An engine's mechanical compression ratio can be defined as the ratio of the cylinder volume above the piston at bottom dead center (BDC) compared to the volume at top dead center (TDC). When the volume of the cylinder at BDC is compressed into the remaining 72cc at TDC, the air/fuel mixture occupies one tenth of the space. Therefore, the compression ratio is 10:1.
A large included valve angle requires a high-dome piston, resulting in large piston-dome and chamber-surface areas. The Ironhead Sporty and Shovelhead engines have large hemi-like combustion chambers with high surface-to-volume ratios, which increase heat loss. Also, high piston domes can impede flame travel and airflow during valve overlap, result in a heavier piston, and usually require more ignition advance.
Nooks and crannies in the combustion chamber, which are located farthest from the point of ignition and generally near the cylinder bore edges, are potential detonation traps. Portions of the intake charge are trapped in these areas and ignite spontaneously due to heat saturation before being ignited by the normal flame-front reaction, thereby resulting in detonation. Eliminate detonation traps as much as possible.
A compact combustion chamber, relatively flat or slightly dished piston top, and centrally located spark plug will provide for short, unobstructed flame travel and rapid combustion. Additionally, a large tight squish band is important for proper turbulence. High turbulence will more thoroughly mix the air/fuel mixture into a more homogenous mixture that will burn more quickly and efficiently, thus improving power. A compact chamber will also require less ignition advance, run on lower octane fuel, and be less prone to detonation and pre-ignition.
A dished or reverse-dome piston machined as a sphere results in the least amount of surface area for a given volume, thereby producing unimpeded flame travel and the least amount of chamber heat loss. A performance gain is often realized when using this shape of piston dome.
Knowing the combustion chamber volume is important for accurately determining the mechanical compression ratio. The only sure way to know chamber volume is to cc it. The spark plug that will be used in the engine is installed, and the head is placed at a slight angle. The chamber is then covered with a 1/4-inch Plexiglas plate with a 1/4-inch hole drilled at its highest position. The plate is sealed to the chamber with a thin coating of light grease. Use a 100cc burette to fill the chamber through the drilled hole with a thin fluid, then record the amount used to fill the chamber.
Dual spark plugs reduce flame travel and unshroud the plug, which can minimize the potential for detonation. Dual plugging is especially helpful with high-dome pistons that shroud flame travel and large-bore engines where the combustion flame has a long distance to travel. The second plug should be located high enough in the chamber for good combustion efficiency and sufficient clearance between the piston and plug electrode. The engine will typically require 5-8 degrees less ignition advance with dual plugs. Test and tune to determine the optimum spark advance for your engine.
A leak-down tester is an excellent tool for checking cylinder sealing and more revealing than a compression tester. A leak-down test can identify leakage past the piston rings or a valve. Stock engines typically have between 6- and 8-percent leakage, sometimes even higher. Strive for no more than 2-percent leakage.
In previous Motor Series installments, we looked at component dimensions and relationships influential for achieving engine synergy, the relationship of torque and horsepower, and airflow factors. In this segment, we'll deal with optimizing engine efficiency. We all know that if you want to go fast, the easiest way to do it today is to drop in a large displacement "crate" motor. With pump gas and a street-legal exhaust system, most of today's typical crate motors are good for about one horsepower per cubic inch, which results in a mild but relatively powerful street engine from just the large displacement alone. Surely these engines offer more power than a well-optimized 80-inch Evo engine from a few years ago that was putting out 90 ponies or so, assuming the owner tuned on it hard enough.
However, if you've had your mildly built street engine for a while and want to take it to the next level without installing bigger cams, higher-flowing heads, or a larger carb or throttle body-which may reduce low-end torque and sometimes make an engine less streetable-what can you do? One thing you can do, regardless of whether you have a big or small engine, is maximize engine efficiency through optimizing the compression ratio, combustion process, and cylinder sealing. In fact, optimizing efficiency improves power on any engine, no matter how big or small it is or how mild or racy its state of build and tune.
