An engine’s compression ratio is a big deal. You never see a low-compression racing engine unless it is arbitrarily limited by some class restriction. Higher compression ratios yield more power in racing engines and street engines. Everyone remembers the anemic low-compression 1970s and nobody wants to repeat them. Once the OEMs gained greater control over fuel and spark with EFI and electronic engine management, compression ratios shot up again because automakers know it makes more power and yields higher fuel efficiency. Higher compression ratios are the main reason diesel engines consistently produce better fuel economy than gasoline engines.
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Performance applications have to consider compression ratios carefully regardless of whether they are normally aspirated or highly boosted via supercharging. We want all the power and efficiency we can get, but a poor combination of parts can unduly influence an engine’s fuel octane tolerance with potentially disastrous results.
It is very important to know or predict compression ratio with a high degree of certainty, so appropriate fuel choices can be made. Now that we have low- and medium-octane gasoline, and high-octane E85 ethanol plus racing fuels, it is more important than ever to match compression ratio to the intended application and the fuel that will be burned. In the case of new engine builds, a suitable mix of components can be tailored to meet a target compression ratio that is either octane friendly or in some cases, sanctioning body mandated.
Octane-limited engines are always at risk for fatal engine damage. That’s why engines in the 1980s began sporting knock sensors that would signal the onboard computer to retard the spark advance when the onset of detonation was detected. Today we have the luxury of engine management controls that allow us to run higher compression ratios, but we still have to calculate them according to specific requirements.
Compression ratio is an effective means of limiting power in some racing series. It is also used to curb the cost of many racing venues. It typically influences piston and cylinder head selection where a particular cylinder head may also be specified by a sanctioning body. When cylinder head and chamber size are dictated, piston configuration, deck height, and gasket thickness must be juggled to chase the compression ratio requirement. Short tracks frequently enforce a 9:1 rule while NASCAR engines are limited to 12:1. Unlimited drag racing and Bonneville engines often exceed 14:1, while stock class drag racers are limited to the original factory compression ratio of their particular vehicle.
Compression-ratio limits can be useful to a degree since they generally dictate flat-top pistons, which encourage efficient combustion while maintaining desirable quench to promote charge turbulence and maintain mixture quality. Hypereutectic pistons are often specified, although forgings are permitted in some series. There is certainly less bang for the buck without higher compression ratios but, given specific parameters, experienced engine builders adjust contributing components to best suit any fixed compression ratio, particularly with an eye toward increasing the effective compression ratio via appropriate camshaft timing and effective inlet tuning.
Factors Affecting Compression Ratio
Quickly name ten or more things that affect or are affected by compression ratio. If you can’t, consider the following :
- Fuel octane
- Fuel mixture quality (droplet size)
- Cylinder volume
- Combustion chamber volume
- Deck height
- Compressed gasket thickness
- Gasket shape
- Piston to head clearance
- Quench area
- Dome or dome volume
- Dish volume
- Ignition timing
- Valve relief volume
- Crevice volume
- Bore chamfer
The formula for calculating compression ratio is pretty simple. We’ll work with some examples in a moment, but first let’s examine the influence of the items on our list, particularly those within our control during an engine assembly process. Of course fuel octane tolerance is a primary concern so we need to know what fuel we will be using. The mixture quality of that fuel is largely determined by air temperature, fuel blend, and the induction components that meter fuel to the engine. These would include the carburetor or the fuel injectors, the intake manifold, cylinder heads, and valves. Even the camshaft timing can have an effect on dynamic compression— or cylinder pressure. Those are all things we can control and so are the items on our list, all of which are right there inside the cylinder exerting their influence on compression ratio. Consider the basic formula.
Compression Ratio (CR) = (V1 + V2) ÷ V2
V1 = cylinder volume
V2 = combustion chamber volume
In practice, V2 is actually called the clearance volume or the compression volume because it incorporates all of the items on our list and actually represents the total combustion space above the piston. This is the space the cylinder volume gets squeezed into during compression. I will call it the compression volume for our discussion. So the formula actually establishes the ratio between the total cylinder volume with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke. Each item on our list changes the value of V2 to some degree or another and that has a profound influence on the actual working compression ratio.
There are two types of deck height: positive and negative. On most engines the piston stops slightly below the block deck surface when it is at TDC, sometimes 0.020 inch or more. This is called positive deck height because the block deck is still above the piston top. No matter how small it might be, this distance contributes additional volume to the combustion space V2 above the piston. This volume must be calculated and added to V2. In some cases the piston protrudes slightly out of the bore. This is called negative deck height and its volume must be subtracted from V2 because it subtracts volume from the combustion space.
