Fuel ManagementChanges in air/fuel ratios have a direct effect on the flame speed and temperature, as well as the reaction time of the end gasses, all factors in the detonation tolerance of an engine. These facts point out that the mixture is an important factor in making the most power with a given octane fuel. 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. Considering an eight-cylinder engine, there are quite a few holes getting filled every time the crank turns, and the mixture reaching each of these holes can vary substantially. Detonation will occur in the lean cylinders, so to compensate, the mixture has to be richer overall.
Now consider what can happen if a finer range of mixture control is achieved. Without having to go richer overall to bring the lean cylinders into the zone, the power is increased, and the detonation limit is raised at the same time. Fuel injection is the most accurate means of evening up the distribution, and better distribution will equate to more output from a gallon of gasoline. A carbureted engine can also benefit from improved distribution, and in fact we've heard from some of our top Engine Masters competitors that getting the cylinder-to-cylinder distribution dialed-in was a major part of their development effort. The emphasis there obviously had nothing to do with economy, rather the effort was aimed at making the most power overall. With distribution held in a narrow range from cylinder to cylinder, the mixture could be optimized for the engine, without some overly rich cylinders dragging down power while compensating for some lean holes that would otherwise tend to detonate. Distribution with a carburetor and wet intake manifold is very tricky to optimize, even with a Lambda sensor in each hole, while with EFI it's practically a given.
Getting InsideUp until this point we have been discussing factors to maximize the pump-gas potential of an engine, without even getting inside the powerplant. Things like temperature management and precise control of the fuel and ignition systems can readily be employed on an existing engine. There are significant gains in pump gas performance up for grabs when the engine is being built. Again, if maximum power is to be had from a given fuel octane quality, the cylinder pressure potential of the powerplant has to be at the maximum tolerable, short of detonation. This limit can clearly be pushed up, if the tendency to detonate is reduced. There are numerous steps that can be taken when coming up with an engine combination to make it less likely to encounter detonation.
Cylinder head design is an area where all the players are not created equal. Just by going 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, there are other factors in the cylinder head design that increase detonation tolerance and allow higher compression ratios. The combustion chamber design is the biggest factor here, with compact chambers featuring small volumes and plugs moved inward to a more central position in the cylinder generally able to get more power out of an octane point. The major reason once again is time. These designs tend to provide a faster burn rate, propagating 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.
Quench It If You CanThe quench effect on engine efficiency has been well documented and researched since early in the last century. What is the quench effect, you ask? Simply put, it is designing in a close clearance between a substantial portion of the piston area and the bottom of the cylinder head when the piston is at top dead center. A closed-chamber head has a large flat area, the quench surface, over a substantial portion of the bore. It has been found that if the piston rises to within .050 inch or closer to the flat of the head, good things happen in the combustion process. The effects here are multifaceted. First, is the squish effect, wherein as the piston closes the gap in the quench portion of the head as it approaches TDC the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence, 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. Since with a tight quench clearance, most of these end gasses are squeezed out near TDC, the chances of auto-ignition (detonation) are greatly reduced. The temperature of autoignition is approximately 1,375ff,,f,, F. 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 gasses at the extremities of the chamber are 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 quench will be more detonation resistant, and it is typical for surprisingly substantial improvements in torque to result from the more efficient combustion. Most builders consider .040 inch or so to be an effective target for piston-to-quench-area clearance, a spec easily obtained with a closed-chamber head, a piston at zero deck, and a standard FelPro .039-inch compressed thickness gasket.
Mechanical ConfigurationWe've discussed how the cylinder heads at the top end of the engine can provide a substantial benefit to pump gas power, but what about the basic mechanical configuration of the bottom end? 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 2.5 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. Shorter rods will result in quicker piston motion in the vicinity of TDC. Proponents of the short rod theory claim there is an advantage to be had by the accelerated volume gain away from TDC as combustion propagates, though we wonder if there are 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.
Coatings Can HelpThere 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 (TBCs) are right on target. TBCs are designed to reduce the transference of heat. That characteristic can be put to effective use inside an engine in a number of ways. Heat is energy and energy is power, so it follows that keeping heat in the combustion space has the potential for an enhancement in power. Aluminum is a very effective heat conductor, and also happens to define a large portion of an engine's combustion surface area. Barrier coatings applied to piston crowns have become popular in an effort to retain more of the combustion energy in producing power, rather than being lost through absorption through the piston. Similarly, TBCs are commonly used to coat the combustion chambers of aluminum heads, again to curb heat loss.
Heat transfer is a two-way street, and sometimes a TBC is just as valuable in keeping heat out as it is in keeping heat in. 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. While the valves are being cooled, the induction charge is being heated, and we've already discussed the negative effects of that. At the other side of the chamber, the exhaust valves get little relief from heat. Temperature builds in the exhaust valve, making it 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.
Thermal barrier coating can reduce the ingress of combustion heat into the valves, with the promise of cooler surface temperatures. Coating 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 TBCs, port surfaces and intake manifolds are among other areas where some builders are employing these coatings.
Valve Timing VariablesWe 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. Obviously, if the camshaft keeps the valves open all the time, the pistons will go up and down and there won't be any compression happening at all. Given that fact, it's clear that the cam has a substantial effect on the cylinder pressure. So 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 closes. Long-duration camshafts will open the intake valve earlier, and correspondingly close the intake valve later, which reduces the cylinder pressure at lower rpm. With long-duration camshafts, the compression ratio should be increased to compensate. Other aspects of the camshaft alter the intake valve closing point, and therefore have a direct affect on cylinder pressure. At a given duration level, a wider lobe separation angle will close the intake valve later, bleeding off some of the cylinder pressure. Similarly, when the installed centerline is altered, the cam timing will change. Advancing the camshaft will increase the pressure, while retarding it will close the intake valve later and reduce it.
So what is the way to go with camshaft events if maximum pump gas power is the goal? The answer here is too specific to the infinite variety of engine combinations possible to make a blanket recommendation. There is an effective way to experiment with different combinations, without the need to try a dozen cams. The trick is to use one of the many engine simulation software programs available, to model the effects of various cam profiles. With a program like Dynomation's Engine Analyzer Pro, any changes of cam timing events can be simulated, and the resultant changes to cylinder pressure are estimated with a good degree of accuracy.
The Compression RatioWith all this talk about pressure and detonation, we've purposely left the topic of compression ratio for last. There is no doubt that compression ratios have been trending up, with OEMs now building engines with ratios on the same level as during the high-octane musclecar days of the late '60s. High compression serves up many benefits, helping to achieve high torque and power numbers, increasing fuel efficiency, and improving throttle response, while the reduced clearance volume also aids in cleaning up the idle. Modern OEM engines are pushing ever closer to the 11:1 mark, with Chevrolet's new LS7 Corvette plant actually at that level now.
Beginning decades ago, when available fuel octane first began to rapidly decline, the trend was to lower compression ratio to 9:1 or even less. These days much higher ratios are again becoming commonplace in high-performance builds. The improvements in technology, particularly in the areas discussed, allow higher compression ratio to be tolerated on pump gas. Building an engine with all of these tricks makes it possible.