dynamic comp
- Thread starter swedefish
- Start date
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blue missile
Well-Known Member
swedefish,
check thread on performance issues about DCR vs octane.I am the original poster, and am gratefull for the continuing discussion which has now also included cranking (cylinder ) pressure as well
Andrew
check thread on performance issues about DCR vs octane.I am the original poster, and am gratefull for the continuing discussion which has now also included cranking (cylinder ) pressure as well
Andrew
moper
Well-Known Member
It's low. I aim for 8.25 for iron heads, 8.75 for aluminum. Anything under 8 will feel sluggish low down. Least that's what I've found. What is the rest of the build?
I belieave that I put the wrong figures in to the calculator,
in this link http://kb-silvolite.com/calc.php?action=comp
I can´t find the accurate number to put in "the intake closing time" column.
It´s a mopar purple 284/484 108* cam. Probably me who can´t read a cam card.
The rest of the build is
383 +.040 B block
linehoned + square decked.
piston to deckheight .020
KB dished pistons 5.00 cc
Steel chim headgasket .020
915 heads shaved for straightness
Above mentioned camshaft
Eagle "stock" rods
in this link http://kb-silvolite.com/calc.php?action=comp
I can´t find the accurate number to put in "the intake closing time" column.
It´s a mopar purple 284/484 108* cam. Probably me who can´t read a cam card.
The rest of the build is
383 +.040 B block
linehoned + square decked.
piston to deckheight .020
KB dished pistons 5.00 cc
Steel chim headgasket .020
915 heads shaved for straightness
Above mentioned camshaft
Eagle "stock" rods
Petzschler
→Welcome backâ†
my combo has a dynamic cr of 7.49:1 with a static cr of 10.98:1.
The calculated compression ratio, as you have given above, presumes that the cylinder is sealed at the bottom of the stroke (bottom dead center - BDC), and that the volume compressed is the actual volume.
However: 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 ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
*RADIO EDIT*
for a more detailed explanation click on the following link:
http://victorylibrary.com/mopar/cam-tech-c.htm
The calculated compression ratio, as you have given above, presumes that the cylinder is sealed at the bottom of the stroke (bottom dead center - BDC), and that the volume compressed is the actual volume.
However: 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 ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
*RADIO EDIT*
for a more detailed explanation click on the following link:
http://victorylibrary.com/mopar/cam-tech-c.htm
3404spdvaliant
Banned
Petzschler
→Welcome backâ†
sort of...
intake closing + exhaust opening points make up the total valve overlap. which is a factor in DCR.
intake closing + exhaust opening points make up the total valve overlap. which is a factor in DCR.
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