grindingears294
Member
what is the compression ratio of an engine? how do you figure it up? and is it better to have a high or low compression ratio?
what is compressing ratio?
- Thread starter grindingears294
- Start date
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74RAT
Veteran Member
it's the ratio of volume,, of the space above the piston when it's down,,,,, to the space above the piston when it's up. bottom of stroke to top of stroke. example,, if you have 1000cc's above the piston at bottom dead center and 100cc's above the piston at top dead center,, then you'd have a 10.0:1 compression ratio. i'd stick arround 10.0:1 with a matching cam for that ratio for a street limit maximum. the higher ratio's can be tuned arround to some extent,,but there are limits on every combo. keep it safe for pump gas,, if that's your direction.
can be altered by changing piston dome volume, block deck height, gasket thickness, chamber volume, bore diameter, stroke length. hope it helps.
andy
can be altered by changing piston dome volume, block deck height, gasket thickness, chamber volume, bore diameter, stroke length. hope it helps.
andy
74RAT
Veteran Member
bore X bore X stroke X .7854 = cubic inches per cylinder.
then X 16.39 = is cubic inches converted to cc's for one cylinder.
.7854 is the constant used,, and 16.39 is the conversion factor to convert cubic inches to cubic centimeters.
can use the same formula for the gasket thickness and deck height when those thickness's are plugged into the stroke of the formula. i use the stock bore size plus .120 for the gasket diameter. close enough for figuring anyway. some gaskets aren't round,, but it'll get pretty close on cc's. gaskets are always bigger than the bore to not have gasket overhang after boring to say .060 over.
then add up these: all of the totals for one cylinder with piston at bdc:
cylinder cc's
deck cc's
gasket cc's
chamber cc's
plus or minus the cc amount for dish or dome
that's the bottom dead center volume.
then add up these,, all totals for above the one piston at tdc:
deck cc's
gasket cc's
chamber cc's
plus or minus the cc amount for dish or dome
this is the total cc's for above at tdc.
then divide the little number into the big number and that's your ratio. hope it helps.
andy
then X 16.39 = is cubic inches converted to cc's for one cylinder.
.7854 is the constant used,, and 16.39 is the conversion factor to convert cubic inches to cubic centimeters.
can use the same formula for the gasket thickness and deck height when those thickness's are plugged into the stroke of the formula. i use the stock bore size plus .120 for the gasket diameter. close enough for figuring anyway. some gaskets aren't round,, but it'll get pretty close on cc's. gaskets are always bigger than the bore to not have gasket overhang after boring to say .060 over.
then add up these: all of the totals for one cylinder with piston at bdc:
cylinder cc's
deck cc's
gasket cc's
chamber cc's
plus or minus the cc amount for dish or dome
that's the bottom dead center volume.
then add up these,, all totals for above the one piston at tdc:
deck cc's
gasket cc's
chamber cc's
plus or minus the cc amount for dish or dome
this is the total cc's for above at tdc.
then divide the little number into the big number and that's your ratio. hope it helps.
andy
Last edited:
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pdq67
BANNED
grindingears294
Member
so my ultimate question is choosing the right heads based on combustion chamber sizes as appose to other attributes as well. My heads are stock and i was looking at edelbrock because of there price and just the brand has done me good but was wondering to get a higher cc head combustion chamber wise cause or the lower one and then a matching cam but then 1.5 or 1.6 roller rockers i know running the 1.6 is like running a bigger cam so would it have the rough idle im kinda looking at a budget build and want the most out of what i have
