By Jeff Smith

“It says so, right there on the box – these pistons are 10.5:1 compression.”

Or – maybe not.

Compression ratio is one of the building blocks of power. Squeeze that mixture of air and fuel a little harder and after the cylinder goes bang, the crankshaft will spin with a little added grunt. But compression is a fickle thing – even with the right parts it’s easy to lose and tough to reclaim. We thought we’d take a close look at where the squeeze comes from and how with a little attention to detail and some proper machining, you can get that last little bit of squeeze out of your engine.

Compression ratio is the mathematical comparison of the volume of the cylinder with the piston at bottom dead center (BDC) versus the piston at top dead center (TDC). If the volume at BDC is 10 times the volume at TDC, then the compression ratio is 10:1. This is pretty simple. But the key is how an engine builder juggles these volumes to custom create the compression that he desires. Let’s start with the some simple observations that may or may not be obvious when it comes to calculating compression.

Stroke

Too few enthusiasts realize that the throw of the crankshaft has a very real impact on compression. If all we do is change the length of the stroke of the engine, we can increase (or decrease) the compression ratio. Let’s use a 5.0L Ford as an example with a 4.030 bore and a 3.00 inch stroke. We won’t get into all the other details, but let’s say this engine has an 9.1:1 compression ratio using a flat top piston. If all we do is increase the stroke to 3.25 inches, the static compression ratio jumps to 9.77:1. Of course, adding stroke also means the engine will need new pistons and perhaps a different rod length to accommodate the longer stroke so new pistons are almost always a requirement. But it’s worth pointing out that stroke is an important consideration.

Combustion Space

Note that we used the term combustion space as our heading. That’s because we’re going to be discussing the net effect of the interplay between the piston, deck height, and combustion chamber volume in regard to compression ratio. In the old days of big, open combustion chambers, the easiest way to increase compression was with a new piston fitted with a giant dome. A domed piston reduces combustion chamber volume and increases the compression ratio. Conversely, adding a dish to the piston adds volume to the combustion space, reducing the compression ratio.

We also mentioned deck height as part of the combustion space. This is defined as the position of the piston in relationship to the block deck. Most pistons are located below the block deck, which will add volume to the overall combustion space. There are some engines, however, that locate the piston above the block deck. Most of the time, this is in response to the use of very thick head gaskets. Placing the piston above the deck will reduce the overall volume of the combustion space. The most common procedure for setting deck height is by measuring the position of the piston and then removing material from the deck to set the proper deck height. What many enthusiasts don’t realize is that the deck is rarely parallel to the crankshaft centerline. The deck may be flat and yet not parallel to the crank. This non-parallelism directly affects the compression in each cylinder. We recently measured a Chevy 454 block that had been professionally assembled only to find the deck height varying by over 0.008-inch front to back. The two rear cylinders were 0.007 and 0.008-inch taller, which changed the calculated compression by as much as much as 0.19 or the difference between 9.74:1 and 9.93:1.

The head gasket is also another combustion space variable. Head gaskets vary in thickness between the very thin steel shim gaskets used for iron heads that measure around 0.015-inch to the latest multi-layer steel (MLS) style gaskets that can be thicker than 0.050-inch. The most popular style of head gasket is the composition gasket that is often around 0.040-inch. This thickness is essentially added to the deck height. Many compression ratio calculators only take into account the gasket thickness and compute the gasket volume by bore diameter and thickness alone. The problem with this assumption is it does not account for the fact that most head gaskets are much larger than the bore and are rarely perfectly round. If you are using a computer program to compute compression ratio, look to see if the program uses mere thickness or if it allows you to input the gasket volume. Many times, aftermarket head gasket companies will supply the gasket’s compressed thickness volume. This isn’t a huge difference, but if you are looking for accuracy, the discrepancy can be as much as 1 cc. It all depends on how accurate you want your calculations.

