building an engine that pushes the detonation tolerance upward. Let's first look at what past scientific research has proven to be fundamentally true when it comes to combustion theory, and then dig into how that theory can be practically applied to our performance cars.
Race-style intake manifolds...
Race-style intake manifolds feature divorced runners, with a separate valley plate isolating the passages from the hot valley below. No exhaust crossover to be found here. Cooler air is denser, adding power, while lower temperatures reduce the tendency to detonate.
Inlet Charge Temperature
Initial temperature has a direct influence on the detonation limit, with a higher initial charge temperature resulting in a greater tendency towards detonation. As the temperature of the inlet charge increases, the temperature of the end gas in the combustion process increases at a given pressure, making the engine more prone to detonate. You can see the practical application of combating this effect in practically any newer vehicle, as nearly all are equipped with some form of cold air induction. A cold air induction provides a dual benefit in performance applications, increasing the charge density for greater power, while allowing the engine to tolerate higher cylinder pressures with a given fuel. In the old days, most vehicles would gulp nothing but underhood air, and it is surprising how many performance cars are set up in exactly the same way today. A cold-air induction system can drop the inlet air temperature from a typical figure of around 180 degrees, to a much more digestible 100 degrees, or lower.
While the air inlet temperature is a factor, it isn't exactly the same thing as inlet charge temperature in the combustion chamber. The thing to consider here is keeping the charge cool once it finds its way past the carburetor or throttle body. Where are the potential sources of heat gain? The biggest potential culprit is the exhaust crossover originally found in many older vehicles, and also built into quite a few aftermarket performance manifolds. The exhaust crossover passage plumbs a constant flow of exhaust gas right through the center of the manifold, often with a substantial area in the passage directly below the manifold's plenum. Imagine the heating effect to the inlet charge of plumbing exhaust gasses of up to 1,000 degrees right through the manifold. In fact, transferring heat to the induction charge is precisely the reason the exhaust crossover was put there in the first place. The heat helps vaporize the fuel for better cold distribution, and actually warms the manifold to prevent part-throttle carb icing in very cold climates. If maximum pump gas performance is the goal, the exhaust crossover is really working against the effort.
While the crossover is an obvious source of manifold heat that is quickly soaked by the induction system, there are other sources of heat that also play a part. The manifold of most V8 engines also serves as the cover for the lifter valley, making the intake subject to all the heat cooking up in the engine's lifter valley. Racers identified this as a potential power loss, and various heat shields have been available to help diminish the effect. Better still, a design originally applied to race manifolds, and showing up more and more on performance street intakes is the divorced runner, with a separate valley plate isolating the valley, and the runners open to the air from below. Examples here include many race single-planes, and Edelbrock's AirGap series of two-planes. While we've covered the most obvious offenders for heat transfer into the induction charge, there are other more subtle heat sources that can be considered. In looking to minimize the heat reaching the intake, we've seen competitors in the Engine Masters Challenge experiment with heat transfer resistant materials for intake manifold gaskets, and even seeking to minimize the heat transfer from the intake manifold and carb fasteners. More conventionally, spacers or isolators under the carb can contribute to reduced charge temperature. Any temperature reduction here will help in making power on pump gas.
Minimizing coolant temperature...
Minimizing coolant temperature helps in the pursuit of power, and raises the detonation limit. A high-quality aftermarket radiator can substantially decrease operating temperature. Flex-a-lite's internal fins perform as heat sinks absorbing heat more quickly from the engine coolant, then radiating it through external fins to dissipate heat twice as efficiently.
Keep it Cool
The effect of coolant temperature on detonation is similar to that of inlet temperature, and the reason is strangely the same: lower end-gas temperature. Here the direction is pretty clear: minimize the coolant temperature, and the potential is there to make more power on pump gas. Anyone who has had the experienced of driving an under-cooled but edgy performance powerplant in scalding weather is pretty familiar with the result. Here the goal is to maximize the efficiency of the cooling system, which by itself will increase power, while allowing more power to be produced on pump gas.
