![]() For example, the Mercedes-Benz A45 AMG may only have a compression ratio of only 8.6:1, yet the manufacturer recommends high octane fuel (98) due to the starting pressure within the cylinders. This is as a result of the high starting pressure within the cylinders caused by the turbocharged setup. Turbocharged engines should always use at least 95 octane rated fuels, as these engines are subjected to higher stresses than a naturally aspirated engine. Generally the recommendation will be for a higher octane fuel at sea level, and a lower octane rated fuel above altitudes of 1200m. Some manufacturers will recommend multiple octane ratings for different altitude locations (Usually on naturally aspirated engines as they suffer from reduced air densities). If this is the case, and your owner’s manual is nowhere to be found, the general rule of thumb is that performance engines require higher octane fuel. But, you might not know what the compression ratio of your car is. However, general guidelines would be that if you car has a high compression ratio, above 8.5:1 you should use 95, and if your car has a compression ratio below 8.5:1 you should use 93. The simple, and most correct, answer to this is to refer to your manufacturer’s recommendation in the owner’s manual – which you receive when buying the car. *Compression ratios given are guideline figures always refer to vehicle ownership manuals for correct fuel requirements. 93 is therefore ideal for lower compression ratio engines (generally low performance engines with a * ratio below 8.5:1), whereas 95 and 98 (* Ratios between 8.5:1 and 10:1) are more stable and suitable for high compression engines (often higher performance engines). 95 and 98 octane fuel can withstand higher amounts of compression respectively without combusting. Fuel can only withstand a certain amount of pressure before it self-ignites, without there being a spark present from the spark plugs.ĩ3 octane fuel is a low-compression fuel, and as such can only withstand lower compression thresholds before it self-ignites. The volume of uncompressed air in relation to its compressed volume within a cylinder is known as the compression ratio. During this stroke, air and fuel are compressed within the cylinder prior to being ignited by the spark plug. One of the strokes of typical internal combustion petrol engines (4-stroke engine), is compression. Simply put: The octane rating is the guide to how much compression a particular fuel can withstand within an engine’s compression stroke. As such, fuels with a higher octane rating (95, 98 etc.) are higher performance fuels than lower rated fuels (93). The higher the octane rating, the more pressure the fuel can withstand within the compression stroke of an engine prior to igniting. This technique is suited to someone who is in the middle of putting an engine together and has the right tools, or someone whose engine is already in pieces.93 or 95, we explain what the numbers mean and which you should use in your car.Īt the beginning of the month, just after pay-day, you’ve probably filled up your car with a healthy dose of 95 octane petrol, and as the month wears on and your wallet gets thinner, you’ve migrated to 93 after all it’s cheaper, it saves you a couple of Rand, so why not? But what exactly do those numbers mean? How does 93 octane petrol differ from 95 octane? Which is more beneficial and which should you be putting in your car?Īn octane rating is defined as the standard measure of the performance of a motor fuel. We're looking at the first of these ways here. Firstly, you can make a math calculation as accurately as possible, or secondly - the more popular method - uses an empty spark plug socket with a pressure gauge inserted. There are two ways of calculating the compression ratio for an engine. The compression ratio is the ratio between two elements: the gas volume in the cylinder with the piston at its highest point (top dead center of the stroke, TDC), and the gas volume with the piston at its lowest point (bottom dead center of the stroke, BDC). The compression ratio of an engine is a very important element in engine performance. How to use this calculator: Simply fill in all the boxes below with the requested figures and click on "Calculate CR" to find your engine's compression ratio. ![]() This compression ratio calculator can be used to work out the compression ratio of your engine.
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