Compression Ratio as a term sounds very descriptive. However, compression ratio by itself is like torque without RPM or tire diameter without a tread width. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel, engine/air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbo-charging can drastically alter compression pressures.

  The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restructure plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restructure plate may see less compression pressure than an 11:1 unrestricted engine. The restructure plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine significantly.

  At one time compression pressure above a true 8:1 was considered impractical. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60’s 11:1 factory compression ratio engines were 11:1 in ratio but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compression up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the high RPM limited cylinder pressure due to the short time available for cylinder filling.

  Before continuing with theory, a little practical compression information is in order. If you have a 10:1 engine with a proper .040" assembled quench and then add an extra .040" gasket to give 9.5:1 and .080" quench you will usually experience more ping at the new 9.5:1 ratio than you had at 10:1. Non quench engines are the exception to this rule. Some racers make the effort to convert non quench engines to quench type engines. Street legal and pump gasoline has been run in everything from race cars to airplanes with various degrees of success. Compression ratios that work are as follows:
 

    PUMP FUEL
     8.5:1 - Quench head engine for tow service, motor home and truck.
     9.0:1 - Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.
     9.5:1 - Same as 9:1 except aluminum head used Light vehicle and no towing.
     10:1 - Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam.
     A knock sensor retard is recommended with 10:1 engines.
    RACE GAS
     12.5:1- Is the highest compression ratio suggested with unrestricted race gas engines.
    ALCOHOL
     15.5:1- Is the highest compression ratio suggested for unrestricted alcohol fuel engines.
 

  Satisfactory use of 14:1 - 17:1 compression engines can be made when restructure plate or small carburetor use is mandated by race sanctioning. High altitude reduces cylinder pressure so if you only drive at high (above 4500') altitude, a 10:1 engine can be substituted for a 9:1 compression engine. General compression rules can be violated but it is usually a very special case, such as a 600 Hp normally aspirated engine in a 1500 lb. street car with a 12:1 compression ratio. The radical cam timing necessary for this level of performance keeps low and medium RPM cylinder pressure fairly low. At high RPM, detonation is less of a problem due to chamber turbulence, reduced cylinder fill time, and the fact that you just can’t leave the above combination turned on very long without serious non engine related consequences.

  Piston temperature and horsepower are interrelated. High Hp per cubic inch engines not only make more Hp, but they make more heat. How the excess heat is handled has a significant effect on total engine power and longevity.

  Various piston, cam, valve, chamber and port configurations have been and are currently being tested to optimize engine internal temperatures. Some engines have ceramic exhaust port insulation liners that allow cooler cylinder head operation, while keeping exhaust temperatures elevated for efficient catalytic converter operation. The same ceramic type insulation on a piston top has not been quite so successful. Ceramic insulation on pistons can insulate the piston too well. The piston stays cool while the very top surface gets so hot that the intake air is immediately heated on contact with the piston. The heated intake charge expands and reduces the air flow into the cylinder. On the compression cycle, the now over heated intake charge offers more resistance to being compressed and, because of the higher compression pressure and temperature, is more likely to detonate during combustion. Ideal piston temperatures in an operating engine would suggest refrigeration during the intake and compression stroke, and incandescence during the combustion and exhaust stroke. The advantage of a hot piston on the power stroke is that less combustion energy is going to be absorbed by the piston. So far, it is not practical to heat and refrigerate a piston 6000 times a minute. However, if the incoming air is not heated by the piston and the piston reflects the heat of combustion, you start to approach ideal conditions. A polished hypereutectic piston will reflect combustion heat back into the combustion process. This reflection, combined with the insulating qualities of the hypereutectic alloy, keeps the heat in the cylinder during the power stroke. A smooth polished piston runs cooler than a non-polished piston, even after combustion deposits have turned both pistons black. A cool, smooth piston will transmit a minimum of heat to the incoming fuel air mix.

  Experimental work to reduce piston heating of the incoming fuel mix has been very limited but, in theory, a thin coating may prove to be beneficial. A thin, smooth coating over a polished piston should still reflect combustion heat while reducing carbon buildup and protecting the piston polish. It is easier for a thin film to change temperature with each engine cycle than it is for the whole piston to do the same. A thin film can be cooled by the first small percentage of inlet fuel mix, allowing the main quantity of fuel mix to remain relatively cool. Tests have shown that polishing the combustion chamber, valves and piston top can increase horsepower and fuel economy by 6%.

  All this polishing probably sounds counter to the practice of dimpling the combustion chamber. Dimpling has been shown to put wet flow back into the air flow and improve combustion. We do not recommend dimpling, but do suggest cutting a small discontinuity close to the valve seat to turbulent wet flow. Some bench flowed cylinder heads encourage fuel separation at the inlet port. If a small step is added at the valve seat to force the wet flow over the resulting sharp edge, fuel will reenter the air stream and give you the same affect as dimpling only without losing the benefit of a completely polished chamber. As you reduce wet flow you will improve combustion and most likely need to install leaner carburetor jets. Leaner jets compensate for the excess fuel that is available when wet flow is put back into the air/fuel mix. Significant additional horsepower gains can be had with careful attention to cylinder-to-cylinder fuel distribution by allowing all cylinders to be set "just right".

  Combustion chamber design work has increased steadily in the last ten years. Some of the work is mandated by the EPA and some is the result of race engine development. Some of the smog work has actually enhanced race engine development. Combustion chamber science is now more concerned with the effects of swirl, tumbling, shrouding of the valve, quench, flame travel, wet flow and spark location. A combustion chamber shaped dish piston can improve the flame travel in the combustion chamber. A dish allows the flame to travel further and expand more before it is stopped by a metal surface. This rapid flame travel makes it unnecessary to run big spark advance numbers. Ideally, we would like to be able to initiate ignition at top dead center since this would reduce negative torque in the engine that is now caused by spark advance. We are some time away from a practical spark ignition system that will make optimum power with a TDC setting. Some day it will happen. Donut go out and buy dished pistons for your open chamber 454. The advantage in flame travel is more than offset by the low compression ratio this combination yields. Small combustion chambers respond well to dished pistons, especially reversed dome or "D" cups. A 400 small block Chevy can use a 22cc D Cup piston and still have 10.4:1 compression. The trend in modern engine design seems to be smaller combustion chambers with recessed piston tops for more HP per cubic inch.

  Ignition timing on most installations should be 34 degrees total with full mechanical advance dialed in. More advance may feel better off the line but the engine lays down as the combustion chamber components come up to temperature. At the drag strip set timing for maximum MPH not best ET. Too much spark advance will shorten the life of any performance engine.