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Message: [QUOTE="Rich Rohrich, post: 1308434, member: 16241"] It sounds to me like someone screwed something up that resulted in the hard starting. The high compression piston has a value if you change the cam and it runs a later Intake Valve Close timing. The point of Intake Valve Close will determine the dynamic or trapped compression ratio of a four-stroke engine. As an example if you took a YZF 444 with a chamber volume of 34.40 CC, a 13.91:1 mechanical compression ratio and closed the intake valve at 50 degrees ABDC you'd have a trapped compression ratio of 12.16 : 1. If you take that same engine and use a cam with an intake valve that closes 20 degrees later at 70 degrees ABDC the trapped compression ratio would only be 10.50. The difference is about 70 psi of pressure (absolute) before combustion. Compression ratio and intake valve close are intimately related and need to be considered as you look to modify an engine. It's pretty simple once the terms are clear. Mechanical or Static compression ratio is the volume difference between the piston at BDC and the piston at TDC . Let's say for example we have a 450cc cylinder with a 36 cc combustion chamber. At BDC the volume is 450+36 or 486cc, and at TDC the volume is only 36cc . If we put it in a formula it looks like this (Cylinder Volume at BDC + combustion chamber volume at TDC) / combustion chamber volume at TDC = Static compression ratio with numbers it looks like this: (450 + 36) / 36 = 13.50 static compression ratio This is the number that the piston manufacturers will list in their catalog because they really have no idea what the cam spec is this is the only number they can list. Plus this number will always be bigger and bigger sells Trapped or Dynamic compression ratio is the volume difference between the piston at Intake Valve Close and the piston at TDC . Let's take that same 450cc cylinder with a 36 cc combustion chamber. If we close the intake valve at 70 degrees ABDC the cylinder volume is only is 334+36 or 370cc when we finally close the intake . If we plug those numbers in our modified formula (Trapped Cylinder Volume at IVC + combustion chamber volume at TDC) / combustion chamber volume at TDC = trapped compression ratio with numbers it looks like this: (334+ 36) / 36 = 10.27 trapped (aka dynamic) compression ratio The later we close the intake valve the smaller the trapped volume and the lower the trapped compression ratio. Lower trapped compression ratios are less likely to detonate and have a lower octane requirement. So that same 13.50 compression piston in the Wiseco catalog will have very different behavior depending on what type of cam is in the engine. The opposite is also true. If you put a high compression piston in an engine that closes the intake valve very early the trapped ratio will be very high and need really high octane fuel to keep from detonating. The older XRs were like this. Some people might be wondering at this point why you would want to close the intake early or late. Well by leaving the intake valve open longer you can fill the cylinder more completely. The obvious trade off is the fact that at low speeds this late IVC will cause major reversion problems in the intake. Basically with the intake valve open too long the mixture that we are trying to get into the cylinder can back up into the intake port and cause all kinds of problems. I believe this is part of the reason that Yamaha used such small valves in the YZF engines. By keeping the intake velocity high with a small valve and small port, they can minimize the negative effects of late IVC while retaining the high rpm, and lower octane requirement advantages. If this seems confusing, think of it this way. The combination of IVC timing, intake port/valve diameter, and the intake tract length will combine together to determine the rpm range that the engine is most efficient at, and volumetric efficiency at various engine speeds. The inertia generated by the incoming fuel/air mix is the major factor that is used to determine these relationships. Higher port velocities from the smaller valves and ports help to increase the overall cylinder filling up to a point. Once port velocities get above about 300fps, there is a trade off between charge density, filling, and pumping loses. It's as simple as that. ;) [/QUOTE]

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