One of the most common questions we get regarding engine builds is, “How much compression can I run?” It seems like enthusiasts intuitively gravitate towards pushing the compression ratio up as high as practical, and for good reason. With increased compression comes increased efficiency, manifested by improved power and economy. Everyone knows the stock low-compression engines of the early emissions era were dogs by most measures, soft on power while guzzling fuel. Those late smog 318’s 360’s, 400’s, and 440’s often came through with actual measured compression ratios of less than 8.0:1, crippling power output as well as economy. When building a performance engine, optimizing the compression ratio is one of the biggest challenges in putting together a successful combination.

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. Optimizing the compression ratio involves a lot more than just running as much ratio as you think you can get away with. Really, 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. What puts a ceiling on the usable cylinder pressure? Usually, it’s the engine’s detonation limit with a given fuel.

Detonation
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 tolerate just enough cylinder pressure before the onset of detonation. Just what is detonation? Detonation is the result of auto-ignition of a portion of the air/fuel charge (sometimes referred to as the end-gasses) that has not yet been consumed by the normal flame travel. What happens is the expanding combustion gases involved in the normal propagation of the flame front raises the pressure and temperature of the remaining gases to a point where the end gas auto ignites. If the auto ignition 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 a temperature rise, which damages engine parts in a hurry.

It might seem easy enough to just figure out how much pressure it takes for detonation to occur, and then just build the engine with less pressure than that. Well, that would be nice, but there are a multitude of factors that directly affect the detonation limit, including the mechanical components of the engine combination and the operating conditions. Fortunately, we can control a number of these factors when building an engine to push the detonation tolerance upward. We’ll look at these factors and how they influence the detonation tolerance, and then dig into how that theory can be practically applied to our performance engines.

Inlet Charge Temperature
Physics dictates that the very act of compressing a gas directly increases its temperature. Since auto ignition is caused by pressure and temperature, starting out at a higher base induction temperature results in a greater tendency towards detonation. As a rough rule of thumb, a reduction of inlet air temperature of 25 degrees is equivalent to a one point increase in fuel octane. Step one to reducing inlet charge temperature is cold air induction as opposed to using underhood air.

Heat gain does not stop once the air enters the engine, with the exhaust cross over being the biggest offender in older Mopars. It’s not hard to imagine the heat imparted by 1,000-degree (f) exhaust gasses blasting through the manifold. This effect is magnified by aluminum manifolds, which transfer heat at about five times the rate of cast iron. Other sources of heat gain include heat from the lifter valley, which can be reduced with internal heat shields, thermal barrier coatings or an Air-Gap type manifold. We’ve seen engine builders looking to isolate the induction charge with heat transfer resistant materials for intake manifold gaskets. Spacers or isolators under the carb can contribute to reduced charge temperature. Any temperature reduction here will help in making power on pump gas.

Cool It
The effect of coolant temperature on detonation is similar to that of inlet temperature, and the reason is not surprisingly the same: lower gas temperature in the cylinder. Minimizing the coolant temperature reduces the tendency towards detonation and allows more pressure and thus power on pump gas. A rule of thumb here is that a 10-degree drop in operating coolant temperature is equivalent to a gain of one octane point. If you’ve ever driven a Mopar with an overheated engine on a scalding day, you’ve likely experienced the resultant detonation first hand. Keeping the coolant temperatures at a minimum by itself will increase power, reduce the tendency towards detonation, while allowing more power to be produced on pump gas.

Many of our classic Mopars came with barely adequate cooling systems when new, and upgrades are often necessary to control temperature. Radiator capacity is key, helped by a good fan and shroud system, and improved with a high-flow water pump. Generally, a 180-degree thermostat is recommended for a street engine, though a 160-degree unit can provide advantages. Often overlooked, many engines are rebuilt with heavy rust and scale remaining inside the water jackets. This acts as an effective thermal insulator, making it tougher to transfer heat out of the engine and into the cooling system.

Lighting It Off
Ignition timing has a pronounced effect on power and how much power can be made without detonating. The optimal ignition point for max power is a function of cylinder pressure versus crank angle in the running engine. However, if the engine wants to detonate, optimal timing may never be achieved. A general rule of thumb in terms of ignition timing settings in relation to detonation tolerance is that 2-degrees of ignition timing is equivalent to an octane point.

In practical terms it pays to be on the conservative side when tuning the ignition curve to minimize the tendency to detonate. Since most engines respond with diminishing returns as optimal timing is approached, lower spark advance can provide a buffer to detonation without a significant reduction in power. For example, if your 440 make peak power with 38-degrees total advance, a reduction to 34-degress can provide two octane points of cushion for what may be a marginal loss in output.

Fuel Management
Changes in air/fuel ratios have a direct affect on the flame speed, temperature, and the reaction time of the end gasses—all factors in the detonation tolerance. The first thing to consider is what the actual air/fuel ratio is in the cylinder. Rich mixtures do tend to suppress detonation, but at the price of reduced fuel efficiency, and that isn’t usually a good trade-off for a street performance application. The factor often overlooked here is the mixture distribution. Detonation will occur in the lean cylinders, so to compensate, the mixture has to be richer overall. A finer range of mixture control and distribution avoids this compromise, leading to improved efficiency, and power at a higher detonation limit.

Fuel injection is the most accurate means of evening up the distribution, however a carbureted engine can also benefit from improved distribution. Distribution with a carburetor and wet intake manifold is very tricky to optimize, even with a Lambda in each hole, while with EFI it’s practically a given.