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Computing Compression Ratios Regardless of what kind of engine you plan to build, it's important to know accurately what the compression ratio is. Thirty years ago, 10:1 or higher compression ratios were common, and the average premium fuel had high enough octane to control detonation. Since the switch to unleaded fuels in the early '70's, most Windsor compression ratios have ranged between 8 and 9:1. With today's performance parts, you can build some serious power with a 9:1 motor, but nevertheless, the important point is that you need to be able to determine what the compression ratio is. Compression ratio is the ratio of cylinder volume when the piston is at Bottom Dead Center to the vlume when it's at Top Dead Center. hence a 9:1 compression means that from the BDC, the air/fuel mixture has been compressed into a volume of 1/9 of the total. The higher the ratio, the tighter the squeeze ont he air/fuel mixture, and that helps make more power. But that isn't the only consideration. High compression needs high-octane fuel, and the 92 octance unleaded is usually only good to about 10:1 compression in a Windsor, and that's with aluminum heads. You can purchase race gas, or Union Oil's 100 octane unleaded, but both of these fuels command premium prices. On a supercharged engine though, you want stock or even a lower compression. This is because the blower is forcing more air/fuel into the cylinder, which offsets the reduced compression. To calculate compression ratio, you need to know two things, the cylinder swept volume and the total chamber volume. First is the formula for finding the swept voluem in cubic centimeters. Swept volume=bore x bore x stroke x 12.8704 For example, we'll use the 302, Swept volume equals: SV=4.00 x 4.00 x 3.00 x 12.8704 The next step is determining the total chamber volume. That means adding the head combustion chamber volume, head gasket volume and piston volume, subtracting if the piston has a dome, or adding volume if there are valve reliefs or a dished piston. A domed piston increases compression, while a dished piston or one with valve reliefs reduces it. For example, let's use a head volume of 60 cc's, with a gasket thickness volume of 9.5 cc's (this volume will be listed by the gasket manufacturer). Valve relief volume is 4 cc's, and the total chamber volume is 73.5 cc's. Now the compression ratio formula is: Compression Ratio=Swept Volume + Total Chamber
Volume 4.00 x 4.00 x .010 x 12.8704 = 2.05 cc's That increase, 2.05cc's, is added to the total
chamber volume figure of 73.5 for a total of 75.55. Rework the formula: Increased deck clearance means more chamber volume, which reduces compression. This is why it's important to get close to zero deck, since .010" is a reduction of .3 of a poiont of compression |