What's Compression Ratio
Strictly defined, compression ratio is the total volume above the piston at bottom dead center (BDC), divided by the total volume above the piston at top dead center (TDC). First, let's look at the things that make up the total volume above the piston at TDC, which is the clearance volume.
1. Chamber Volume: This volume is measured in cc's, usually by checking the volume with a graduated burette. A burette is a long glass tube with cc's marked off on its length and a shut-off valve at the bottom. Many cylinder heads have typical cc numbers listed by the manufacturers or other references, but when it's important to know the true volume, measuring the heads is the only way to be absolutely sure.
2. Gasket Volume: This volume is also measured in cc's, and is published by the gasket manufacturer. The published numbers are generally accurate, but if no numbers are available, the cc's can be estimated by figuring 2.2 cc per .010 inch of thickness for small-blocks, and 2.5 cc per .010 inch for big-blocks.
3. Piston Dome Volume: The piston's net dome volume is typically published by the manufacturer, and is comprised of the dome-or material above the piston deck-minus the valve notches. Domed pistons lessen the clearance volume, since the net of the dome and notches takes up space above the piston's deck. With flat-top pistons, the volume considered is just the valve notch clearance, which will add to the total clearance volume. If the pistons have been massaged with custom cutting or profiling on the domes, about the only way to accurately get the cc volume is to measure the domes directly (see How to measure).
Deck clearance is easy to...
Deck clearance is easy to measure, using a deck bridge or dial indicator, as shown on this 318 short-block. Just zero the indicator at the deck, park the piston at TDC, and measure the distance from the piston to the deck as near to the piston's center as practical. Try rocking the piston in the bore in the axis of the piston pin and see how the measurement varies. If the rocking alters the reading, use an average number.
4. Deck Clearance Volume: This is the clearance created by the piston's position in the bore. If the piston is way down in the hole at TDC, there will be a loss in compression due to deck-clearance volume. To figure this volume, you can do the math, πr2(h) where r is half the bore size, and h is the piston deck clearance. Then, multiply the result by 16.387 to convert from cubic inches to cc's. Most handy online calculators let you just enter the piston-to-deck measurement in thousandths of an inch and will do the math for you.
5. Crevice Volume: This is the volume of the area between the top of the piston to the top of the ring. The volume here is fairly small, and the crevice volume is usually neglected when the ratio is calculated. You can add about 1/2 cc to account for it.
With the total volume above the piston at TDC calculated, getting the total volume above the piston at BDC is just a matter of adding in the swept volume. The swept volume can be obtained by again doing the math. By using the formula πr2(h), where r = 1/2 the bore and h = the stroke, and again multiplying by 16.387 to get cc's. To get the compression ratio, simply divide the total volume above the piston at BDC by the total clearance volume at TDC.
Get Real, Dulcich
Ok, some of the above may be a little more math-heavy than a lot of guys want to get into. Luckily, these days there are numerous Web sites with calculators that do the heavy number crunching for us, and it's free. These still need the basic numbers to punch in, but they do away with having to figure the volumes associated with deck clearance and swept area. One of our favorites is http://www.race-cars.net/calculators/compression_calculator.html. This handy calculator is easy to use and greatly simplifies calculating compression. Just plug in your bore, stroke, chamber cc's, piston dome cc's, and the gasket thickness in thousandths, plus or minus the amount the piston is in the hole. The calculator will spit out the compression ratio in an instant. It is especially handy in quickly comparing how various changes will affect the ratio, without all the longhand-math and slide-rule action.
Here we have an Edelbrock...
Here we have an Edelbrock small-block head, which has been modified with some porting and chamber work. To find the cc's, we installed a spark plug in the chamber to be measured. We then dropped in the two valves using a little grease at the valve seats to provide a fluid-tight seal. The head was leveled with a slight angle to the plug side from which we will fill it.
How To Measure
No compression-ratio calculation is worth spit if the numbers used to calculate it are wrong. In the old days, a lot of the chamber volumes published for stock heads were NHRA minimum-legal blueprint specification figures for Stock and Super Stock racing classes, which were miles away from production reality. These days, aftermarket head manufacturers are fairly honest about chamber volumes. For the most part, using published catalog numbers will get you close in calculating compression ratio-unless, of course, the heads have been altered by milling, porting the chambers, different valves, or the like. To find the true chamber volume, the heads need to be cc'd. Here's how to do it.
