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Compression does vary amongst the years. A good shop manual will give them to you. There is no such thing as an 'ideal compression' as it depends upon the given engine (more compression = more torque & hp [and heat]). The compression ratio is a numerical explanation of how much the maximum volume one cylinder has. When the piston is at the top of its stroke, where the volume in the cylinder is the least, this volume representation is denoted as 1, the number found on the right side of the expression (number:1). Now, when the piston is at the bottom of its stroke the volume that the cylinder contains is more. So, you take that first small volume and see how many times it takes to fill the current volume -- basically you'd be dividing the big volume by the small volume and the result is the number on the left of the expression (7.6:number). I'm not sure why the right number, the 1, is still used as every compression ratio I've seen for a car/truck uses the number 1 but it's probably just a holdover from days gone by -- maybe Jim could tell us...bawhahaha, just givin' you a hard time, Jim ;) Compression can be changed by piston size, cylinder size, head size (cylinder heads, silly, not ego!), stroke, turbo or supercharging aspiration, and spacers between the heads and cylinders (to give less compression). Low compression doesn't mean the engine will burn oil if it was designed as a low compression engine. You are confusing a compression check to cylinder compression. The compression ratio will not change for an engine as it's a measurement of volume, however, the compression as measured in Pounds per Square Inch (psi) can vary, say by going low if the rings are worn or a valve is burned. Yes, generally a higher compression engine requires a higher octane than a lower compression engine. This is to help reduce detonation (the sudden explosion, not burning, of the fuel mixture in the cylinder). Remember, even a low compression engine can use a high octane fuel without harm. You may notice that non-naturally aspirated engines, like turbos and superchargers, have a low compression ratio. But when the turbo or supercharger starts "kicking in" they will be pumping more fuel mixture into the cylinder and effectively increasing the compression ratio. Nitrous oxide (NO) engines can keep their stock compression ratio but when the NO kicks in the compression ratio will increase. Toby Erkson air_cooled_nut@pobox.com <-- Please use this address for email '72 VW Squareback 1.6L bored and stroked to 2.0L, Berg five-speed '75 Porsche 914 1.8L, ORPCA member Portland, Oregon, http://www.geocities.com/MotorCity/8501/ >-----Original Message----- >I am trying to figure out what exactly is the meaning of the >compression ratio. As far as I can figure a low compression = >engine will >burn oil ( I take this is because the seal around the pistons >are worn, I >don't reallyknow). Also having a high ratio means one has to >use higher octane >gas and >the engine runs hotter. Also people talk about adjusting the >compression, how >is this done. What do each of the numbers refer to and what is >the ideal >compression for t3's ( I take they are different for different >engins). ------------------------------------------------------------------- Search old messages on the Web! Visit http://www.vwtype3.org/list/