There are also a couple of other considerations often missed when considering cooling. Hot-rodded engines make more power and are almost always tougher to keep cool than a stocker. Most of us have rebuilt, over-bored engines, which are generally harder to keep cool, depending on the remaining cylinder wall thickness. Finally, perhaps more than 95 percent of engines rebuilt are simply hot tanked or jet-cleaned, which does nothing to clean the surface rust scale inside the water jacket beyond the loose crust. Acid-dipping is about the only effective means to really clean the jackets of a used engine, an operation seldom performed. The rust layer is an effective thermal insulator, making it tougher to transfer heat out of the engine and into the cooling system.
All of this adds up to a real need for an exceptional cooling system. The most obvious component here is the radiator. The secret to effective cooling is effective heat transfer, and the path to getting there is capacity. The radiator is the workhorse of the cooling system, with the job of dissipating the heat carried by the coolant into the air. To do its job, an effective fan is a must, along with a shroud. Radiators are not all created equal, and can vary in their construction, size and efficiency. Until recent years, automotive radiators were of copper/brass tube-and-fin construction, and most manufacturers varied the cooling capacity of different models by radiator size, both in cross-sectional size, and in core thickness. The rule for selection here is a no-brainer: bigger is better both in cross section and thickness.
What's happening in the water...
What's happening in the water jackets is an important aspect in cooling efficiency. For optimal heat transfer, the water jackets should be clean of scale at the time of a rebuild if a used block is used. An additive like Royal Purple's Purple Ice helps heat transfer from metal engine components into the coolant.
Aluminum radiators have become quite popular in recent years, and for good reason. Aluminum radiators look good, are lighter, and have been found to be very efficient in cooling. Technically, the rate of heat dissipation for brass is higher than aluminum, however one of the primary advantages of aluminum is mechanical. Aluminum is much stronger than brass, allowing for the tubes to be built with a much more efficient elongated shape. This is a primary reason for their outstanding cooling capabilities.
The radiator and fan don't work alone when it comes to keeping the engine cool. The water pump is another part that needs consideration. The stock water pump is fine for a stock or mild engine, but there are circumstances where an upgrade can be an advantage. High-compression, increased cylinder pressure, rpm, and power output all increase the demands on the cooling system. A high-efficiency water pump is an effective way to increase the cooling capacity under such compromised conditions. The increased coolant flow and pressure will help "scrub" heat from localized hot spots, improving cooling. There are a number of manufacturers that produce high-capacity replacement water pumps, including Milodon, Edelbrock, and Weiand, which move significantly more water at lower rpm, and are designed to lessen the impeller cavitation at higher engine speeds. A cooling system needs a system control, and that's handled by the thermostat. Thermostats are rated by temperature at the point when they begin to open. As the engine's temperature rises, the `stat opens to allow more coolant flow through the engine. A lower rated `stat won't make the engine run cooler if the rest of the system can't dissipate the heat fast enough. However, if the capacity is there, a cooler thermostat will result in a cooler-running engine. Generally, a 180-degree thermostat is the recommended piece for a street engine, though a cooler 160-degree unit can result in a slight power increase, and reduced tendency toward detonation, if the cooling system has the capacity to keep the temperature down.
Conventional reverse-poppet valve thermostats are designed in such a way that the valve opens against the flow of the engine. This creates a situation wherein the coolant flow pressure has an effect on the thermostat's opening. With high-flow water pumps, the pressure differential can be enough to cause a change in the operational opening rate of the thermostat. Improved "balanced flow" `stats utilize a balanced sleeve design, which results in equal pressure at both sides of the valve, so that its operation is solely regulated by coolant temperature. A balanced sleeve thermostat, like one of those offered by Milodon, is highly recommended with a high-flow water pump, since the added pressure can try to hold a conventional thermostat's valve closed.