Cylinder Block Tuning
In the introductory article of this section we sought to define what it means to tune a car as well as to highlight what you need to be aware of if you wish to take part in this exciting feat. As we have already pointed out, every serious tuning process begins with improving engine performance, and then dealing with other segments that ought to support higher speeds and acceleration in an optimal way.
Our story begins with the engine and it will be divided into a couple of segments, starting from the cylinder block. The cylinder block is roughly composed of the engine block, crankshaft, oil pump, pistons with piston rings and piston rods. For this composition to be able to handle engine tuning, each of these elements requires special attention.
The purpose of the crankshaft is to transform linear motion into circular motion, which is then transferred by means of the flywheel to the transmission mechanism and ultimately to the drivetrain. This is why it’s critical for the crankshaft to do its job perfectly within the specified tolerances, especially when we are dealing with a used engine (because of microscopic displacements that happen over time). The right tolerance is important because, as the engine works, the rotating crankshaft must never touch the surface of the slide bearing, but rather “slide” on the oil film coming from the high pressure oil pump. A common expression for this type of malfunction is that the engine has “gone berserk”.
The process by which all the shafts in the crankshaft are being aligned is done in specialized engine machining workshops.
The Connecting Rod
The connecting rod is an element that connects the piston and the crankshaft (hence the name). The bearings inside it are one of the elements that need to support higher power of a tuned engine. If we take a closer look at the connecting rod bearings, we will see that there are two halves. Going upwards, the piston seeks to compress a certain volume to a 10 to 12 smaller space, so the upper half is subjected huge resistance. When the explosion takes place, after reaching the upper dead centre when the piston starts to push the crankshaft downwards, the pressure is again on the upper half of the bearing. When we talk about bearings used in race cars whose quality allows higher pressure and power, certain manufacturers make an equal number of standard and special halves. For example, if we have a 4-cylinder engine, out of 8 halves produced in total, the 4 special halves (usually marked by a dot) are installed on the side of the big end, while the 4 standard halves are placed on the side of the bearing cap. Other manufacturers see to it that both halves be of higher quality, at adequately higher prices. Such bearings have no marking, but each manufacturer does recommend switching the halves’ places after reaching a certain mileage.
Higher-quality bearings are usually of the tri-metal type. Unlike standard bearings, they are composed of three different materials that secure better endurance and less wear.
For a tuned engine, it’s essential to have each set composed of a piston and a connecting rod absolutely equal in terms of weight. Serially-produced engines can tolerate 2 to 7 grams of difference in these sets, depending on the engine capacity. On the other hand, for a race engine that is not a good practice. The rotary assembly of an engine needs to be perfectly balanced to eliminate the occurrence of irregular vibrations for that could lead to engine’s demise.
The position of pistons in relation to the cylinder block is calculated and performed once the following conditions are met: defining correct crankshaft bearing tolerances, performing crankshaft alignment, modifying cylinders in relation to pistons and adjusting the piston rings (cylinders and piston rings will be explained in more detail further in this text).
Namely, standard cylinder blocks have a higher base than racing cylinder blocks because of a lower compression ratio. Your tuner will calculate what degree of compression is needed to achieve the desired performance. After that, the height of the cylinder block is being adjusted - slightly reduced, leveled with the piston crown or even reduced below the level of the piston crown - depending on the tuner’s idea. This topic will be further explored in one of the following articles that talk about compression.
The Piston Rings
The piston rings are elements on the pistons that collect burnt products of air and fuel, some of which also perform lubrication. Each piston has a sequence of rings (most often 3) in which, looking downward, the first ring compresses, the second one wipes, and the third one lubricates.
All three rings perform a very serious task and therefore demand special attention.
Every piston ring has its own beginning and ending, between which there is a small gap. The size of the gap is strictly defined, and needs to be reduced almost to 0 (but never to 0) once the engine reaches its working temperature and causes the rings to expand. Whether this gap will be 2, 3 or 4 tenths of a millimeter depends on your tuner’s skill. Why does it matter? When combustion takes place, causing the piston to move downward, all the burnt air and fuel will pass through the tiniest space available and leave the combustion section. Hypothetically speaking - the bigger the gap, the weaker the blow on the piston crown. It is in your interest to maximize the blow, having the compression ring collect as much energy as possible to push the piston downward, and this will be possible only if your tuner makes adequate gaps on the rings. For example, if the piston’s diameter is up to 80 mm, the expected gap will be maximally 0.2 mm. If the piston’s diameter is wider, say 900 or 100 mm, the gap should be 0.25 mm because bigger mass exposed to high temperatures means greater expansion.