 

Beyond piston domes and dishes, valve reliefs are another variable in the combustion space that must be accounted for when calculating compression ratio. Let’s take the example of a flat-top piston small-block 340 Chrysler (4.07 bore and 3.31 inch stroke) with a flat top piston and a 65cc chamber. A pure flat top piston with no valve reliefs computes to 10.33:1 compression but add 6 cc for the reliefs and the compression drops to 9.64:1 with all the other variables remaining the same. In the case of the stock replacement pistons for a small-block Chevy 350, pistons usually come with four valve reliefs so one piston can be used on both sides of the engine. Those extra pair of valve reliefs cost about 4cc’s, which can push the compression down from 9.04 to 8.70:1, which is more than a quarter of a point. It’s small things like this that make a difference.

Yet another combustion space variable is something called crevice volume. This is the area between the top ring and the top of the piston. For nitrous and supercharged engines that will experience very high cylinder temperatures and pressures, piston manufacturers routinely move the top ring down on the piston to expose the top ring to less direct heat. This space is very small, but if you really want to account for everything it’s a variable that deserves to be included.

Let’s say we have a custom-built big-block Chevy where we’ve purchased a domed piston for our 0.030-overbored 454. The company that makes the piston literature lists this at 10.5:1 compression. Next, we deck the block but not in relation to the crankshaft centerline. By measuring the deck height of the four corner pistons, we discover piston deck height changes by as much as 0.010-inch. Plus, we’d like to account for some minor massaging we did to the piston tops to increase the depth of the valve reliefs. Even with all these variables we can still calculate with a high degree of accuracy the actual compression ratio for each of these cylinders.

What we have to do is mount one piston on its rod and connect it to the crank with its top ring attached. Smear some white grease on the bore to help the ring seal and then place the piston far enough down in the bore so that the dome is below the deck. In our case, we set the piston 0.300-inch below the deck.

Next, we used a burette and filled it with rubbing alcohol colored with some blue food dye to make it easier to see. We made our own square plastic lid out of clear plexiglass with a 0.250-inch hole drilled in it on one corner to allow us to fill the cylinder with liquid. We measured the volume of that cylinder at 45 cc’s and the calculated the volume of a normal cylinder at 4.280-inches round and 0.300-inch tall (4.28 x 4.28 x0.300 x 0.7854) which equates to 4.31 cubic inches of volume. Next we converted that to cc’s by multiplying times 16.387 to come up with 71 cc’s. By subtracting our true volume of 45 cc’s from the calculated volume of 71 cc’s, we now know that the true dome volume is 26cc. The piston manufacturer listed our piston as having a 23 cc dome. Despite the seemingly insignificant 3 cc difference, this changes our calculated compression ratio from 9.93:1 with 23 cc to 10.19:1 with a 26cc domed piston. That’s more than a quarter of a point!

Compression Ratio Programs

In the following sidebar, we’ve listed the formula and explanations on how to perform the calculations in long hand, although we don’t expect that anyone will actually do the math. That’s because there are probably a dozen or so different sites that offer a compression ratio calculator that will do the math for you in as much time as it takes to enter the data.

Our particular favorite is a free compression ratio calculator from Performance Trends.com. Kevin Gertgen owns the company and this was one of his very first computer programs. The reason we like this program is because you can download the program for free from his website – just click on the Downloads button on his home page. You can download the program for free into your hard drive and then run it anytime you like without having to go online. The idea behind the free use is that you will like how simple this program is to run and that you might like to run the slightly more complicated Dynamic Compression Ratio upgrade, which will mean you have to purchase the full Compression program.

The beauty of any of these compression ratio programs is that they allow you to experiment with different combinations to create the desired compression ratio. This will also show you which combinations don’t work, but used as a tool, the program can save you time and expense by showing you which parts will work the best. As an example, we built a stroker LS engine about five years ago. At the time, we built the engine like a small-block Chevy with a dished piston 0.005 inch below the deck. Only after the machine work was completed did we discover that the only available piece was a 0.053-inch compressed thickness MLS gasket. Now, Fel-Pro makes a 0.041-inch thick MLS head gasket that is 0.012-inch thinner, which will bump the dished piston combination from 9.8:1 to 10.05:1. This also demands the warning that you keep in mind that a thinner head gasket also moves the head closer to the piston and the valves closer to the pistons. In this case, our piston-to-head clearance is 0.041 + 0.005 = 0.046-inch which is more than adequate. This is also part of the report that even the free part of the Performance Trends program will give you.