Any gearhead knows that pistons affect compression ratio. Two factors come into play when determining the ratio-compression height and crown configuration.
Compression height is the distance from the centerline of the wristpin to the top deck of the piston. A piston with a short compression height will sit lower in the bore as the crank reaches TDC. Estimating the piston deck height is fairly simple. Add half the engine's stroke to the rod length and the piston's compression height. After that, subtract the deck height to get the piston deck, or how far down in the hole the piston should end up at TDC. For example, a 360 has a stroke of 3.58 inches. Half of that is 1.79 inches, which is the up-stroke from the crank centerline. Add the length of a standard small-block rod at 6.123 inches and you get 7.913 inches. Now, if we are considering a Speed-Pro H116CP piston, which has a compression height of 1.670 inches, we add that in to get 9.583 inches. Since an uncut Mopar small-block cylinder measures 9.600 inches from the crank centerline to the deck, and our up-stroke/rod/piston numbers add up to 9.583, the pistons should be .017 inch down from the top of the block at TDC (9.583-9.600 = -.017).
Crown configuration is fairly self-explanatory. Valve notches and dish designs add clearance volume and cost compression, while domes take up clearance volume and add ratio.
When building an engine, one of the most important choices is selecting the correct piston for the job. For the typical stock-stroke, pump-gas small-block, a zero-deck piston is usually the best choice. Typical low-cost, stock-replacement, flat-top or dished pistons may look like a bargain, but the price you pay in compression ratio is anything but cheap. Take, for instance, a stock-replacement 318 cast piston. These have a compression height of 1.720 inches, putting the piston a good .100 inch in the hole at TDC. The valve clearance notches cost another 1.5 cc. With standard 64cc 318 heads, you're looking at 8.0:1 compression, while a set of higher-flowing 360 heads drops the ratio to 7.75:1. There's no way to make power at that ratio. Milling the 360 heads .060 inch and using a thin, .025-inch-thick MP gasket will just score an 8.6:1 ratio, but now the pushrod geometry is messed up, the intake face will need to be milled, and you've spent more money than you would have by getting higher compression pistons to begin with. The same 318 with a set of Keith Black PN 167 pistons can be set to zero deck with a little block milling. This will net a 9.3:1 compression with higher-flowing 360 heads, or over 10.2:1 with late-model 302 or Magnum heads using a standard .039-inch-thick gasket-bagging an ideal quench clearance (see below) in the bargain.
A Plexiglas plate is sealed...
A Plexiglas plate is sealed to the chamber with some grease, and a burette is used to fill it with fluid from a hole positioned to chamber's edge. Just fill it up, and measure how much fluid volume it takes to fill. These chambers measured 65cc's.
Let's look at 360 engine combos, probably the most popular small-block for performance applications. With a stock-replacement dished piston, the compression ratio works out to be 7.9:1 with a stock chamber size of 68 cc and a standard .039-inch-thick gasket. Now, chasing compression by milling the heads .060 inch and using a thin head gasket will get 8.7:1. That's better, but it's far off the mark for serious performance-and be prepared for the typical problems that arise from heavy milling. Again, choosing a more performance-minded piston, such as Speed-Pro's 116 CP flat-top-which can be set up to zero deck when building the engine-provides an easy 10.2:1 ratio with a stock 68cc chamber head, and a .039-inch-thick head gasket.
|318 2V castings||1968-84||64cc|
|318 #302 swirl port||1985-91||59 cc|
|MP Magnum R/T|| ||60 cc|
|MP Aluminum Magnum||53 cc|
|MP W2 closed chamber||55 cc|
|MP W-5-race||59 cc|
|MP W-5-street||60 cc|
|MP W-9||62 cc|
|Edelbrock 6077||63 cc|
|Edelbrock 6017||65 cc|
|Indy 360-1||63 cc|
|Indy 360-2||63 cc|
|Brodix B1BA||65 cc|
Getting the volume on domed...