Please bear in mind that the size of the gap will also depend on the type of material it was made of.
The oil ring, the last one in row, serves to lubricate the cylinder from the inside. We will now explore the course of action that allows proper lubrication. It is the so called “honing”, which needs to be done differently for the purpose of building a race engine. Honing is done with a special machine on the inner side of the cylinder and it’s basically creating tiny crosshatch imperfections that aid lubrication by retaining a small amount of oil when the lubricating ring passes through the cylinder. Your tuner can opt for a fine and dense hone, or a somewhat coarser variation, depending on their intention. You will have a clearer picture of the importance of the hone pattern once you understand the function of the wiper ring.
When the air and fuel mixture starts burning and the piston heads downward, the wiper ring will “scrape” excess oil from the cylinder. The small amount of oil that remains in the hone serves to improve the sealing of the compression ring inside the cylinder. As the compression ring moves downward and high temperature following its path burns the oil in the hone, sealing is over and ready for a new cycle.
Fine fitting of piston rings
Why not simply buy corresponding rings - many of us will wonder. It is virtually impossible to buy replacement rings that fit perfectly to the needs of a tuned engine, and the reason is this… Manufacturers are (rightfully) afraid that too tight a gap could cause friction, as rings exposed to high temperatures expand and tend to close the gap. Friction produces even higher temperatures, which can lead to pistons and piston rings cracking or damage the cylinders permanently. To avoid this, manufacturers make replacement rings with bigger gaps.
Your tuner’s task is to fit the gap to a factory recommended size. One of the most frequently used methods is buying wider rings that will go through the process of fitting. Fitting is usually done by means of a special machine that does not form magnetic poles on the ring’s ends. Magnetic poles are harmful because they collect particles that leave vertical abrasions in the cylinders.
Cylinders are, roughly speaking, cylindrical tubes of a defined diameter through which pistons move up and down. In order for a tuner to calculate the diameter of the cylinder, he/she first needs to measure the diameter of a cold piston. Then, the piston is heated to approximately 100 ℃ to expand, and the diameter is measured again. It is essential that all the pistons be of the exact same diameter, with a precision of a 100th part of a millimeter. The diameter of the cylinder will be the sum of the diameter of the heated piston, plus an added tolerance that will enable the piston to move seamlessly through the cylinder (for example, if the piston is 85.00 mm wide, and the measured expansion at 100 ℃ is 0.08 mm, the cylinder needs to be honed at 85.09 mm, which is one hundredth part of a millimeter wider than the maximum width of the piston).
Like the crankshaft and the cylinder block, the cylinders are also machine-modified, following a specification provided by a professional tuner who has previously made all the necessary measurements. In all segments of measurement - these being three points in height (close to the top, in the middle, and close to the bottom) and perpendicularly at an angle of 90° - the cylinder holes need to form a perfect circle. If the shape is oval, the cylinders need to be machined again. Why does the circle lose its shape? By pressing the clutch pedal we put enormous pressure to the crankshaft via the clutch and the flywheel in the direction of the engine, basically using the crankshaft end float. By hitting the clutch pedal countlessly, we make the crankshaft move for a tenth millimeter forward and backward, moving all the pistons accordingly. Continuous micro-movements of the pistons practically cause the cylinders to take on an oval shape.
The better your tuner sets tolerances, the longer your engine will last. If the tolerance is bigger than necessary, cylinders will get deformed much quicker.
The Oil Pump
The role of the oil pump is to supply oil to all the parts that need to be lubricated in order to prevent wear. The pressure of the oil pump sets a limit to how much one can force an engine, without the crankshaft ever touching the bearing. The greater the rpm, the bigger the pressure. But, this is not where you want to exaggerate. Exaggeration can cause the oil filter to burst and destroy the engine, and in most cases you won’t even be able to tell the cause.
The most frequently used method to alleviate the load on the oil pump is to install a pressure relief valve that will direct all excess oil back into the oil sump. When the pressure drops (the need for higher rpms drops), the valve closes and establishes adequate pressure again.
In the next article, we will be dealing with the cylinder head. Follow us and feel free to write for any questions, suggestions and unknowns.