differences in compression
Thread Starter
Melting Slicks
Joined: Dec 2007
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From: Olivia mn
differences in compression
hey everybody, What is the differance between dynamic and static compression. I have heard that cams with long duration can affect how much compression you can run in an engine, can anyone tell me if this is true or false? thanks
Burning Brakes
Joined: Jan 2006
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From: NC
2022 C2 of the Year Finalist - Modified
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
Vastly oversimplified, SCR = (cylinder displacement at BDC / combustion chamber displacement at TDC), while theoretical "DCR" = (cylinder displacement at intake valve closing / combustion chamber displacement at TDC). Of course, arriving at these stats is a bit more involved.
For a good primer on "DCR" and comprehensive SCR/"DCR" calculator you can download...
http://www.empirenet.com/pkelley2/DynamicCR.html
Do bear in mind that such "DCR" calculations based solely on when the intake valve closes are strictly theoretical (hence the quotation marks), as true Dynamic Compression Ratio = SCR x VE (volumetric efficiency); the latter of which varies significantly with load, rpm, etc... (hence the term dynamic). Also, with very healthy breathing engines max VE will often exceed 100%, so you shouldn't take for granted that you can get away with high compression just because the calculations say you're "DCR" is fine due to a huge cam being installed. Thus, one must develop some sense of when and how far to back away from theoretical "DCR" limits so as to not build too much cylinder pressure where max efficiency is achieved. My next pump gas, solid roller 427 BB (w/alum heads) will have no more than 8.0:1 "DCR" for this very reason.
That said, knowing "DCR" is a good reference to have. Just don't get caught up in it as if it were the holy grail of engine design.
For a good primer on "DCR" and comprehensive SCR/"DCR" calculator you can download...
http://www.empirenet.com/pkelley2/DynamicCR.html
Do bear in mind that such "DCR" calculations based solely on when the intake valve closes are strictly theoretical (hence the quotation marks), as true Dynamic Compression Ratio = SCR x VE (volumetric efficiency); the latter of which varies significantly with load, rpm, etc... (hence the term dynamic). Also, with very healthy breathing engines max VE will often exceed 100%, so you shouldn't take for granted that you can get away with high compression just because the calculations say you're "DCR" is fine due to a huge cam being installed. Thus, one must develop some sense of when and how far to back away from theoretical "DCR" limits so as to not build too much cylinder pressure where max efficiency is achieved. My next pump gas, solid roller 427 BB (w/alum heads) will have no more than 8.0:1 "DCR" for this very reason.
That said, knowing "DCR" is a good reference to have. Just don't get caught up in it as if it were the holy grail of engine design.