Getting the volume on domed pistons that have been radically altered, either by profiling or fly cutting, will require a direct measurement. To do that, place the piston down in the bore a precisely measured amount (usually .500 inch), and add grease to the ring land area to seal the piston to the bore. Now measure the total cc's it takes to fill the cylinder to the deck with a burette. Compare the measured volume (cc's) to the mathematical volume of the cylinder at .500-inch depth. The difference is the dome volume or clearance. In this case, a 4.350-inch bore at .500 inch down has a volume computed mathematically of 121.7 cc. With the piston deck set at .500 inch down, we measured 114 cc of fluid to fill it. The missing 7.7 cc is the net dome volume.
Quench And Compression
Pre-Magnum small-block heads were a fully open-chamber design, with a recessed relief of over .100 inch cast into what is normally the quench side of the chamber. Modern performance and production heads are typically a quench design, in which the head's flat deck extends over a substantial portion of the cylinder bore. As the piston approaches TDC, the space between the quench portion of the head and piston rapidly closes up to the designed minimum clearance. As the piston is approaching TDC, the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence.
Secondly, a cooling effect is imparted on the remaining gasses in the closed quench area, since the hot gasses are in close proximity to the cooler surfaces of the chamber. This cooling of the end gasses-which are normally the last gasses reached by the flame front and the most likely to detonate-is properly referred to as the quench effect. The quench effect materially enhances detonation tolerance.
Quench heads are sensitive to the clearance between the head and piston at TDC. As quench clearance is lessened, the velocity of the gasses pushed out of the cylinder goes up. Cut the quench clearance in half, and the mixture is expelled at four times the velocity. Though we've heard some guys brag about running the quench so tight the pistons kiss the chambers (this will occur somewhere between .030- and .020-inch clearance), .040 inch is considered a safe and effective quench clearance. Open the quench gap over .060 inch, and most of the benefit is gone, and at some critical point the detonation tolerance will be substantially reduced.
Beyond enhancing the detonation limits, a tight quench clearance is credited with measurably enhancing torque production. Building an engine with an effective quench comes down to selecting a piston-and-head combination with the required clearance. Since about .040-inch clearance works so well, a flat-top piston at zero deck combined with a closed chamber head and a typical .039-inch head gasket is the easiest and most straightforward route to building effective quench into an engine.
Displacement Effects And Strokers
Stroker engines greatly increase displacement, but generally the heads that top them were designed for much smaller displacement engines. With a stroker, the clearance volume at TDC remains about the same as with a smaller engine, but the total volume goes way up as a result of the far greater swept area. Essentially, you now have a far greater volume of gas compressing into the same amount of space at TDC, and the result is a much higher compression ratio. With more cubes, higher compression ratios become easier to achieve. In fact, often the problem with strokers is to select components that prevent the compression ratio from ending up too high. For example, a 416 small-block stroker with a zero-deck piston, topped with an Edelbrock head and Fel-Pro gasket, will have a ratio of 12.2:1. The exact same combo of a zero-deck flat-top in a 340 will have a ratio of 10.25:1. As can be seen, sometimes with large-cube strokers, the challenge is getting compression ratio out of the engine.
There are several ways to get a stroker's ratio down to a pump-gas level of around 10:1. Dropping the pistons in the hole, using dished pistons, running thick gaskets or big, open chamber heads will all accomplish a lower ratio, but will preclude building an effective quench. The best solution is to use a piston with D-cup, which has a dish over half of the surface to reduce the ratio, while the far side remains at zero deck for an effective quench.
Cams and Effective Compression
What does a cam have to do with compression ratio? Nothing really, but most guys have heard that the cam and compression ratio need to be considered together when working out an engine-build combo. While the camshaft doesn't change the engine's compression ratio, it does have a dramatic effect on the compression pressures the engine will see. Why? Some guys think it has to do with overlap, but actually it is the intake-valve closing point. We all know that a piston goes up from BDC toward TDC on the compression stroke, compressing the air/fuel mix. The intake valve, even with a stock cam, closes after the piston is on its way up from BDC. Obviously, there is very little compressing going on while the intake valve is still open. As the cam's duration is increased, the valves stay open longer, and the intake valve will close even later in the compression stroke. Remember, compression doesn't begin until the valves are shut.