Although increased displacement, airflow, and rpm are keys to making more power, more power will not be realized unless combustion chamber pressure is optimized and harnessed for maximized pressure on the piston. We all know that the internal combustion engine is nothing more than an air pump. However, what we must not forget is that it is a pump designed to generate heat and harness cylinder pressure. Making heat and controlling cylinder pressure is the secret to making power. For optimized cylinder pressure to occur, heat must be contained and converted into usable pressure on the crankshaft while minimizing heat loss to the cooling system. Of course, a balance must be achieved between maximizing heat and minimizing its harmful thermal and mechanical effects on components. When designing and building an engine, remember that it is critical to maximize cylinder pressure on the power stroke while minimizing pressure on the exhaust stroke. This starts with optimizing the compression ratio to the engine combination.
Corrected compression takes into account the point at which the intake valve closes after BDC on the compression stroke. The later the valve closes, the lower the corrected compression ratio. For an optimized engine, the corrected compression ratio should be matched to the vehicle's application and fuel octane. In general, the later the intake valve closes, the higher the mechanical compression ratio must be to maintain a given corrected compression ratio. To determine corrected compression, the mechanical compression ratio and the cylinder displacement remaining from the time the intake valve closes must be known.
Cylinder pressure is crucial to achieving a powerful and optimized engine. Increasing volumetric efficiency with big cams, higher-flowing cylinder heads, or larger induction are a few ways to increase cylinder pressure, but raising the engine's mechanical compression ratio (also known as static compression) is another. Up to a point, raising the compression ratio increases thermal efficiency because heat and combustion pressure are increased for greater gas expansion. Therefore, assuming an optimized exhaust valve opening, more potential energy can be extracted from the expanding gases. Since the speed of combustion increases as the square of combustion pressure, doubling combustion pressure through higher compression increases the speed of combustion by a factor of four. This has several implications for the optimum ignition timing and the required fuel quality.
An engine with aluminum heads can usually support a compression ratio of roughly 1.0 to 1.5 points higher than an equivalent cast-iron head engine without incurring detonation. This is because the aluminum acts as a heat sink and pulls heat from the combustion chamber. From a power standpoint, this means that all things being equal, including the compression ratio, an iron head engine will make more power than an aluminum head engine because more heat will remain in the combustion chamber, thus producing higher cylinder pressures. To make up for lost heat and power, an aluminum head engine must run a higher compression ratio than an equivalent iron head engine. As an example, it takes roughly a 15.5:1 compression ratio with an aluminum head Evolution engine to match a 14:1 compression ratio with an Ironhead Sportster. Another factor that is affected by cylinder head material is fuel octane. For a given compression ratio, an aluminum head engine requires less fuel octane to avoid detonation than an equivalent iron head engine.
Combustion chamber design, thermal barriers, and ignition timing all affect thermal efficiency. Although increased compression raises power over the entire rpm range, it is particularly effective for low and midrange power increases. Though many variables are involved, the maximum mechanical compression ratio is determined by the engine's detonation threshold. This threshold is primarily influenced by fuel octane, volumetric efficiency, combustion chamber efficiency, and cylinder head material. Engine compression is usually discussed in terms of mechanical, corrected, and dynamic ratios. We'll take a brief look at mechanical and corrected ratios.
Mechanical CompressionAn engine's mechanical compression ratio can be defined as the ratio of the volume above the piston at bottom dead center (BDC) compared to the volume at top dead center (TDC). The Harley V-Twin engine responds exceptionally well to an increase in mechanical compression. In fact, many engines are poor performers because they lack sufficient mechanical compression.
The first thing to consider when determining the mechanical compression ratio is whether the engine will use 92-octane pump gas or high-octane race gas. If the engine will be run on pump gas, it will be limited to somewhere between 9.0:1 and 10.5:1 mechanical compression, depending on variables such as cam timing, combustion chamber design, ambient temperature, bike weight, and gearing. If race gas is the fuel of choice, the effective maximum compression ratio will be limited to roughly 17:1, again depending on several variables. Keep in mind, however, that once the compression ratio goes beyond about 16:1, thermal efficiency starts to drop and parts breakage potentially becomes a major problem.
Ambient temperature plays a big part in where detonation sets the engine's power limit. One reason a drag racing engine can run very high compression is that it only operates for a short period and doesn't get very hot. On the other hand, a street engine heats up significantly on a hot summer day, especially while idling for long periods in traffic in the late afternoon. Where a street engine may not encounter detonation during the cool spring and fall months, detonation may be rampant during summer months, especially when riding double and launching from a stop. If you are building a street engine, keep this point in mind when determining the engine's mechanical compression ratio.
