What to use when figuring CR? Static or Dynamic
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TECH Enthusiast
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Joined: May 2002
Posts: 747
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From: East Tennessee
What to use when figuring CR? Static or Dynamic What should I consider when trying to achieve a certain compression ratio? This is not a daily driver (especially since the engine bay is empty!). I was shooting for a static CR of 11.75. Using the following values I come up this:
Cylinder Head Volume 57cc
Piston Head Volume 11cc (valve relief)
Gasket Thickness .054
Gasket Bore (in.) ? used 3.903
Cylinder Bore Diameter 3.903
Deck Clearance (in.) -.010
Stroke (in.) 4
Rod Length (in.) 6.125
Intake Closing Point (degrees) ABDC @ 0.050 lift (TRex) 47
STATIC CR 11.656
DYNAMIC EFFECTIVE CR 10.433
The dynamic seems awfully low. If I change the Piston Head Volume to 2cc (valve relief) I get:
Static CR 12.602
Dynamic CR 11.271
Is that too much static? I am trying to get the highest CR with out having to get custom pistons. 8 and 2 cc valve relief pistons are stock sizes.
Cylinder Head Volume 57cc
Piston Head Volume 11cc (valve relief)
Gasket Thickness .054
Gasket Bore (in.) ? used 3.903
Cylinder Bore Diameter 3.903
Deck Clearance (in.) -.010
Stroke (in.) 4
Rod Length (in.) 6.125
Intake Closing Point (degrees) ABDC @ 0.050 lift (TRex) 47
STATIC CR 11.656
DYNAMIC EFFECTIVE CR 10.433
The dynamic seems awfully low. If I change the Piston Head Volume to 2cc (valve relief) I get:
Static CR 12.602
Dynamic CR 11.271
Is that too much static? I am trying to get the highest CR with out having to get custom pistons. 8 and 2 cc valve relief pistons are stock sizes.
TECH Enthusiast
Joined: Feb 2004
Posts: 743
Likes: 0
From: Foley, Alabama-southern Alabama
Hey BB, most guys go by static cr.Since you are using a rather big cam you can get away with that much cr. As more duration wiill bleed you cr at lower rpm's.I think you'll be fine! 
Thats stadic!! 