Interestingly, the piston's motion from BDC is accelerating, while we tend to think of increases in cam timing as a constant in terms of (crank) degrees. Remember that the piston is accelerating as it comes off the bottom of its stroke, so the actual change in piston position for each degree of later intake closing becomes more pronounced. In other words, as cams get bigger, the loss in trapping efficiency accelerates rapidly. Adding mechanical ratio compensates for the loss of pressure, so typically the bigger the cam, the higher the compression ratio should be.
Duration is only part of the equation as far as when a given cam's intake lobe will close the valve. The other factors are the lobe separation angle and the installed centerline. The chart below shows the effects of different cam characteristics on trapping efficiency or cylinder compression pressure.
Cam Speed Characteristic Compression Pressure Effect
|Installed Centerline||Advanced ||Higher|
Building Detonation Tolerance
It is widely recognized that higher compression builds more power, and this is true up until the point the engine begins to detonate, which is highly destructive. In the years following the phase-out of high-octane leaded pump gas, most of the advice we've run across in regard to ratio is to wind it way down to avoid detonation. The idea of avoiding detonation is still valid, but dropping the ratio always costs efficiency and power. OE manufacturers have been steadily raising compression ratios, and in fact many current production cars carry compression ratios much higher than the actual ratios used in the '60s. Many things can be done when building an engine to increase its detonation tolerance and to allow a higher ratio to work without detonating. Build an engine with steps to reduce the tendency to detonate, and you can run more compression than the next guy. Here are some of the most effective ways.
Quench: We have already talked about quench as a separate topic, and it is nothing new in the world of engine design. The desirable effects of an effective quench area have been documented since early in the last century. One of the key reasons quench affects detonation tolerance is because the most likely part of the charge to auto-ignite and begin detonation are the gasses remaining in the far end of the chamber away from the plug. Again, about .040-inch piston-to-head clearance works really well in a street application. As the clearance is increased, the effect diminishes, and by about .060-.070 inch, the effect is lost. In fact, an engine with a dead quench area with a large volume of end gas can actually induce detonation.
Thermal Barrier Coatings: Coating technology has come a long way, and these days, thermal-insulating metallic ceramic-coatings are fairly common. In terms of detonation tolerance, the biggest bang for the buck is to have the chamber face of the valves coated. The hot valves, particularly the exhaust valve, impart a large degree of detonation inducing heat to the mixture. Coating the valves greatly reduces this heat transfer and increases the detonation tolerance. A set of coated valves can be worth as much as running fuel with three points higher octane.
Heat: Another key component in detonation tolerance is induction-charge temperature. Taking in a cooler intake charge makes the engine less likely to detonate. A cold-air intake is a step toward this goal, but it's important not to neglect how much heat is added to the charge on the way into the engine. The exhaust crossover in the intake manifold can add tremendous heat to the mixture. Blocking it-especially when using an aluminum intake manifold-can dramatically lessen the potential for detonation. Coolant temperature has a similar effect, though to a lesser degree. A high-capacity cooling system and cool thermostat reduces the likelihood of detonation
Spark: When it comes to setting up advance curves for performance, the common advice has typically been to speed up the curve so that full timing is in very early, like 1,600-2,000 rpm. Why? Detonation is far more likely to occur at low engine speeds since the cycle time is much longer, and mixture motion is much poorer. Delaying full advance has little effect on performance, and can greatly diminish the tendency toward detonation. Think about it-what good is having the timing "all-in" by 1,600 rpm, when even a stock Mopar torque converter will flash to 2,600 rpm at WOT? Another factor is total timing. We see guys pushing it up as high as they can go, when really the better approach is to back it off as much as possible until power begins to go away. If the engine makes about the same power at 34 degrees total as it does at 40, run 34 and the engine will be much happier.
Fuel: The air/fuel mixture also affects detonation tolerance, with richer being more forgiving. Running fat is a poor way to control detonation, but a very lean mixture can push an engine into the ragged edge of detonation unnecessarily.
Cam: We have already talked about cam timing as it relates to cylinder pressure, and cylinder pressure directly affects the detonation tolerance. Study the principles listed above to get a handle on how different cam specs can raise or lower the cylinder pressure.
There you have it-a deep and theoretical approach to the question "How do I figure my compression?" Although this article will definitely promote discussion and present more questions, it should give you a better understanding of the principal behind the idea.