Compression ratio is an index that represents the ratio of the volume of its combustion chamber, from its largest capacity to its smallest capacity, represented in ε as per international criteria. It is the ratio between the volume of the cylinder and combustion chamber. When the gasoline engine operates, a mixture of gasoline-air is drawn in. The higher the compression ratio, the higher the  pressure and temperature of the eventual mixture, the more completely the gasoline molecules of the mixture vaporize, the quicker and more complete the combustion will be, therefore, it is desired to have a engine with higher power, because it is more cost-effective and the exhaust quality is improved. On the contrary, the combustion of the engine with lower compression ratio is elongated, which leads to increase in energy consumption, thus decrease in power output.

Higher compression ratio is normally accompanied by aggravated vibration of the engine at work (It becomes less evident after special tuning of the engine nowadays). Instead of improving combustion, excessively high compression ratio will give rise to “detonation” or” surface ignition” or other abnormal combustions. Detonation will overheat the engine, lower the power, increase the energy consumption, or even damage the engine. Surface ignition will also aggravate the burden of the engine and shorten its service life. In addition, the compression ratio is subject to limitation set by the laws and regulations for emission pollution.

Normally low compression ratio means that below 10 while the compression ration above 10 is considered as high compression ratio. The gasoline grade required is based on the compression ratio of the vehicle engine. Generally speaking, the higher the compression ratio is, the higher the gasoline grade is required. Using gasoline with lower grade than recommended may give rise to “knocking”, aggravated vibration or uneven run, it may also be harmful to the engine performance and shorten its life. Normally, when the compression ratio is below 7.5, 90# gasoline shall be used; 90# or 93# gasoline for compression ratio 7.5~8.0, 93# for 8.0~8.5, 97# for 8.5 plus.

Other factors shall be considered for a specific automobile. When it’s hard to tell, general practice is to use higher rather than lower grade product. However, this doesn’t mean that higher grade gasoline is recommended. As the compression ratio, Spark Advance and other parameters of an engine has been set up at factory, and the computer program has set up adaptive fine-tuning program for the gasoline with poor anti-knock performance rather than for high grade gasoline, blind use of high grade gasoline is not only a waste of money, also may result in weak acceleration because its anti-knock advantage can not be best utilized. It is advisable to select the right grade of gasoline according to the users’ manual or the marks on the tank cover.