![]() If the valves are open at all during compression stroke, the engine produces zero compression pressure. In other words, the valves stay open longer, during the compression stroke. The more aggressive cam you have, the more valve timing and overlap it has. It is not a ratio, it's a solid, measurable pressure reading.ĭynamic peak pressure in a car engine is much lower than the dynamic compression pressure inside a closed chamber, because the valves are open during the compression stroke, allowing air to escape. That is what CR means.ĭynamic compression is a measure of maximum pressure inside the cylinder & combustion chamber before ignition. That is how much the air WOULD compress, if it was squeezed into a closed chamber. The numbers I wrote above are Compression Ratio numbers. ![]() (nobody ever talks about it, although it could be calculated somehow I'm sure, after you measure actual compression pressure) There is no such thing as dynamic compression ratio. Scott the viking wrote:That's a dynamic compression ratio? ![]() So, perfect world, 12 to 1, no head temp increase is the new rule. Anything past that and the act of the compression alone would affect the head temp. So.I guess we could say."if the laws of physics ceased to exist on our VW engines for a day.then when could run as high as 12 to 1 before the heat of the compressed air affected the engine head temp. The problem with that is that it does not really apply to most guys building a air-cooled VW's unless he is a cylinder head designer and could make a combustion chamber piston combo that would safely run more compression without detonation. So.if you run a certain compression ratio.with a certain head design, then yes.it will be just fine. So if your argument is.compression is not the tonation is.that is true.until you get to a certain point, then compression becomes the heat builder. They have figured out how to run a gob of compression (due to new head designs) and still run pump gas.Īt this particular point in time.there are still limits to how much compression we can run. A good example of this would be today's four stroke dirt-bikes. Many auto makers are experimenting with all sorts of different head designs in an attempt to run as much compression possible without the usual adverse affects. does exist, thus.no real reason to test things in a world where it does not exist. If no detonation existed.an engine running 11 to 1 compression (or higher) would be great. You don't need to find any test.I can tell you.than in a perfect world were detonation would never exist.High compression would not raise heat enough to matter. General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminum cylinder heads.Probably the reason you don't find much about it.is that it is somewhat unrealistic. However, that same 11:1 static compression ratio engine with the radical 259/269 duration camshaft would have a dynamic compression ratio in the neighborhood of 7.5:1, totally acceptable to run on pump gas. Therefore with the mild cam it will have a high dynamic compression ratio, probably 9.5:1+ which would be way too high to run safely on 91 octane gas. If you put a very mild camshaft (194/204 duration this cam will have an "early" IVC (intake valve closing point), and will "bleed off" less compression than a radical camshaft with 259/269 duration and a considerably "later" IVC. To give an example, lets say you're considering only pump gas for your engine, and it has a static compression ratio of 11:1. Unlike Static Compression Ratio, Dynamic Compression Ratio takes into account camshaft timing by considering the intake valve closing point in relation to the piston position. Static Compression Ratio numbers are the ones you hear thrown around the most ("10:1 compression"), and it takes into consideration the full sweep volume of the cylinder in regard to its range of crankshaft stroke. This calculator will calculate both Static and Dynamic compression ratioĬompression Ratio is the ratio of an engine's cylinder volume vs.
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