tag:blogger.com,1999:blog-6582429758941507301.post954990558107200720..comments2024-03-18T08:25:40.019-05:00Comments on Knucklehead Theology: Measuring Compression RatiosSt. Leehttp://www.blogger.com/profile/01238671101561121436noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-6582429758941507301.post-52164606268894198912008-06-01T17:30:00.000-05:002008-06-01T17:30:00.000-05:00Overlap has no effect on compression directly, but...Overlap has no effect on compression directly, but the correct amount will improve volumetric efficiency at higher speed, which increases the effective cylinder pressure.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6582429758941507301.post-66584118455020726692008-04-26T18:26:00.000-05:002008-04-26T18:26:00.000-05:00Thanks, that was what I was refering to...the valv...Thanks, that was what I was refering to...the valve overlap.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-6582429758941507301.post-88923647525224097632008-04-26T13:29:00.000-05:002008-04-26T13:29:00.000-05:00I am not entirely sure I get the meaning of your q...I am not entirely sure I get the meaning of your question, but I'll take a stab at it. Keep in mind what I described in this post was mechanical compression ratio. That is the difference in volume between the cylinder/chamber when the piston is at TDC (Top Dead Center) verses the voume when it is at (Bottom Dead Center). This measurement does not take the cam into account whatsoever, but is important in choosing a cam.<BR/><BR/>Technically cam overlap is the period near TDC at the end of the exhaust stroke when the exhaust valve is not quite closed, but the intake valve has already started to open. <BR/><BR/>Cranking compression (measured in PSI) is the measurement obtained with a compression gauge when turnig the engine over with the starter. This measurement will be greatly affected by cam timing as well as ring and valve seal. <BR/><BR/>An engine will only be building compression when both valves are closed and the piston is coming up. By increasing the duration (time the cam holds the valve open) you have decreased the degrees of rotation that the engine is actually building compression. That is why "Hot" cam with a lot of duration needs a higher mechanical compression ratio to run at its best. All of this is related to RPM also, since for a given duration, at higher RPMs there is less time for the air to enter and exit the combustion chamber. That is part of the reason a longer duration cam is needed for high RPM operation; to get good cylinder fill.<BR/><BR/>So, to maybe answer your question, no one cam will be the best over all RPMs. It all comes down to making the right compromise in cam and compression ratio for the power output required, the riding style, the cubic inches available, the cylinder head flow, the longevity required, the fuel to be used, and probably a bunch of other factors I missed.St. Leehttps://www.blogger.com/profile/01238671101561121436noreply@blogger.comtag:blogger.com,1999:blog-6582429758941507301.post-49687063273075296122008-04-26T08:43:00.000-05:002008-04-26T08:43:00.000-05:00one question here... will a cams overlap actually ...one question here... will a cams overlap actually build compression over the rpm range?Anonymousnoreply@blogger.com