Compression Ratio
Posting Guru
Joined: Sep 2005
Posts: 2,238
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From: Kiefer, Northeastern Okla
The cam doesn't have anything to do with it.
CR=Cyl.Volume @ BDC/ Cyl.Volume @ TDC
CV=Clearance Volume
CR=(CV+Swept Volume)/CV
CR=1+ Swept Volume/CV
CR=1+ (Swept Volume/Vol@TDC)
CR=1+ (0.7854 X Bore X Bore X Stroke)/ (CCV + HGV + PDV)
Note: If volume is given in cc's then / by 16.4 to get cubic inches.
HGV= Head Gasket Volume, in cubic inches
HGV= Head gasket compressed thickness X 0.7854 X bore X bore
PDV= (0.7854 X bore X bore X deck to piston distance)+ (volume of piston depressions - volume of piston bumps)
CV= CCV + HCV + PDV
Example: What is the CR of the engine if heads have 72 cc chamber, head gasket is compressed to 0.040 inch and flat top pistons give 0.025 deck clearance @ TDC?
CCV= 72/ 16.4= 4.39 cubic inches
HGV= 0.040 X 0.7854 X 4.030 X 4.030= 0.51 c.i.
PDV= 0.025 X 0.7854 X 4.030 X 4.030 / (4.39 + 0.51 + 0.32)
CR= 1+ (0.7854 X 4.030 X 4.030 X 3.48/ ($.39 + 0.51 + 0.32)= 1 + (44.39/ 5.22)= 9.5cr.
Did that help?
CR=Cyl.Volume @ BDC/ Cyl.Volume @ TDC
CV=Clearance Volume
CR=(CV+Swept Volume)/CV
CR=1+ Swept Volume/CV
CR=1+ (Swept Volume/Vol@TDC)
CR=1+ (0.7854 X Bore X Bore X Stroke)/ (CCV + HGV + PDV)
Note: If volume is given in cc's then / by 16.4 to get cubic inches.
HGV= Head Gasket Volume, in cubic inches
HGV= Head gasket compressed thickness X 0.7854 X bore X bore
PDV= (0.7854 X bore X bore X deck to piston distance)+ (volume of piston depressions - volume of piston bumps)
CV= CCV + HCV + PDV
Example: What is the CR of the engine if heads have 72 cc chamber, head gasket is compressed to 0.040 inch and flat top pistons give 0.025 deck clearance @ TDC?
CCV= 72/ 16.4= 4.39 cubic inches
HGV= 0.040 X 0.7854 X 4.030 X 4.030= 0.51 c.i.
PDV= 0.025 X 0.7854 X 4.030 X 4.030 / (4.39 + 0.51 + 0.32)
CR= 1+ (0.7854 X 4.030 X 4.030 X 3.48/ ($.39 + 0.51 + 0.32)= 1 + (44.39/ 5.22)= 9.5cr.
Did that help?
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
The cam has an effect on the dynamic ratio, but not the static ratio. If your engine now has a 9.0 to 1 ratio ( static) with 64cc heads, going with a .040 overbore and less 3cc heads will only raise it at best, 1/2 point to 9.5 to 1. Of course this is assuming the 40 over pistons have the same deck clearance the stock pistons had. Many don't. Pin heights range from 1.585 to 1.610 for 5.0/302's. The shorter the piston, the less compression it'll have.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Originally Posted by petes79f150
Nice catch, baddad. I forgot to get into piston variables.-P.
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