Conclusion

There’s even more subtle nuances to the compression ratio game, but we’ll save those for another story. If you want to take this discussion to the next level, think about the concept of Dynamic Compression. Static compression ratio compares the ratio of the two cylinder volumes, but dynamic compression takes into account the variable of when the intake valve closes. One upgrade to the Performance Trends compression ratio software does this calculation for you. Dynamic compression just adds a layer of sophistication to the job of building and tuning high performance engines. Stay tuned for that piece.

Doing the Math

Compression ratio computer programs do the math for you, which is convenient. But it’s important to know how to get there with a pencil and paper. The following is a breakdown of the math with examples. In the old days, engine builders had to crunch through these variables perhaps a dozen times to generate a preferred combination.

Notice that we’re using a shortcut with the standard formula for the volume of a cylinder. The accepted formula is: Pi x radius x radius x height. The shortcut is Bore x Bore x Cyl. height x 0.7854. Keep in mind that this result will be in cubic inches – not cc’s.

Our examples are based on a 350ci small-block Chevy with a 4.00 inch bore, 3.48 inch stroke, a 64cc chamber, a flat top piston with 4cc’s worth of valve reliefs, a compressed gasket thickness of 0.041-inch, and a deck height of 0.010-inch. Abbreviations include cubic inches (ci) and cubic centimeters (cc).

Displacement: Bore x Bore x Stroke x 0.7854 x No. of Cylinders
Example: 4.00 x 4.00 x 3.48 x 0.7854 x 8 = 349.8 ci – or 350ci
Cylinder Volume in cc: Bore x Bore x Height x 12.8704 = Volume in cc
Example: 4.00 x 4.00 x 3.48 x 12.8704 = 716.6 cc
Head Gasket Volume: Bore x Bore x compressed gasket thickness x 0.7854 (Gasket bore is 4.060)
Example: 4.060 x 4.060 x 0.041 x 0.7854 = 0.5307 ci
Convert ci to cc: CI x 16.387 = CC 0.5307 x 16.387 = 8.69 cc
Deck Height Volume: Bore x Bore x Deck Height x 0.7854 = Vol. in ci, Bore x Bore x Deck Height x 16.387 = Vol. in cc
Examples: 4.00 x 4.00 x 0.010 x 0.7854 = 0.1256 ci, 4.00 x 4.00 x 0.010 x 12.8704 = 2.06 cc

We have now calculated the volume of the cylinder, cc’d the chamber, and calculated the volume of the head gasket and the deck height. We also need to know the volume of the piston top and whether to subtract the dome volume or add the dish volume to the chamber. Knowing all these variables, determining the compression is a matter of dividing the total cylinder volume with the piston at the bottom (BDC) by the volume of the cylinder with the piston at the top (TDC). To make the math simpler, subtract piston dome volume from the chamber or add dish (or valve relief) volume to the chamber volume. We left the piston dish volume in the formula below for clarity.

When crunching the numbers, remember to do all volumes in either ci or cc. These values are not interchangeable and all values must agree for the math to be accurate. This is basic algebra combined with a little geometry and right at the limit of our working knowledge!

Compression Ratio Formula
(Cyl. Vol. at BDC)+(Chamber Vol.)+(Gasket Vol.)+(Deck Ht. Vol.)+(Piston Vol.) /
(Chamber Vol.)+(Gasket Vol.)+(Deck Vol.)+(Piston Vol.)

Example:
(716.6cc)+(64cc)+(8.69cc)+(2.05cc)+(4cc) = 795.3 /
(64cc)+(8.69cc)+(2.05cc)+(4cc) = 78.74

Compression Ratio= 10.1:1

Sources

Performance Trends
(248)473-9230
www.performancetrends.com

Powerhouse Products
(800)872-7223
www.powerhouseproducts.com