With a compression ratio of 14:1, it will be interesting to see if this car requires 91+ Octane (Premium Unleaded). Higher compression ratios are a great way of

With a compression ratio of 14:1, it will be interesting to see if this car requires 91+ Octane (Premium Unleaded). Higher compression ratios are a great way of squeezing more power out of the same amount of gasoline, but raising the compression ratio also raises the risk of detonation. Detonation occurs when the pressure and temperature inside the cylinder cause the air-fuel mixture to literally detonate, rather than burn (relatively) slowly and evenly as desired. If this occurs before the spark, it is called pre-ignition. When it occurs after the spark, while the flame front is progressing through the cylinder, it is referred to as detonation. Higher octane fuels are more resistant to detonation and pre-ignition, and usually go hand-in-hand with higher compression ratios. As a side note, E85 Ethanol has a ridiculously high octane rating, among other things, which allows for higher compression ratios and much higher boost in forced-induction engines. I keep hoping E85 will catch on outside of the corn belt so I can effectively get race gas performance from pump gas in my turbocharged motor, but no such luck yet. There are plenty of tricks that can be played to reduce the risk of detonation using low-octane fuels in high compression motors. Efficient head design with excellent cooling will reduce cylinder temperatures and minimize hot spotting within the cylinder head, reducing the risks of detonation and pre-ignition. Getting the heat out of the valves is particularly challenging as they only meet the cylinder head in a relatively small contact patch. The valves can be filled with sodium, which turns to liquid as the motor reaches operating temperature and circulates within the valve, cooling the valve and further reduce hot spots (pre-ignition points) within the motor. Also, designing the piston and head to thoroughly mix and swirl the intake charge helps create a quick, even, clean burn. However, to get up to 14:1 on a production motor, I wonder if Mazda is playing serious games with the intake camshaft timing to reduce the effective compression ratio. If you leave the intake valves open long enough after the piston begins the compression stroke, it will actually push a tiny amount of air back out of the intake valves, reducing the effective compression ratio of the motor at top dead center. With variable valve timing, Mazda could just be doing this near full load, where detonation and pre-ignition are problems. At partial load, you can get away with much higher compression ratios (up to 20:1 in certain cases, IIRC) with associated increases in efficiency (and thus higher MPG). Either way, I'm glad to see more efficient internal combustion engines. The internal combustion engine is a 100+ year old technology, so significant improvements are hard to come by without drastically increasing the cost of the motor. Good stuff.