Lighting It Off
It's impossible to try and make the most of pump gas power without considering the time factor. Ignition timing has a pronounced effect on both power and how much power can be made without detonating. There is an optimal ignition point for max power, reflective of the cylinder pressure versus crank angle in the running engine. However, the optimal timing may never be achievable, if the engine wants to detonate. As rpm rises, cycle-to-cycle time is decreased, and the time available for the end gasses to auto-ignite and propagate is progressively lower. For detonation to be possible in an engine, the reaction time of the unburned mixture must be shorter than the time for normal flame travel through the mixture, again a reduced likelihood as rpm increases. What does this mean to the hot rodder? Simply put, at higher rpm, detonation becomes less of an issue, and the engine can operate at higher cylinder pressures and power levels. One of the main reasons for advance in a spark ignition engine is to lower the octane requirement at lower engine speeds.
Whether or not the end gas auto-ignites is primarily a question of end-gas temperature and compression time. The influence of end-gas pressure is secondary and a much smaller factor in itself, outside of the resultant heat gain of the end gasses through the compression process itself. Since the propagation of the normal combustion front is the primary compressive factor on the end gasses, the ignition timing can control the compression time of the end gasses, reducing the tendency to detonate. This control is especially attractive at lower rpm, where the potential for detonation is most acute. With fine control of the engine's advance curve, the overall output can be optimized. The best examples here are some of the OE engine management systems in modern performance cars, where the timing is constantly adjusted to just under the point at which the engine will detonate. A feedback system like this is pretty difficult to compete with when working with mechanical springs and weights in a conventional distributor, however, skilled tuning of the advance curve is a sure path to making more power with pump gas.
A more recent development is the electronic programmable ignition system found on high-end ignitions such as the MSD Digital 7. With a box like this the advance rate can be tuned to any imaginable curve, and adjusted electronically. Tuning it for the maximum advantage requires trial and error testing, preferably with some dyno time, but it provides a level of flexibility in optimizing an engine package unavailable with conventional ignitions.
Fuel Management
Changes 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 Inside
Up 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 Can
The 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 1,375 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.
The chamber's partner in the quench effort is the piston. To be effective, tighter piston-to-head clearance is better, but pushing it much closer than 0.040 inch becomes risky for a street application. Note the quench pad on this dished piston, a much-preferred setup as compared to a quenchless full dish. | | |