Compressed Gasket Thickness
Head gasket volume also adds to the compression volume. It is dictated by the compressed gasket thickness, the gasket bore diameter, and the gasket shape. Many head gaskets are slightly larger than the cylinder bore diameter and they often have irregular shapes. Deck height and gasket thickness also influence piston-to-head clearance which must be considered, especially in high-RPM applications. Steel connecting rods don’t really stretch, so you can get that piston right up close to the cylinder head (without consequence to improve quench). Quench is where the flat top portion of the piston rises very close to the head, which tends to force or squirt the charge toward the spark plug with high chamber turbulence to improve the burn.
Aluminum connecting rods have some degree of elasticity, so they require increased piston-to-head clearance to avoid physical contact and the ensuing damage at high engine speeds.
These requirements can influence your choice of gasket thickness and thus compression ratio. Often you have to juggle the combination to get what you want. Calculating it ahead of time helps you make those choices.
Dome Volume and Dish
Volume If the piston has a raised dome to increase compression, the dome’s volume must be considered in the compression ratio calculation. Dome volume must be subtracted from V2 since it reduces compression volume. Dish volume is added to V2 since it adds volume. And while you’re figuring dome and dish volumes, you also have to account for the volume of any valve reliefs in the piston top.
And if you really want to pick nits, you can include the crevice volume above the top piston ring and the chamfer volume at the top of the cylinder bore. While infinitely small, they still contribute to the total volume of V2 in the equation. Crevice volume is the tiny space between the piston and the cylinder wall above the top ring. Typically only a few thousandths of an inch, it is still multiplied by the bore circumference and has a volumetric value. And if the cylinder bore also has a large chamfer to aid piston installation, it also contributes volume to the combustion space. Crazy huh?
Some of these volumes are inconsequential in most cases, but you should know about them so you can decide whether to incorporate them in your calculations. If you are building a high-performance engine, you’ll be repeatedly measuring and altering many of these volumes during pre-assembly mockups. Proper clearance between rapidly moving parts is essential and unforgiving so you have to establish them first. An awareness of their effect on compression ratio helps you consider your alterations and parts selection accordingly.
Compression ratio is no simple thing, especially when you break it down into all its contributing factors. Still, it’s manageable and there are many ways to look at it. While this is primarily an engine math book, it is still important to comprehend all the factors and the manner in which they affect engine performance. Compression ratio is simply a measure of how tightly the incoming charge is squeezed before the spark plug ignites it. It is created by the combined volume of the cylinder and the compression volume when the piston reaches TDC. In reality it is controlled by the swept volume of the cylinder and any combination of the various combustion space volumes that make up the compression volume V2. Since that is where all the variables are, that is where you have to concentrate your efforts to achieve the compression ratio you want.
To see how much influence these factors have, let’s compare the basic formula to the same formula with all factors considered. As previously discussed, the various contributing factors are either additive or subtractive from the total compression volume. The combustion chamber is the primary value. All other volumes are either added to it or subtracted from it prior to working the basic equation.
CR = V1 + V2 ÷ V2
This comparison between a domed piston and a dished piston illustrates how the dome projects into the combustion chamber to raise compression by reducing chamber volume while the dished piston increases combustion space volume to reduce compression ratio. Determine combustion chamber volume by filling the chamber with water or alcohol from a graduated burette calibrated in cubic centimeters (cc). Tighten a spark plug in the chamber with both valves installed. Then use a light grease to seal the deck surface. Place the plastic cc plate over the chamber and position the head so the fill hole is at the highest point. Fill the chamber and read off the burette. Divide by 16.4 to convert to cubic inches.
Note that V1 is constant, but V2 can vary by a considerable degree when you start adding and subtracting the various values that contribute to it. In the simple formula, V2 is called chamber volume, but we know it is really compression volume because it incorporates other factors. If you add all the other factors, it makes a very long equation. You can break it down by calculating absolute V2 before plugging it into the equation. This requires precise measurements although, in practice, published values for gasket volume, dome and dish volume and valve relief volumes are often substituted. Crevice volume and chamfer volume are typically ignored because they are so small. The following list is called the V2 stack.
To find absolute V2, begin with the measured chamber volume with cubic centimeters converted to cubic inches, then:
add deck volume (or subtract if deck is negative)
add compressed gasket volume
add dish volume (or subtract if dome)
subtract dome volume (or add if dish)
add valve relief volume
add crevice volume (if desired)
add chamfer volume (if desired)
This is simple, but somewhat tedious to measure and calculate so many engine builders choose to measure it all at once by cc’ing a cylinder with a piston in it. I will explain how to do that in a moment, but first let’s discuss how to determine all the individual volumes that make up V2.
Calculate deck volume as if it were a very short cylinder. The positive or negative deck measurement represents the height dimension in the formula which uses the displacement constant 0.7854.