General guidelines for conservative mechanical compression ratios on 92-octane pump gas with the V-Twin engine are as follows: With Twin Cam and Evo Big Twin engines, you can run between 9.5:1 and 10.5:1 mechanical compression without encountering detonation. The lighter weight Evo XL can handle between 10:1 and 11:1 mechanical compression. For the Ironhead XL and Shovelhead, expect 9:1 to 10:1 as the maximum mechanical compression ratio on pump gas. Fuel-injected models may work well with up to a half point higher mechanical compression than the values listed above. Several other variables, such as combustion chamber design and cam timing, will also influence the actual compression limit. Since cam timing affects cylinder pressure when the engine is running, cam events and compression must be closely coordinated to maximize power for a given parts combination. This is where corrected compression ratio comes into play.
When the mechanical compression ratio is determined, it is assumed that the intake valve is closed when the piston reaches BDC at the beginning of the compression stroke. If this were the case, the total cylinder volume would be compressed on the compression stroke. In reality, however, intake valve closing is delayed beyond BDC, and this results in the piston being part way up on the compression stroke when the valve closes. Therefore, less than a complete cylinder volume is compressed, so the actual compression ratio is correspondingly less. Calculating compression based on intake valve closing is called corrected compression. Corrected compression is a more realistic way for determining an engine's compression ratio.
Since cam timing has a major influence on compression, cam events and compression should be coordinated when designing an engine. An engine with a long-duration cam will experience a significant loss of low-end torque unless the corrected compression is matched to the cam timing, specifically intake valve closing. Many street engines are deliberately built with a late-closing intake valve to bleed off cylinder pressure at low rpm to stave off detonation. However, that hurts low-end torque. Therefore, it is important to coordinate gasoline octane, mechanical compression ratio, and cam timing when designing an engine. All three variables should be determined as a coordinated combination instead of three separate variables.
For a well-running V-Twin engine, ideally the corrected compression ratio should be no less than 9.0:1. Experience has shown that with an optimized combustion chamber, a Twin Cam or Evo Big Twin engine running on 92-octane gas can support between 9.0:1 and 9.5:1 corrected compression ratio (calculated at 0.053 in. tappet lift) before encountering detonation. Therefore, to maximize performance with pump gas, it is important to design an engine having about a 9.2:1 corrected compression. However, keep in mind that many factors, such as ambient temperature, barometric pressure, altitude, humidity, dual spark plugs, gearing, and total bike weight can affect an engine's actual detonation limit. With dual spark plugs and optimum conditions, the limit can be as high as 9.5:1. But overall, 9.2:1 corrected compression is a reasonable guideline to use.
The most important points to take away from this discussion are not a specific corrected compression ratio to use, but the concept of corrected compression and a general range to shoot for when using pump gas. The mathematical formula for calculating corrected compression is somewhat complex and beyond the scope of this discussion. However, the easiest way to calculate corrected compression is to use the "Accelerator for Windows" engine simulator program or a program specially written for calculating corrected compression.
Compression Ratio Considerations
Following are several points to consider when determining an engine's mechanical compression ratio:
- Match the mechanical compression ratio to fuel octane, cam timing, volumetric efficiency (VE), combustion chamber design, gearing, and total bike weight.
- Aluminum heads generally tolerate higher compression than iron heads.
- Power-adder applications (supercharged, turbocharged, and nitrous) require less compression.
- At high altitudes or with low VE, increase the compression ratio.
- Dual spark plugs may allow a higher compression ratio for a given fuel octane.
- Higher compression requires an earlier exhaust valve opening.
- A lower compression ratio requires a later opening exhaust valve.
- A low compression ratio benefits from a higher flowing exhaust port.
- Higher compression offsets some negative affects of a large low-velocity intake port.
- Higher compression requires less ignition timing.
- Good fuel atomization reduces the potential for detonation, thus allowing a higher compression ratio.
- Higher compression improves throttle response.
- Minimizing combustion chamber heat loss is most important with low compression.
- Higher compression may require a high-torque starter motor.
