Really you can run high boost as long as everything is tuned properly

Fuel/air Ratio

Gasoline can be ignited in a non-stratified charge type engine between limits of roughly 11 to 1 (rich) and 20 to 1 (lean) air/fuel ratios. Most gasolines will burn fastest at ratios in the 17 to 1 range. The stoichiometric or chemically correct ratio is around 14.7 to 1 which also results in the lowest average emissions. Best power is obtained with a rich mixture of around 12 to 1.

Charge Density

Charge density is affected by the pressure and temperature of the charge. The higher the charge density, the more rapidly it will burn.

Charge Homogeny

This refers to how uniformly the air and fuel molecules are distributed in the charge. This is a very important factor with regards to successful ignition. If there is a big lump of fuel molecules with no air present between the spark plug electrodes or vice versa, at the time that the spark jumps, there will be no ignition. If the charge between the electrodes is leaner than 20 to 1 or richer than 11 to 1, there is little chance for ignition. Ideally, the molecules throughout the whole charge should be evenly spaced and distributed. This allows for a smooth rate of burn. If the charge is randomly mixed, there will be local variations in flame front propagation rates which will not produce maximum power as these may advance or delay when PCP is achieved. This phenomenon is known as ignition probability.


Charge Turbulence

Because the charge is in constant motion from the valve and port flow characteristics along with inertial effects and piston motion, the mixing of fuel and air molecules is dynamic. From one split second to the next, the actual mixture and molecular distribution changes. This can mean in some instances, that if a very short duration spark was initiated at one instant, the mixture might not ignite, whereas only a half millisecond later, conditions might be perfect for ignition. For this reason, very short duration sparks are undesirable. A long duration spark or multispark ignition system will ensure the highest ignition probability.

Fuel Characteristics

Low compression engines usually run well on low octane fuel because they have relatively low charge densities and the burn rate within these confines is usually predictable. A low compression engine switched to 118 octane race fuel will always lose power unless the ignition advance is increased to compensate for the slower burn rates. Even then, a low CR engine may lose power with the timing optimized for high octane fuel.

A high compression or turbocharged engine operates with much higher charge densities and consequently faster burn rates. The high octane fuel permits these rapid burn rates because it has far less tendency to autoignite and detonate under these conditions. As a result, high compression and turbo engines cannot realize their full hp potential without high octane fuel.

Inert Effects

Inert effects constitute 2 areas. Residual exhaust gasses left over from the last exhaust stroke tend to dilute the fresh charge and slow down burn rates. Camshaft timing, port flow and exhaust backpressure will affect charge dilution. Nitrogen is the major constituent of air and is essentially inert in the combustion process. Its presence substantially lowers the burn rate but since there is little that we can do about it, it is generally ignored. Nitrous oxide can be injected along with extra fuel to increase charge density as it contains a much higher concentration of oxygen than does air. Oxygen is the essential element in the combustion process.

The second inert effect concerns the relatively cold, metal engine parts in direct contact with the combustion gasses. Combustion will not easily take place in areas where the gas temperatures are well below the ignition temperature. This property is often used to advantage on engines to reduce the tendency to knock.

On many engines, a squish or quench area is used to negate combustion in certain areas to avoid knock. By having a matched area where the piston and combustion chamber come in close proximity at TDC, the gasses are kept cool enough so that they will not ignite until the piston has moved down the bore and cylinder volumes are increasing. This keeps the rate of pressure rise below the knock limit. Some people are dismayed when they install a thicker head gasket to lower the CR and have knocking worse than before. This is because they have negated the designed-in quench effect. A large squish area also tends to promote increased chamber turbulence which is important for mixing and power at high rpm.

Combustion Chamber Shape and Spark Plug Location

Combustion chambers and spark plug location and the number of plugs will have a marked effect on the time required to complete the combustion process. A large open chamber like a hemi which has a high surface to volume ratio, will combust more slowly than a wedge or modern pentroof chamber simply because it has more cold, metal molecules in contact with the combustion gasses which tends to slow reaction rates. For this reason, these chambers will require that the spark be initiated sooner to achieve PCP at the correct time.

The slowest combusting chamber would be an open chamber with a large bore size and the spark plug at one edge of the chamber. The flame front has a long distance to cover to complete combustion. By placing the plug in the center of the chamber, you halve the distance that the front needs to travel and will be able to reduce the spark advance needed to achieve maximum power. Another solution would be to add another spark plug to create two flame fronts which would also require much less time to combust. This is the solution in most aircraft engines where big bores and poor fuel distribution and homogeny require solutions to increase ignition probability.

Modern 4 valve engines with shallow pentroof chambers and a central plug location are fast, efficient combustors, requiring minimal advance for maximum power.