Example: for a 0.020-inch-positive deck height on a 4-inch bore
42 x 0.020 x 0.7854 = 0.251328 ci
This will be added to the V2 stack because it increases the compression volume. If the deck measurement were negative (piston above deck), the result would be subtracted from the V2 stack because it reduces compression volume. An interesting fact is that all small-block Chevys are positive-deck engines, but the newer Gen III engines all have negative decks.
Combustion chamber volume is measured directly by cc’ing a chamber with a graduated burette. Note that chamber size in cubic centimeters must be converted to cubic inches. Divide by 16.4 to make the conversion. This will be your base volume for calculating compression ratio. All other relevant volumes are either added to or subtracted from the chamber volume to determine the compression volume.
In most cases gasket volume is published by the gasket manufacturer and it is safe to add (+) to the V2 stack. When no published number is available, builders often fudge by calculating the volume based on a perfect circle (just like deck height volume). The problem is that the gasket bore diameter is often larger than the cylinder bore diameter and frequently irregular in shape. If it is perfectly round, you can calculate using the cylinder volume formula with the appropriate diameter and compressed thickness.
If the shape is irregular, you can fudge it or use the string-and-tape method to find the true circumference of the gasket bore and then treat it as a perfect circle for the calculation. Tape the gasket to a flat surface and use small bits of tape to secure a thin string around the perimeter of the gasket bore opening. Once you reach the starting point, carefully cut the string and measure its length.
Using the formula for the circumference of a circle, you can find the appropriate diameter to use in the gasket volume calculation. Suppose you have a 4-inch cylinder bore, and your gasket bore is noticeably larger with an irregular D-shape around the valves (which is typical of many head gaskets). You carefully string the perimeter and get a length of 131⁄16 inches. Convert to decimals and you have 13.0625 inches. Now plug this dimension into the formula.
Circumference = 2 π r, or C = π d
r = radius
d = diameter
d = C ÷ π
13.0625 ÷ 3.14 = 4.16 inches
That’s your true gasket bore diameter and it can now be plugged into the gasket volume formula:
True Gasket Volume = 4.162 x gasket thickness x 0.7854
Dish volumes are typically published so you can usually plug them right into your V2 stack. But let’s say your block has already been decked a couple of times and it’s a little shorter than normal so the piston has a negative deck by some amount that is more than what you are comfortable with for piston-to-head clearance.
Most pistons tolerate some degree of piston deck shaving (up to 0.100 inch or even more in many cases) so you decide to cut them down to achieve a zero deck (piston flush with block deck surface). This is easy to do with flat-top and dished pistons; it’s slightly more complicated with dome pistons (rarely done).
If your piston is dished and you cut it down by some amount, you can cc the dish and add the new volume to your V2 stack. Or you can use the cylinder volume formula to calculate the difference if you have an accurate measurement of depth and diameter. In practice this is never easy because the dish is not always perfectly round and is often D-shaped and curved at the bottom.
Dome Volume Dome volumes are also published by piston manufacturers. They are pretty accurate so you can safely subtract that volume from your V2 stack as long as you haven’t modified the dome by fitting it to the chamber shape, cutting deeper valve reliefs or cutting a fire slot for the spark plug. Sometimes during mockup assembly you identify a small spot where the piston dome contacts the chamber roof during rotation. These spots are typically cut down to achieve the minimum clearance and that alters the dome volume, which then requires you to measure it. Moroso sells a simple tool for cc’ing dome volumes and it comes in handy in this situation. Remember, dome volume gets subtracted from you final V2 stack.
Valve reliefs are easy enough to cc on a flat-top piston and most manufacturers already publish the volumes for all their pistons. Here again, you only need to measure if you have significantly plunge cut the reliefs to gain adequate piston-to-valve clearance. Whatever the volume, it is an additive value to your V2 stack.
Crevice volumes are minimal and are not frequently incorporated in compression ratio calculations, but some builders find reason to do so. Some are simply detail freaks. Crevice volumes have long been known to affect emissions because they provide a hiding place for small amounts of fuel mixture that didn’t quite participate in the combustion process. This is mostly important to chemists and combustion engineers but if you want to include it, here’s how.
CV = (d1 – d2) x c x r
d1 = bore diameter
d2 = piston diameter at top ring land
c= bore circumference
r= top ring depth from piston deck
So, with a 4.00-inch bore, 0.010-inch piston-to-wall clearance above the top ring and the ring 0.125-inch down the bore we calculate:
CV = (4.00 – 3.990) x 12.56 x 0.125 = 0.0157 ci
12.56 is the circumference of the bore and is found by multiplying bore diameter times pi. If you want to be precise, add the result of your final calculation to your V2 stack.
