| Well I was thinking about it and I realized that there maybe some confusion on compression ratios and how they are affected with after market parts. First what is the compression ratio and how is it calculated? Compression ratio = (cylinder volume + clearance volume) / clearance volume clearance volume = combustion chamber + head gasket volume + piston dish volume + Piston Ring volume + piston skirt clearance. For simplicity I suggest we forget about everything except for head gasket thickness. However to my point. If you start with 87mm stock pistons and upgrade to say 89mm pistons you are going to increase your compression ratio. Simply because you just increased your cylinder volume in bore and also piston dish volume. For reference I show an effective increase of compression from stock of 8.5:1 to 8.72:1 with everything else remainging the same. Specifically if you are using the same size head gasket. But if you deck the head and the block you will be decreasing the head volume and therefore bringing up the compression ratio even more. Lets assume that decking the head lost you a mere 2cc and you were using the same 89mm piston your new compression ratio would be 8.97:1. Now for of those who have done chamber work like radius job and removed the spark plug boss and etc. You can effectively be lowering your compression ratio. So be careful when you are putting your engine back together and make sure you consider everything when ordering your pistons and head gasket and doing your head work. If you were to say remove 2cc of material in the head and use the same setup as before your new compression ratio would be 8.40 if you were to use a stock head gasket In my case with all my head work and 87.5mm cylinder size I had to use a .038" or 0.9652mm which thinner than stock to keep my compression ratio at 8.5:1 (which I am now waiting for to be made unless I can find someone who has in stock a metal gasket). Anyways I hope that sheds some light on those building their motors and make sure they don't make the mistake on there final compression ratio. Keep in mind what is the ideal compression ratio is dependant what kind of environment the motor is going to be in. That is a discussion for another time tho. However the ideal situation would be were your optimize your boost levels to the max efficieny of the turbo itself. It has been already shown that 9:1 compression ratio with Sport 500s produced some favorable numbers. For reference here are the numbers I used.. Bore: 3.452 Diameter in Inches
Stroke: 3.2677 Distance In Inches
Cylinders: 6 Number of Cylinders
Head Gasket: .0551 Thickness In Inches
Head Gasket Bore: 3.465 Diameter in Inches, usually larger than Bore!
Piston Deck Height: 0 Piston above(-) / below(+) Deck in Inches
Piston Ring Height: 0.360 From top of Piston to First Ring
Piston Skirt Clearance: 0 In Inches
Piston Dome\Dish Volume: 2 In CC's ("-" Dome), ("+" Dish -> include Valve Relief)
Cylinder Head Chamber Volume: 56.3 Number in Cubic Centimeters
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