Assuming an engine is properly designed for optimized cylinder fill, then the combustion process, scavenging, and ring seal become keys to making power. Improving combustion efficiency is important for optimizing power. Good combustion chamber design will optimize combustion and thermal efficiencies. An engine with optimized combustion efficiency is identified by crisp throttle response, improved fuel efficiency, and a low brake-specific fuel consumption (BSFC). A compact chamber, relatively flat or slightly dished piston top, and centrally located spark plug will provide for short, unobstructed flame travel and rapid combustion. Additionally, a large, tight squish band is important for proper turbulence. High turbulence will more thoroughly mix the air/fuel mixture into a more homogenous mixture that will burn more quickly and efficiently, thus improving power. It will also require less ignition advance, run on lower-octane fuel, and be less prone to detonation and pre-ignition.
The optimum squish clearance (clearance between the squish band areas on the piston and cylinder head) varies depending on the engine components and use of the bike. In general, tighter is better, but not so tight that it impedes combustion. In addition, the piston can never be allowed to touch the head during engine operation. Crankcase, rod, cylinder, and piston expansion must be considered as an engine warms to operating temperature. An engine built with aluminum cylinders can have tighter squish clearance than an iron-cylinder engine because aluminum expands more than iron. Some factory crate engines have been measured with as much as 0.072-inch squish clearance, which is way too much.
With an aluminum cylinder on a street engine, the tightest clearance recommended is about 0.030-inch, with 0.040-inch maximum. For iron cylinders, 0.040-inch is recommended. Race engines built to close tolerances and closely monitored street engines can be setup with slightly less squish clearance, but it is on a trial-and-error basis, and you have to pay attention to what you are doing and monitor the engine closely.
Piston-to-bore clearance is another factor in determining squish clearance. All pistons tend to rock as they pass over TDC, and the rocking movement reduces the piston-to-head clearance. Large-bore pistons with loose piston-to-bore clearances rock more than small-diameter pistons with tight piston-to-bore clearances. Additionally, pistons with heavily notched skirts rock a bunch. The greater the piston rock in the bore, the greater the squish clearance must be.
For a given crankcase, stroke, rod, and piston, squish clearance can be juggled by changing cylinder length, head gasket thickness, or cylinder base gasket thickness. Squish clearance cannot be changed by milling the head if the squish band is located between the top of the piston and the head's head-gasket surface. However, combustion chamber design can change this. For instance, if the squish band is located between the piston dome and the combustion chamber (not the head gasket surface), such as with the Shovelhead and some modified heads, milling the head may reduce squish clearance.
The components of ring seal are: (1) cylinder, (2) ring, (3) piston ring groove, (4) piston stability in the cylinder, and (5) lubricating oil. It is impossible to tune an engine correctly unless it has excellent ring seal. Better cams, carbs, exhaust systems and all the hard-to-find stuff will not show power improvements if the engine's ring seal is not correct. Since maximum cylinder fill and cylinder pressure occur near peak-torque rpm, peak-torque power numbers will drop down first with a poor-sealing pump. Round, straight, and rigid cylinders, accurately machined piston ring grooves, piston stability in the bore (read: tall cylinders and long rods), and ring/cylinder wall finish compatibility are all crucial for achieving proper ring seal.
Compression ratio is a key factor in power production, efficiency, and detonation tolerance. More compression will improve power throughout the engine's rpm range, which makes for snappier acceleration and increases fuel mileage because the engine is more efficiently converting energy into power. However, once the limits of detonation are reached for a given fuel octane and combustion chamber design, so are the realistic limits of compression. Detonation is spontaneous, uncontrolled, and potentially disastrous combustion in the combustion chamber. Sometimes if an engine is on the edge of detonation, just insulating the fuel line with household copper water pipe insulation can eliminate the detonation.
Compression can be raised by replacing the pistons, milling the head, modifying the combustion chamber or increasing the displacement. The engine designer/builder has a delicate balancing act of optimizing compression for maximum power without encountering power robbing detonation or resorting to race gas on a street engine. Some of the major tools he has to work with are corrected compression ratio, combustion chamber efficiency, camshaft timing, carburetor/intake manifold efficiency, ignition timing, and gearing.
As you should realize by now, ending up with an optimized engine involves more than just selecting a bunch of parts. The next time you are planning on freshening up, rebuilding, or building a new engine, don't forget to go the extra mile and optimize its efficiency while your at it. Paying attention to details, such as optimizing engine components, can set you apart from the crowd and put you at the front of the pack.
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