Ok yall, Here goes. This is my first tech thread. I must tell you that I have as a passion of mine, since high school, experimental physics. When I sat down to design the new car I approached it with that same passion. While we all have our own opinions, nothing can undo or discount the laws of physics. In each phase of the design process I did research at the above and beyond level. I am not sure if yall want information at this level. If not I will not be offended and will try to keep my posts free of such lengthy answers. In either case I will try to keep my opinion to a minimum, unless asked. I have read many threads that try to address the issue of octane requirements vs compression ratios in gasoline powered engines. I too became obsessed with this question. Here’s what I found out. First the fuel does not care about static compression ratio but only is effected by the actual compression that occurs after the valves are all closed. This is referred to as dynamic compression. While DCR is effected by the factors that you use to find your static ratio the DCR can be manipulated by several factors, more on that later. Now I have seen that the guys from Canada say that there numbering system is different. If (R+M)/2 is the method that is used, and it should say so right on the pump, the numbers are the same. If not I have not found an official conversion in the petroleum industry archives, but I will keep looking. This first bit is from those archives and is about ten years old if that matters to anyone. 7.2 What is the effect of Compression ratio? (The compression ratio talked about here is actual or as we say dynamic compression ratio. AC) Most people know that an increase in Compression Ratio will require an increase in fuel octane for the same engine design. Increasing the compression ratio increases the theoretical thermodynamic efficiency of an engine according to the standard equation Efficiency = 1 - (1/compression ratio)^gamma-1 where gamma = ratio of specific heats at constant pressure and constant volume of the working fluid ( for most purposes air is the working fluid, and is treated as an ideal gas ). There are indications that thermal efficiency reaches a maximum at a compression ratio of about 17:1 for gasoline fuels in an SI engine . The efficiency gains are best when the engine is at incipient knock, that's why knock sensors ( actually vibration sensors ) are used. Low compression ratio engines are less efficient because they can not deliver as much of the ideal combustion power to the flywheel. For a typical carburetted engine, without engine management [27,38]:- Compression Octane Number Brake Thermal Efficiency Ratio Requirement ( Full Throttle ) 5:1 72 - 6:1 81 25 % 7:1 87 28 % 8:1 92 30 % 9:1 96 32 % 10:1 100 33 % 11:1 104 34 % 12:1 108 35 % Modern engines have improved significantly on this, and the changing fuel specifications and engine design should see more improvements, but significant gains may have to await improved engine materials and fuels. Based on this information I extrapolated the following expansion of the octane chart DCR Octane # 7.1 87.5 7.2 88.0 7.3 88.5 7.4 89.0 7.5 89.5 7.6 90.0 7.7 90.5 7.8 91.0 7.9 91.5 8.0 92.0 8.1 92.4 8.2 92.8 8.3 93.2 7.3 What is the effect of changing the air-fuel ratio? Traditionally, the greatest tendency to knock was near 13.5:1 air-fuel ratio, but was very engine specific. Modern engines, with engine management systems, now have their maximum octane requirement near to 14.5:1. For a given engine using gasoline, the relationship between thermal efficiency, air-fuel ratio, and power is complex. Stoichiometric combustion ( air-fuel ratio = 14.7:1 for a typical non-oxygenated gasoline ) is neither maximum power - which occurs around air-fuel 12-13:1 (Rich), nor maximum thermal efficiency - which occurs around air-fuel 16-18:1 (Lean). The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor in the exhaust. Conventionally, enrichment for maximum power air-fuel ratio is used during full throttle operation to reduce knocking while providing better driveability . An average increase of 2 (R+M)/2 ON is required for each 1.0 increase (leaning) of the air-fuel ratio . If the mixture is weakened, the flame speed is reduced, consequently less heat is converted to mechanical energy, leaving heat in the cylinder walls and head, potentially inducing knock. It is possible to weaken the mixture sufficiently that the flame is still present when the inlet valve opens again, resulting in backfiring. 7.4 What is the effect of changing the ignition timing? The tendency to knock increases as spark advance is increased. For an engine with recommended 6 degrees BTDC ( Before Top Dead Centre ) timing and 93 octane fuel, retarding the spark 4 degrees lowers the octane requirement to 91, whereas advancing it 8 degrees requires 96 octane fuel . It should be noted this requirement depends on engine design. If you advance the spark, the flame front starts earlier, and the end gases start forming earlier in the cycle, providing more time for the autoigniting species to form before the piston reaches the optimum position for power delivery, as determined by the normal flame front propagation. It becomes a race between the flame front and decomposition of the increasingly-squashed end gases. High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly. The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel-air mixture burns in such a way that maximum pressure of the burning charge is about 15-20 degree after TDC. Knock will occur before this point, usually in the late compression - early power stroke period. The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used ( several octane numbers below the vehicle's requirement at optimal settings ), both performance and fuel economy will decrease. The actual Octane Number Requirement depends on the engine design, but for some 1978 vehicles using standard fuels, the following (R+M)/2 Octane Requirements were measured. "Standard" is the recommended ignition timing for the engine, probably a few degrees BTDC . Basic Ignition Timing Vehicle Retarded 5 degrees Standard Advanced 5 degrees A 88 91 93 B 86 90.5 94.5 C 85.5 88 90 D 84 87.5 91 E 82.5 87 90 The actual ignition timing to achieve the maximum pressure from normal combustion of gasoline will depend mainly on the speed of the engine and the flame propagation rates in the engine. Knock increases the rate of the pressure rise, thus superimposing additional pressure on the normal combustion pressure rise. The knock actually rapidly resonates around the chamber, creating a series of abnormal sharp spikes on the pressure diagram. The normal flame speed is fairly consistent for most gasoline HCs, regardless of octane rating, but the flame speed is affected by stoichiometry. Note that the flame speeds in this FAQ are not the actual engine flame speeds. A 12:1 CR gasoline engine at 1500 rpm would have a flame speed of about 16.5 m/s, and a similar hydrogen engine yields 48.3 m/s, but such engine flame speeds are also very dependent on stoichiometry. The web site LS1tech.com has more on doing the math and a gentleman named Pat Kelly has a DCR calculator that I will attempt to give you the link to. He also states that keeping the DRC below 8.25 will keep you in the pump gas range. All of this discussion does not take into account extra measures such as thermal barrier coatings and the difference between quench and non-quench setups. I live in Texas where 93 octane gas is everywhere. I am designing for 92 octane gas because that is the lowest premium gas found in most states. I am into cruising and long road trips, so that’s what I am designing for. There is another factor which corresponds to DCR and that is cranking pressure. I have found it to be generally agreed that a cranking pressure under 180psi will keep you on pump gas. There is a very good article in “Hot Rod” Aug 2005 on all of these issues which continues to back up all of the data.