 IS300 Compression Ratios - Lexus IS Forum

Old 02-24-2005, 10:46 AM Thread Starter

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IS300 Compression Ratios

Special thanks to TurbOeast for the info below

Heres 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. Heres 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  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  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 Im 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

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.

Addition Combustion Chamber Volume = 11.944cc  1.79161cc

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

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

Thats 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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