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.
Inlet Charge Temperature: You won’t find any modern vehicle huffing hot under hood air, an
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.
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.
Cool It: Keeping the coolant temperature to a minimum helps power output and reduces the t
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.
Lighting It Off: Modern electronics make tuning the ignition curve a much simpler prospect
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: Less variation in cylinder to cylinder fuel distribution allows more powe
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.
Up until this point we have been discussing factors to maximize the pump-gas potential of an engine, without even getting inside the engine. The cylinder head plays a key role in determining detonation tolerance. Just by switching to an aluminum cylinder head, a useful increase in compression ratio of up to a full ratio point of compression can be employed. Beyond the material itself, the combustion chamber design is a factor here, with compact chambers, small volumes and plugs moved inwards to a more central position generally able to get more pressure and power out of an octane point.
Heads Up: Cylinder heads and combustion chamber design play a major role in keeping detona
Squish/Quench: An engine’s detonation tolerance is substantially enhanced with squish/quen
Cool Coatings: Thermal barrier coatings are effective in improving heat management within
These designs tend to propagate the burn more quickly, decreasing the potential for end gas light-off. It’s not at all unusual for an efficient aftermarket head to require substantially less total timing to give maximum power, a direct indication that the burn rate is materially quicker. Small closed chambers, which are the norm in today’s heads, provide another benefit, increased quench area.
Mechanical Configuration: Cam timing and especially the intake valve closing position has
The squish/quench effect on engine efficiency has been well documented and researched since early in the last century. What is this effect, you ask? Squish/quench is achieved by building in a close clearance between a substantial portion of the piston-to-head area at top dead center. Research shows that if the piston rises to within .050 inch or closer to the flat of the head, combustion is improved. Squish occurs as the piston closes the gap in the quench portion of the head as it approaches TDC, rapidly displacing the mixture in this area, creating combustion-promoting turbulence and speeding the burn.
In the compression process, the gasses in the chamber reach a very high temperature. As the propagating flame front expands, the pressure can get high enough to auto-ignite the end gas at the far side of the chamber. With a tight quench clearance, most of these end gasses are squeezed out near TDC, reducing the chances of auto-ignition (detonation). The temperature of auto-ignition is 1375 (f), so clearly the cylinder head temperature is significantly cooler than the end gas temperature at or near autoignition levels. Due to the temperature differential, the thin layer of detonation-prone hot gasses at the extremities of the chamber is actually cooled by the proximity to the head, further diminishing the tendency to detonate. It is from this cooling effect that the term “quench” is derived.
An engine with an effective squish/quench will gain significant detonation resistance, as well as improved torque production. Consider .040-inch to be an effective target for piston to quench-area clearance.
The Ratio: Cranking compression can give an indication of the trapping efficiency of the c
We’ve discussed how the cylinder heads at the top-end of the engine can provide a substantial benefit to pump gas power through detonation tolerance, but what about the basic mechanical configuration of the bottom end? Here there are numerous theories, and opinions, but in most instances, little empirical data. Testing at MIT clearly established a relationship between bore size and octane requirements, and the results indicated larger bores increased combustion time and temperature, increasing the tendency to detonate. However, that testing covered substantial variations in bore size, from 21⁄2 to 6 inches. We doubt there is significant enough an effect to warrant consideration in the range of bore sizes typical of high performance street engines.
Another aspect of engine configuration that has gained a following is the use of shorter rods. These will result in quicker piston motion in the vicinity of TDC. Proponents of the short rod theory claim there is an advantage gained by the accelerated volume gain away from TDC as combustion propagates, though we wonder if there are power losses due to accelerated pressure decay. We’ve seen no empirical data to quantify these theories. We do know that shorter rods increase internal friction, and cylinder wall loading. The bottom line is we wouldn’t lose any sleep over trying to find an edge in unusual engine configurations.
There are several types of coatings that have gained popularity in recent years, and it’s for good reason—they work. From the standpoint of making the most power with pump gas, Thermal Barriers Coatings (TBC’s) are right on target. TBC’s are designed to reduce the transference of heat. Valves, having limited means to transfer heat via the seats and guides, tend to be among the hottest component inside an engine. Intake valves are cooled considerably by the air/fuel charge rushing past, but as the valves are being cooled, the induction charge is being detrimentally heated. Temperature builds in the exhaust valves, making them the hottest component in the combustion space. Exhaust valve heat has been found to be a major contributor to end-gas temperature rise, and the resultant detonation.
The exhaust gas crossover is designed to add heat for fuel vaporization in cold weather, b
Thermal barrier coating of the combustion face of the valves has become popular to reduce the valve’s operating temperatures, and our own experience has indicated a genuine reduction in the tendency for the engine to detonate. While valves, pistons, and chambers are the most common applications for TBC’s, port surfaces and intake manifolds are among other areas where some builders are employing these coatings in an effort to reduce induction charge temperature.
Valve Timing Variables
We all know that pressure in the cylinder plays a critical role in making power, but we also realize that too much pressure will contribute to the onset of detonation. The camshaft plays a vital role in determining just what operating pressure the engine will see. How does cam timing relate to detonation tolerance, power, and cylinder pressure? We’ll examine some of the key points.
From the standpoint of cylinder pressure, the most relevant event in cam timing is the intake valve closing point. The intake valve closes while the piston is on its way up on the compression stroke, and there isn’t any compressing going on until the intake valve is shut. Intake closing is affected by duration, lobe separation angle, and installed centerline. Long duration, wide centers, and later installed centerlines all allow more compression due to the reduced trapping efficiency of the later intake closing point. As a rule of thumb, half a point of compression can be added for every 10-degrees of duration over 220-degrees at .050 inch.
Detonation can be encouraged by the use of inappropriate components. The plugs must dissip
Aluminum cylinder heads have much better thermal characteristics than iron, and will typic
With all this talk about heat, pressure, and detonation, we’ve purposely left the topic of compression ratio for last. Compression ratios in production engines have been trending upward as OEM’s search for improved power and efficiency, with many new performance engines including the SRT-8 Mopars pushing upwards of 11.0:1 stock. So how much ratio can you run in your Mopar build? If you do everything wrong, we’ve seen plenty of Mopar engines ready to cough their guts out at 8.0:1. Real world experience taking the information here to heart, we have consistently run 10.0:1 compression in short duration street applications, focusing primarily on the thermal management factors detailed here. In a more serious effort with cold air induction, awesome engine cooling, aluminum heads, coated valves, ignition and carb tuning and quench, I’ve run over 11.0:1 on pump gas with nary a trace of detonation.
The bottom line here is that it comes down to the execution of the overall combination, and as they say, your success will vary. It really isn’t that hard to build a 10-11:1 true street Mopar that lives happily on today’s pump gas, but it takes attention to the details. Building an engine with all of these tricks makes it possible.