Need some help figuring CR

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This one always seems to work fine for me......

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I could never understand, & still don't, why the crevice volume is included in the CR calculation. By crevice volume, I mean the volume from the top ring to the piston crown. The theory is that the piston OD is smaller at the crown, but this expands from the heat under operating conditions & closes up this gap. Then you have the carbon formation on both the top of the bore & on the piston above the top ring; this would ensure no discernible gap.
 
Bewy said:
I could never understand, & still don't, why the crevice volume is included in the CR calculation. By crevice volume, I mean the volume from the top ring to the piston crown. The theory is that the piston OD is smaller at the crown, but this expands from the heat under operating conditions & closes up this gap. Then you have the carbon formation on both the top of the bore & on the piston above the top ring; this would ensure no discernible gap.
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I'm wondering if that's a safeguard for assuming loss of compression concerning an airgap when an engine cylinder wall is worn down by .010 or so, and just reringing with oversized rings?? Idk
 
There is no wear in that area of the bore [ above the top ring ] so not applicable.
 
Bewy said:
I could never understand, & still don't, why the crevice volume is included in the CR calculation. By crevice volume, I mean the volume from the top ring to the piston crown. The theory is that the piston OD is smaller at the crown, but this expands from the heat under operating conditions & closes up this gap. Then you have the carbon formation on both the top of the bore & on the piston above the top ring; this would ensure no discernible gap.
Click to expand...
The space present around the top of the piston down to the top ring is included because it is there. The piston does not expand from combustion heat to close up that space. It is a trend for OE and some aftermarket pistons to move the top ring higher on the piston to eliminate some of that space as unburnt fuel can hang out there resulting in a loss of full potential power and increased emissions. Raising the top ring location is a balancing act between reducing the space and exposing the top piston ring to more heat.
Not all compression calculators include the space down to the top ring as an input. For those that don’t it is either fudged in or ignored. The space is there, but it is very small in comparison to the full piston stroke.
High end Diesel engines like those in over the road class 8 trucks have a feature called a ‘scraper ring’ that sets in the top of the cylinder sleeve and closed up that space down to the top ring when the piston is at TDC and compression ignition occurs. The diesel ‘scraper ring’ protects the top ring from the soot formed in diesel combustion and really helps the long term sealing of the top ring.
 
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Bewy said:
There is no wear in that area of the bore [ above the top ring ] so not applicable.
Click to expand...
Yep...that makes sense too. The piston at full TDC relative to cylinder wall wouldn't have any wear at that point.
 
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