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...
Heads Up: Cylinder heads and combustion chamber design play a major role in keeping detonation at bay. A compact chamber with ample quench area and a deep, central, sparkplug position, will generally provide a faster burn and reduce the total timing requirement.
Squish/Quench: An engine’s...
Squish/Quench: An engine’s detonation tolerance is substantially enhanced with squish/quench. To be effective the piston must achieve a close proximity to the quench surface of the head, which improves mixture motion and combustion efficiency, while cooling the remaining end-gasses. A piston to head clearance of approximately 0.040-inch is an effective target.
Cool Coatings: Thermal barrier...
Cool Coatings: Thermal barrier coatings are effective in improving heat management within the engine. While these coatings are applicable to the ports, chambers, pistons, and manifold, I have found coating the chamber sides of the valves especially effective in improving detonation tolerance.
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 a direct influence on cylinder pressure and therefore the detonation limit. Computer modeling can predict the effects of a variety of cam events as an aid to component selection.
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...
The Ratio: Cranking compression can give an indication of the trapping efficiency of the cam/compression combination, especially at lower engine speeds. How much pressure can be run on pump gas? Commonly, a cranking compression of 180 psi is considered the upper limit for pump gas octane. With attention to the factors discussed here, we have run 200 psi successfully, and are looking for more by employing every trick in the book.
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...
The exhaust gas crossover is designed to add heat for fuel vaporization in cold weather, but unfortunately it adds considerably to charge temperature and substantially increases octane requirement in the heat. With the naturally high heat conductivity of aluminum, the problem is magnified. Performance applications will generally benefit from blocking the crossover.
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...
Detonation can be encouraged by the use of inappropriate components. The plugs must dissipate sufficient heat for the application. Performance engines often benefit from a cooler heat range as the power level and cylinder pressure increase.
Aluminum cylinder heads have...
Aluminum cylinder heads have much better thermal characteristics than iron, and will typically allow a full point more compression than iron castings. Iron heads can be improved by polishing the chambers and removing sharp edges to minimize potential hot spots.
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.