# MOD BASICS - COMPRESSION RATIO

Right then. You got your engine apart and it's an old heap... You got all the parts and are putting in together, but you need to write down the actual measurements in order to figure out your new or revised compression ration. In this example, we are installing deeper dish pistons in order to try and achieve the 8.5 CR the guys stated up on ebay. Being a turbocharged engine, we aim to lower compression to allow more boost, to achieve more power safely. So you ask google for a compression ratio formula, and the answer is very simple - total volume at BDC divided by the compressed volume at TDC. Pretty simple right? Wrong. You need to have all the needed measurements and then plug them into the more detailed formula... [cylinder volume aka displacement + deck clearance volume + piston volume dish-positive/dome-negative + gasket volume + chamber volume] divided by/ [deck clearance volume + piston volume dish/dome + gasket volume + chamber volume]. Right then. Get your caliper out.

Fresh bore.

For the simplicity of this exercise we are ignoring ring depth since if you're usually messing about with metal parts and installing oversize pistons, you'll end up honing & boring your block, and usually the fight will be snug to say the least, since the new bore is tailored to fit your new (usually forged) pistons set. So the volume that can fit in between the block walls and pistons, coming down to the depth of the first compression piston ring, is negligible to say the least.

So, the above example... stock bore*stroke was 86x86mm. The volume formula is bore*bore*0.7854*stroke/depth/height. To make it simple, we deduct the total displacement to be 86*86*0.7854*86 mm3 / 1000 to get to ccs = 499.55cc*4 = 1998.23cc total engine. Yes, it'a a two liter. Or it was... now we have 87mm forged pistons, i.e. 87*87*0.7854*86/1000=511.24cc*4=2044.97cc. It's a bit more than a 2 liter now. But, coming back to compression. Stock specs showed us an 8.8 CR from the factory. Now we have the poor man's handy tip! Easy way to calculate your piston dish volume and combustion chamber volume. If you have dome pistons (N/A usually with high compression ratios), you could make a fast clay mold of your piston's head.

The head. Old, but refurbished. Removing the studs to make room for new ones ;-)