IS300 Compression Ratios - Lexus IS Forum
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IS300 Compression Ratios

Special thanks to TurbOeast for the info below


Hereís one way to calculate the compression ratio. We know the stock bore and stroke is 86mm, and the stock compression ratio is 10.5 to 1. I also know that the stock head gasket thickness is .3mm thick, and has a bore size of 87.2mm. Hereís how we figure it out:


First we need to figure out combustion chamber volume (also know as cc). To do that use the formula:


Chamber Volume = Cylinder Volume/ (Compression Ratio Ė 1.0)


Cylinder Volume = (pi/4 x (bore x bore) x stroke) / 1000 (To convert from mm to cc)


Cylinder Volume = (.7853982 x 7396 x 86) / 1000


Cylinder Volume = 499.557 (If we multiplied this by 6 we would have 2997.34cc or about 3 Liters)


Now back to Chamber Volume:


Chamber Volume = Cylinder Volume/ (Compression Ratio Ė 1.0)


Chamber Volume = 499.557 / (10.5-1.0) or 499.557/9.5


Combustion Chamber Volume = 52.58494


Now we need to calculate the stock gasket cc (volume) and the 2mm gasket cc. The stock gasket has a bore of 87.2mm and a thickness of .3mm. Note Iím converting our numbers to centimeters to make the math easier. We simply move the decimal place and now our numbers are 8.72 and .03 centimeters.


Gasket Bore Volume = pi/4 x (bore x bore) x gasket thickness.


Gasket Bore Volume = .7853982 x 76.0384 x .03


Gasket Bore Volume = 1.79161cc


To figure out Gasket Bore Volume for our 2mm head gasket we just need to exchange some numbers. Our bore is the same 87.2, and our thickness is 2mm. We will convert to centimeters, and our numbers are now 8.72 and .2.


Aftermarket Gasket Bore Volume = .7853982 x 76.0384 x .2


Aftermarket Gasket Bore Volume = 11.944cc


Now we need to subtract the stock gasket volume from the 2mm ga sket volume to calculate the difference, or the additional volume we are adding to the combustion chamber.


Additional Combustion Chamber Volume = 2mm Gasket Volume Ė Stock Gasket Volume


Addition Combustion Chamber Volume = 11.944cc Ė 1.79161cc


Addition Volume added to the Combustion chamber with our 2mm head gasket is 10.15239cc.


Now we take and add that volume to the original combustion chamber volume.


Original Chamber Volume + Additional Chamber Volume from 2mm head gasket


52.584cc + 10.15239 cc = 62.73639


Now we calculate compression ratio.


Compression Ratio = (cylinder volume + chamber volume) / chamber volume


Compression Ratio = (499.557 = 62.73639) / 62.73639


Compression Ratio = 8.962 to 1 or as we round up 9 to 1


Thatís how we calculated for compression ratio based on the data and measurements we have. I would bet with engine tolerances you could be between 9.1 and 8.9 to 1.
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