Re: Static VS Dynamic compression
[Re: emarine01]
#378995
07/18/09 10:53 AM
07/18/09 10:53 AM
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Joined: Jan 2003
Posts: 1,025 Andrews,In. U.S.of A.
67_Satellite
super stock
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super stock
Joined: Jan 2003
Posts: 1,025
Andrews,In. U.S.of A.
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Trial and error is right. Just think of all the variables that effect the amount of air coming into the cylinder before that intake valve touches the seat.Carb.size plenum volume,runner distribution variance,runner entry shape,taper&size,valve size,lift rate,seat angles,port shape& flow,chamber shape,valve shrouding, piston acceleration rate&velocity,brand of beer consumed while assembling the engine,etc.etc. It boggles the mind.
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Re: Static VS Dynamic compression
[Re: emarine01]
#378997
07/18/09 11:42 AM
07/18/09 11:42 AM
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Joined: Mar 2008
Posts: 5,746 Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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This one has been pretty close from my experiences http://www.rbracing-rsr.com/comprAdvHD.htmI think many engine builders aim at 185 max with aluminum heads for 93 octane? I ran my old motor at 195 (193 to 202) on sunoco 94?? Iron heads maybe 160 and attention to any sharp high spots that can glow is a must! Tryng to get this JWD closer to 190 I'm at 160 low now and 170 high was at 150 low and 160 high so advanced the cam 4 more degs??
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Re: Static VS Dynamic compression
[Re: emarine01]
#379006
07/18/09 05:29 PM
07/18/09 05:29 PM
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Joined: Mar 2008
Posts: 5,746 Ontario, Canada
Dodgem
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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They vary slightly but it is a good general guide. tight lash compression goes down loose it goes up ever so slightly!!
Last edited by Dodgem; 07/18/09 05:31 PM.
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Re: Static VS Dynamic compression
[Re: emarine01]
#379007
07/18/09 06:33 PM
07/18/09 06:33 PM
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Joined: Aug 2003
Posts: 8,716 Baltimore/Denver
64Post
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master
Joined: Aug 2003
Posts: 8,716
Baltimore/Denver
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Quote:
Good question, in theory the intake valve has to be closed, so the on line calcs vary , but what about valve lash, lobe ramps, solid compared to hyd cams it all effects DCR by exactly when the intake seals the chamber
I made essentially the same point in the other thread.
I guess the smart guys took the day off, which leaves us.
I'll be corrected, but I think Crane came up with the .050" measurements originally, or at least started listing their cams @ .050 first. I think the story was that, empirically through their research, cylinder and charge pressure loss/gain were negligible at .050, and that air really didn't start moving until the valve was at .050". If you subscribe to that line of thinking then use the 50 number to establish ICP and calc from there.
The numbers in my case at 5300 ft.
Static CR = 12.2 ICP = 58* @.050" Dynamic CR = 9.0 Dynamic CP =185 V/P = 176
Last edited by 64Post; 07/18/09 06:34 PM.
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Re: Static VS Dynamic compression
[Re: emarine01]
#379009
07/18/09 08:12 PM
07/18/09 08:12 PM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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Quote:
Is there a ratio between the two that works best?
"Dynamic Compression" is just a description of re-calculating the compression ratio from the point where the intake valve closes. The assumption is that at low engine RPM, like cranking, the intake valve will close at a point after BDC "trapping" the standard atmospheric pressure, and then compress the trapped air (with no losses) to estimate the cranking cylinder pressure. Usually if the calculated cranking pressure is around 160 psi on run on pump gas. Below 160 psi, usually means you are leaving power on the table by not having enough static compression. Above 160 psi is still doable on pump gas, but you would want a good quench chamber and aluminum heads. This is just a rough guide, as there are many variables that determine what octane level a specific engine will need.
This is the Victory Library tech article on this: http://victorylibrary.com/tech/cam-c.htm
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Re: Static VS Dynamic compression
[Re: emarine01]
#379010
07/19/09 11:58 AM
07/19/09 11:58 AM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
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Joined: Oct 2007
Posts: 4,219
New York
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in theory the intake valve has to be closed
Actually, the stoppage can be from the closed valve, or simply the difference in pressure between the inertia of the intake flow vs. the lower static pressure of the charge in the cylinder ATDC. At low speeds, there isn't enough inertia to get flow to enter after the piston changes direction, and flow not only stops at the valve but reverses. At higher speed, inertia keeps it going until IVC.
DCR is useful to estimate how knock-prone an engine will be a low to mid speeds, but (as said above) at high RPM the full stroke length is captured and compressed (not just the stroke after IVC) at the beginning of the torque range and DCR becomes irrelevant.
Boffin Emeritus
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