Most machinists put a chamfer at the top of the bore to help guide the rings into the bore during assembly. Sometimes it is quite substantial so you may want to include it in your calculations. Chamfers are generally 40 to 60 degrees and even at these small dimensions, you can pretty much treat them as squares or rectangles when viewed end on. Use the same formula as for crevice volume, but start with the larger outside dimension where the chamfer starts (see fig. 1, page 35)
If it is about 0.060 larger than the cylinder bore:
CV = [(4.060 – 4.000) x 12.748 x 0.060] ÷ 2 = 0.022 ci
Note that the “c” dimension has changed because we now have a 4.06-inch outer diameter (4.06 x 3.14 = 12.748). The depth is only 0.060 inch and we have to divide the result by 2 to complete the formula for the area of a triangle and thus the volume when the length is added.
The result is more than the crevice volume, but still nothing substantial, which is why most engine builders exclude crevice volume and chamfer volume from their calculations. If you use them, remember that they are additive and thus are added to your V2 stack. Crevice volume and chamber volume partially occupy the same space, but it is more convenient to calculate them separately.
Now let’s review our V2 stack with calculated values based on the following dimensions:
Bore/Stroke, 4.00 x 3.00 inches ………………37.699 ci
V2 Chamber volume, 64 cc…………………………3.902 ci
Deck height, 0.020 positive ……………………0.251 ci
V2 + Gasket thickness, 0.015 (published) ……….0.194 ci
V2 + Flat top (or dish/dome) …………………………0.000 (flat) ±
Valve relief, 4 cc (published) ………………….0.243 ci
V2 + Crevice volume, calculated ……………………0.015 ci
V2 + Chamfer volume, calculated ………………….0.022 ci
V2 + Total 4.627 ci = V2
V1 + V2 ÷ V2 = CR
(37.699 + 4.627) ÷ 4.627 = 9.14 CR
That’s adequate, but perhaps a little low for street performance. If you zero deck the block and eliminate the deck height dimension from V2, you can raise the compression ratio to 9.61:1, just about right for a street engine. That small change illustrates just how much influence all the small volumes that make up V2 have on the final compression ratio.
The concept of displacement ratio is not frequently used, but it should be understood because it can sometimes help us estimate the amount of combustion chamber milling that will achieve a desired compression ratio. As we have seen, compression ratio is the combined volume of the swept cylinder volume and the compression volume divided by the compression volume (see sidebar, page 37). Displacement ratio is simply the swept cylinder volume divided by the compression volume:
Compression ratio = V1 + V2 ÷ V2
Displacement ratio = V1 ÷ V2
Note that the compression ratio is always 1 greater than the displacement ratio. By rearranging the displacement ratio formula we can calculate a new compression volume V2 that will yield the desired compression ratio.
New V2 = V1 ÷ displacement ratio
Now we can derive a formula for cylinder head milling:
Mill Cut = [(new displacement ratio – old displacement ratio) ÷ (new displacement ratio x old displacement ratio)] x stroke
Recall that we previously calculated a compression ratio of 9.14:1 for a 4.00-inch bore and a 3-inch stroke. Since displacement ratio is always 1 less than compression ratio, we use 8.14 for the displacement ratio in our formula. We already saw that eliminating 0.020 inch of deck height raised compression to 9.61:1. Now let’s see what reducing the combustion volume does. Since we want to raise the compression to 9.61:1, our displacement ratio is 8.61.
Mill Cut = [(8.61 – 8.14) ÷ (8.61 x 8.14)] x 3 = 0.0201 inch
That is almost exactly the same amount as the deck height we eliminated in our previous calculations, but is it correct? Not exactly. In removing the deck height dimension, we accounted for the entire bore diameter of the cylinder. But the D-shaped combustion chamber on our small-block Chevy is only about half the size of the bore. We have to make a deeper cut to get the same result. In this case, about 0.040 inch gives us the desired result. We have to double the cut because we’re only dealing with half the area. These are relatively straightforward procedures, but you have to think carefully about them to avoid costly mistakes.
Cranking compression is often confused with compression ratio. While compression ratio is a relationship of volumes within a cylinder, cranking compression is actually a measured cylinder pressure taken at the spark plug hole while the engine is bring cranked with the throttle plates held open. The coil wire is removed during this operation to prevent the other cylinders from firing. Cranking compression is the peak pressure achieved within the cylinder during cranking. Higher compression ratios can affect cranking compression, but the two are not related.
Cranking compression is used as an indicator of engine condition and also of the relationship of intake and exhaust valve opening and closing points. Depending upon the condition of the piston rings and valves, a healthy engine typically has a cranking compression between 150 and 180 psi. A good performance engine can easily have a cranking compression of more than 200 psi. Some are a little higher and a few are much lower. The important thing is that all cylinders should read the same during a compression test. A low reading on any cylinder typically indicates leaky valves or piston rings. Big camshafts with a lot of valve overlap can also affect cranking compression but not by any great amount. As long as all the cylinders match within 5 or 10 psi, you probably have a healthy engine. Inexpensive compression gauges are available at any auto parts store.
Written by John Baechtel and Posted with Permission of CarTechBooks