Compression Lessons

Performance engines are built to push the envelope. A bigger cam, a single-plane intake manifold, a monster carburetor, and large-tube headers all contribute to making more power. Everyone also wants to push the compression as well. Certainly, the harder you squeeze what’s between the combustion chamber and the top of the piston, the more power you’ll make. But compression isn’t always the big power guarantee that many think it is. Let’s take a look at compression, how to figure it, and how it plays into the big picture power scheme.

Let’s start out by defining compression ratio, which is the ratio of the cylinder volume with the piston at the bottom of its stroke (bottom dead center or BDC) compared to the cylinder volume when the piston reaches the top of its stroke (top dead center or TDC). If we measured a small-block with a cylinder volume of 45 ci at BDC and a cylinder volume of 5 ci at TDC, then 45 divided by 5 equals 9, giving us a compression ratio of 9:1.

While this sounds simple, it takes accurate measurement and some patience to come up with these values. We’ll go over how to measure these different volumes and then show you an easy way to calculate compression. There are several volumes you must measure to determine compression. The largest is the volume of the cylinder. The other volumes that affect compression include the combustion-chamber volume, the deck height (the height of the piston above or below the block deck), the head-gasket volume, and the piston design that will either add volume (a dished piston) or subtract volume (a domed piston) from the combustion chamber. All of these components have a direct affect on compression. For example, as the bore becomes larger, compression increases. If you use heads with a smaller combustion chamber, the compression will increase. Even changing to a thinner head gasket will increase the compression ratio.

Number Crunching

Let’s take a look at how all these volumes interact by detailing the simple math that’s used to determine compression ratio. The equation for the volume of a cylinder is (i.e., 3.1417) / 4 x Bore x Bore x Stroke. This can be shortened to 0.7854 x B x B x S = volume of a cylinder. For a standard-bore 350ci engine, this computes to 0.7854 x 4 x 4 x 3.48 = 43.73 ci. Since most of the measurements we will be dealing with in the engine are measured in cubic centimeters (cc), we will convert cubic inches into cubic centimeters by multiplying ci times 16.39, which in this case is 43.73 x 16.39 = 716.7 cc.

Rather than try to compute the volume of a complex combustion-chamber shape, it’s best to measure the volume with a burette graduated in cc’s. For our example, let’s say our small-block head comes out to 76 cc. Next, we’ll need to measure the piston’s deck height. The best way to do this uses a dial indicator on the piston of an assembled short-block. Virtually all small- and big-block Chevy deck heights will place the piston below the deck surface. Often, production engines can have as much as 0.020 to 0.040 inch of clearance between the top of the piston and the deck. Machining the block surface can reduce this distance, an operation referred to as decking. Reducing this distance increases the compression ratio. This can be computed exactly the same way as the cylinder volume, except substituting the deck height for the stroke. Using our 350 example, let’s say we’ve measured the deck height at 0.010 inch. This computes to 0.7854 x 4 x 4 x 0.010 = 0.1256 ci x 16.39 = 2.06 cc.

Piston shape also plays a big role in compression. A pure flat-top piston does not affect compression—but rarely occurs in the real world. Even a “flat top” piston incorporates valve reliefs in the piston top that add volume. A typical small-block, four-eyebrow piston contributes roughly 4 to 6 cc’s worth of volume. This is the same as adding that amount to the combustion chamber size.

Dished pistons perform the same function by increasing the volume above the piston, adding between 10 to 20 cc of volume that reduces compression. Conversely, a domed piston increases compression by displacing volume from the combustion chamber. For example, a piston with a 10cc dome is effectively the same thing as a pure flat-top piston with a 10cc-smaller combustion chamber.

Finally, head gaskets play a critical role in compression ratio and offer the easiest and least expensive route to changing compression. However, head gaskets can be a bit deceiving. You might think that all you have to do to compute the volume is to treat the gasket like deck height. If we compute volume for a standard 350 Chevy gasket like the Fel-Pro 1003 with a compressed thickness of 0.041 inch, we come up with 0.515 ci, which equals 8.44 cc. But this assumes the gasket bore is both round and the same size as the cylinder bore. In reality, the 1003 gasket is 4.166 inches in diameter and is not round. The better way to determine compression is to use the manufacturer’s published gasket volume. Fel-Pro’s published gasket-bore volume for the 1003 gasket is 9.1cc, roughly 0.7cc larger than our computed volume. While this isn’t overly critical, it does affect the accuracy of the final result.

Speaking of accuracy, there is another small volume that is usually ignored but also contributes to compression. This is called the crevice volume and is the tiny volume between the compression ring and the top of the piston. Typically this will not increase total volume by more than 1 cc, but if you’re looking for complete accuracy, it should be included. The best way to account for crevice volume is to measure the entire piston/cylinder assembly with the engine assembled. Using a flat plastic plate with a hole drilled in the top, you can place the piston ½ inch down the cylinder and then fill that volume from a measured burette. Then compare the volume of a perfect cylinder to the amount you measured. The difference will be the combination of any piston top valve reliefs and the crevice volume.

Continuing with our 350 engine example, with a 4-inch bore and the piston top ½ inch down the bore, a perfect cylinder would measure 0.7854 x 4 x 4 x 0.50 = 6.28 ci x 16.39 conversion = 103 cc. Measuring that same cylinder using a four-eyebrow piston resulted in a 110cc volume for a total difference of 7cc. Combining this crevice volume with the more accurate manufacturer’s gasket volume produces a much more accurate static compression ratio that can often be 0.10-ratio lower than computed using the less accurate method.

The Easy Way

The sidebar Doin’ the Math details the longhand method you can use if you don’t have a computer. But for the lazy ones like us with a computer and Internet access, there’s a faster way to compute. Performance Trends is a Detroit, Michigan-area company that has been creating excellent high performance automotive software for many years. If you go to the company’s Web site (www.performancetrends.com), you can download an easy-to-use compression ratio program for free. The basic program does the math for you instantaneously, which allows you to experiment with all the variables. The program offers several other optional features. If you want the bells and whistles model, it’ll cost $40, which is still inexpensive as a time-saver. Just stick it in your hard drive and start calculating.

If you’ve never computed compression ratio before, this is worth experimenting with on a number of levels. For example, stroke has a much greater effect on compression than bore, which is why the short stroke/small displacement engines require such small combustion chambers. Other areas worth investigating are variables like gasket thickness. Replacing a thick 0.051-inch composition head gasket on a 350 with one of Fel-Pro’s 0.024-inch rubber-coated head gaskets can pump the squeeze factor from 8.68:1 to 9.16:1.

Another way to use this program is to play with various volumes to create a suitable combination. Let’s say you’d like to use 64cc chamber Vortec iron heads on a 383ci small-block. Unfortunately, this small chamber combined with a flat-top, 6cc valve relief piston, a 0.015-inch deck, and a 0.041-inch thick gasket creates an excessive 10.6:1 ratio—too much for an iron-head street motor on pump gas. Using the software, we discovered that a 20cc dished piston drops the compression to a more 92-octane–friendly 9.2:1 compression.

The speed of this Performance Trends compression ratio program allows you to play tons of what-if games without spending a dime or cursing your calculator. So if learning a few compression lessons sounds like fun to you, log onto Performancetrends.com and download the program. It’s fast, it’s easy to run, and best of all—you might learn something.