84 TBI question
Racer
Joined: May 2006
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What is "dynamic compression"? Are you referring to the dynamic compression RATIO of an engine? If so, then NO!. The static compression ratio is the comparison of the cylinder volumes, with the piston at TDC and the piston at BDC. The dynamic compression ratio is a similar comparison of volumes; however, instead of comparing TDC volume to the full, BDC cylinder volume, it is only comparing it to the effective cylinder volume, after the intake valve closes, where compression can actually begin. The comparison of measured physical volumes is not effected by types of induction. Volumetric efficiency (VE) is another topic of discussion.
Team Owner
Joined: Aug 2004
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From: Park City Utah
This is not true: intake valve closure (sealing the cylinder) always takes place after BDC, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "dynamic compression ratio".
This is also why larger duration cams not only allow, but require more static compression; they "leak" more if the compression back in to the intake than a shorter duration cam.
Racer
Joined: May 2006
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Originally Posted by Tom400CFI
The calculated compression ratio (refered to as "static compression"), presumes that the cylinder is sealed at the bottom of the stroke (BDC or bottom dead center), and that the volume compressed is the actual volume.
Originally Posted by Tom400CFI
This is not true: intake valve closure (sealing the cylinder) always takes place after BCD, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "dynamic compression ratio".
Originally Posted by Tom400CFI
This is also why larger duration cams not only allow, but require more static compression; they "leak" more if the compression back in to the intake than a shorter duration cam.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
What is you point besides arguing about the existance of a term that does exist which explains a condition that does exist?
Last edited by Tom400CFI; Jan 21, 2007 at 09:42 PM.
Racer
Joined: May 2006
Posts: 293
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Originally Posted by Tom400CFI
What is you point besides arguing about the existance of a term that does exist which explains a condition that does exist?
Originally Posted by VetNutJim
Will changing from a 2 barrel carb. to a 4 barrel carb have any effect on the dynamic compression [ratio] of an engine?
Fact: A compression ratio (static or dynamic) is a comparison of volumes.
Fact: Changing the carburetor doesn't affect the dimensions (volume) of the insides of the engine.
That's the point. That's all.
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
No kidding. But it does affect cylinder pressure, unless it's so large that it's not a limiting factor.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
I stand corrected:
Dynamic compression IS indeed a calculation that uses only two factors: volume of cylinder when intake valve closes, and volume at TDC. Obiviously those factors do not change with a carb swap (though cylinder pressure could), so changing a carb doesn't change DCR.
My bad.
-Tom
Dynamic compression IS indeed a calculation that uses only two factors: volume of cylinder when intake valve closes, and volume at TDC. Obiviously those factors do not change with a carb swap (though cylinder pressure could), so changing a carb doesn't change DCR.
My bad.
-Tom
Racer
Joined: May 2006
Posts: 293
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Thank you. 
Back in post #41 I asked what VetNutJim meant by the term "dynamic compression", and I continued on after explaining that I was talking about the "dynamic compression ratio". In my last post, I even high lighted, in blue, where I had amended the question to read as I thought it was intended to read. Your statements were (are) correct as to some of the effects that can occur due to changes in DCR. After your second to last post, I was about to point out that it seemed as though you were confusing the terms compression "pressure" and compression "ratio". Then I read that you "got it". Great!
Back in post #41 I asked what VetNutJim meant by the term "dynamic compression", and I continued on after explaining that I was talking about the "dynamic compression ratio". In my last post, I even high lighted, in blue, where I had amended the question to read as I thought it was intended to read. Your statements were (are) correct as to some of the effects that can occur due to changes in DCR. After your second to last post, I was about to point out that it seemed as though you were confusing the terms compression "pressure" and compression "ratio". Then I read that you "got it". Great!
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Le Mans Master
Joined: Sep 2000
Posts: 6,303
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From: Bartlett IL
Tom,
The discussion was interesting to me and I did think that the issue was a confusion regarding compression(lbs/sq.in.) v ratio. One of the reasons I swapped cams was to lower the compression in my cylinders because it was at the level where I felt I could use more overlap. With over 200lbs. in all cylinders, I felt the intake valve was closing too soon and I was losing power due to pumping losses. Now I fully expect that my hypothesis could be inaccurate. But the cam swap has proven to add more power and extend the rpm band 5-600rpms.
The discussion was interesting to me and I did think that the issue was a confusion regarding compression(lbs/sq.in.) v ratio. One of the reasons I swapped cams was to lower the compression in my cylinders because it was at the level where I felt I could use more overlap. With over 200lbs. in all cylinders, I felt the intake valve was closing too soon and I was losing power due to pumping losses. Now I fully expect that my hypothesis could be inaccurate. But the cam swap has proven to add more power and extend the rpm band 5-600rpms.
