Compression Ratio

[Genopsyde]

How do you determine the compression ratio on an engine. other than looking it up in a book. Are there test that you can run physically, or is there a formula to figure it by using the combustion chamber size, and bore and stroke figures?

[89IROC&RS]

compression ratio = cylender volume(cc) / combustion chamber volume(cc)

[89IROC&RS]

oh sorry, messed that up, its actually.....

CR = (CV + CCV) / CCV

CR = Compression Ratio
CV = Cylender Volume (cc's)
CCV = Combustion Chamber Volume (cc's)

[4onFloor]

dont you also figure in the thickness of the head gasket or is that part of the ccv?

[Genopsyde]

yea, because I want to up my compression by using the 58cc chamber size like the stock are, but with a steel shim gasket. and i'll need to figure out the new compression ratio for when I get the chip burned.

[Genopsyde]

am I making any sense, or is the constipation effecting my mind?

[4onFloor]

better make sure you heads and block are perfect before using steel shims. i've never head good luck with those

[Genopsyde]

like, perfect how?

[Genopsyde]

like, perfect as in perfectly flat for perfectly meeting surfaces? it doesn't hafta be steel shim, just something thinner than stock. I hear the stock one i got is pretty damn thick.

[89IROC&RS]

well yeah, i was just giving the short of it, so here is the complicated part.

CV = [3.14(r)^2] x (S)

where r = the radius or Bore/2 and S = the stroke. This gives you the volume in cubic inches, which you will have to multiply by 2.54^3 to get cubic centimeters (cc's)

CCV = (Combustion Chamber volume) + (gasket volume) + (piston to deck colume)

Combustion Chamber Volume can be found by contacting the manufacturer, or measuring it with alcohol and a burrett.

gasket volume is found by the same method as finding cylender volume. it is as follows

GV = (3.14(r)^2) x (T)

where r is the radius of the gasket, normally 4.1 / 2 or 2.05inches. and T is the thickness of the gasket in inches, for factory applications its normally around .045inches, and steel shim gaskets are normally in the .015-.020inch range. i use .015inches for my work. again this answer will be in inches, so youll want to convert it to cc's by multiplying it by 2.54^3

Piston to Deck volume is the volume above the piston and below the deck line of the block at TDC. and again you use the same formula that we used for gasket volume. where r = the radius of the bore, but in this case the T value is found by the following calculation

T = (9.025) - [(stroke/2)+(rod length)+(piston compression height)]

so in a small block chevy, with a 3.48in stroke, 5.7in rods, and pistons at 1.56in compression height it would look like this.

T = (9.025) - [(1.74)+(5.7)+(1.56)]
= .025in

meaning the piston is .025in below the deck at TDC. this calculates to a volume of .314 cubic inches or 5.14554 cc's with a 4in bore.

zat help??

[4onFloor]

^
my calculator just overheated

[89IROC&RS]

lol, whadda ya mean? i do that in my head

[Genopsyde]

uhhhh........i'm gonna just print that and let my dad figure it all out

[Genopsyde]

bah, maybe i'll just leave it alone, that way I can run some boost.

[89IROC&RS]

lol, oh and i also forgot, that when calculating all that, you have to take the piston dish into account, as part of the combustion chamber volume. but thats a no brainer, you can get that from the manufacturer, and you have to just add or subtract cc's baised on the final answer. but its honestly not that big of a deal, i mean its kinda sloppy on here cuz of how its laid out, but if you write out the equations yourself, its pretty simple.