It forms the combustion chamber floor, which means several things. The piston head will determine the quality of intake air swirl or turbulence, flame dispersion, and fuel distribution. Automotive piston manufacturers make these auto parts in different head designs. Each design has its best attributes. The downsides, too. To give you an idea of these variations, here are the piston type names based on the design of the piston head.
As the name suggests, this piston type features a flat top. The design enables the piston to achieve one of the most efficient combustion processes. With the head flat, the flame spreads evenly, and fuel burns more effectively. Because of the reduced surface area and uniform combustion, the flat top piston creates a high amount of reciprocating force. These piston types are easy to make.
It reduces the piston price but also the amount to rebuild or purchase an engine. This piston type is also known as bowl piston.
It features a plate-like shape with raised outer edges. Because of the increased combustion chamber volume , dish pistons achieve a lower compression ratio. Although a drawback, this characteristic is an advantage in some situations where high reciprocating forces are not necessary. Dish pistons are often used in engines with turbo charged or supercharged combustion.
They help to prevent knock or detonation that would be caused by the boosted compression. In some engines, pistons help to contain the fuel spray, helping to enhance the combustion process. In older engines, the process will be influenced by several factors: piston type, carburetor design, and the volume of the cylinder bore. In terms of design, this piston type is the opposite of the dish piston. As its name implies, the piston assumes the shape of a raised center.
This increases the surface area of the head significantly. The combustion gases have to travel further. As a result, dome pistons are known to create a poorly performing combustion chambers with insufficient burning.
This lowers the compression ratio. The reduced compression may be a disadvantage of the dome piston. However, it is a requirement in some engines. A piston can be pounded into a pretzel, but it won't shatter into chunks.
The high rate of thermal conductivity of low-silicone aluminum will transfer more heat into the skirts. Coupled with the material's higher expansion rates, pistons generally need significantly more bore clearance. Lacking the high silicone content, a piston is more subject to wear in the ring lands and skirts.
These are racing pistons, not the slugs to put in a truck expected to go ,plus miles. If the plan calls for serious nitrous, boost, or rpm, and bulletproof is what you are after, a piston is the right choice. Modern machining and design technology have improved the piston by allowing for more sophisticated skirt designs. Manufacturers taking advantage of these advances have been able to reduce the bore clearance considerably compared to traditional requirements.
Stroker Considerations Building Mopar stroker combinations has never been more popular, and why not? With the availability of low-cost crankshafts, the door has been blown wide open in the small-block world, while in the big-block camp, popular combos include the tried and true that uses a crank in a low-deck.
Other combos are supported by readily available aftermarket crank combinations. Another contributing factor is the ever-improving cylinder-head situation. The aftermarket is offering plenty to feed all those extra cubes and make the prospect of building a stroker combo worthwhile.
Unlike some of the competing engine designs, Mopar blocks were endowed with generous cylinder deck dimensions. To fill that space, Mopar engines were fitted with relatively long connecting rods and tall pistons. The distance from a piston's wristpin centerline to the top of the piston is referred to as the compression height. Mopar engines have excessive compression height designed in them, making the pistons heavier than they need to be.
In a stroker combination, the added stroke pushes the piston higher up the bore-exactly half the distance of the increase in stroke. To compensate, the piston's compression height is reduced a corresponding amount. With some engines, there simply isn't enough room to shorten the compression height without the piston pin encroaching into the oil-ring area of the piston.
In fact, Chevrolet used a shorter connecting rod in its production long-stroke small-block, aggravating an already bad rod ratio. There's little worry about not finding the room in a Mopar engine. Stroke a Mopar big- or small-block, and the piston mass is considerably reduced. Buying Right With so many pistons to choose from, it's kind of tough to narrow down the choices and decide on an appropriate piece.
Really, the two factors to consider are application versus dollar. Given a choice, we can't think of a reason, other than price, anyone would prefer a cast replacement piston to a modern hypereutectic or forging. But again, it's important to consider the whole picture. If the goal is an economical rebuild in a low-rpm, low-compression, low-output engine, these cheap castings will be fine.
On the other hand, if you're looking to build horsepower, but the build is on a shoestring budget, those cast replacement pistons are dirt cheap, but the low compression ratio is going to cost output and efficiency. For the couple of hundred bucks saved, you're stuck with compression, and the combustion efficiency is hampered because the quench area is too big.
Plus, you'll have a potential time bomb if the thing ever does make real power or turns some higher rpm. Some engine builders use much higher ratios, but they have the necessary control to do it. Piston-to-cylinder bore clearances are another concern that will vary with the engine application and type of pistons used.
One manufacturer said they recommend. Another manufacturer said they recommend only. A third manufacturer said some coated hypereutectic pistons may be installed with as little as. On a Chevy LS1, they recommend. So it all depends on the application, the type of piston, the alloy used and the piston profile. Piston-to-valve clearance is also a very important consideration especially with long duration cams and oversized valves. The standard rule of thumb is to allow at least.
The valve pockets that are machined into the piston must also be the right one for the type of heads that are used. Consequently, aftermarket heads that have a different valve angle than the OEM heads will typically require different pistons. Smaller, thinner rings reduce drag for more usable horsepower, yet some racers remain skeptical about how well the low drag rings will hold up over the long run or what effect they have on piston cooling.
Pistons with only two rings a top compression ring and an oil ring are also available, but are only used in drag motors. Eliminating the second ring reduces friction, but also heat transfer and the ability to control oil. Therefore, two-ring pistons are not recommended for any kind of street engine or endurance racing. Several piston manufacturers also mentioned the importance of machining the ring grooves as precisely as possible.
The flatter the ring groove, the better the ring seal and transfer of heat. Some performance pistons feature gas ports to help seal the top ring. Combustion pressure blows through the port to help seal the ring from behind and underneath. Vertical gas ports have holes drilled from the top of the piston to the top ring groove just behind the ring.
Lateral gas ports are drilled through the bottom side of the top land and extend to the back wall of the ring groove. Gas ports work best at high rpm above 7, rpm and are not recommended for street engines. The added space traps blowby gasses and helps prevent the top ring from unseating and fluttering. The latest thinking on ring end gaps calls for opening up the second ring groove a bit so blowby gasses can escape before they unseat the top ring.
This trick works best on engines that are running a dry oil sump and pull a vacuum in the crankcase. One manufacturer said they recommend increasing the end gap on the second ring. For naturally aspirated engines, the same manufacturer recommends. For a turbocharged or supercharged engine, the top ring gap should be. For a nitrous oxide engine, the top end gap should be opened up even more to. Some manufacturers have decided to take the risk out of selecting piston rings by offering custom matched piston and ring sets.
The people who supply these type of rings claim they typically allow less than 1 cubic foot per minute cfm of blowby, which is good for up to 10 to 15 horsepower or more on a typical street engine. On an alcohol-fueled circle track engine, using a gapless ring in the second groove helps keep alcohol out of the crankcase. Features: Justina Reusch, Reusch Diesel. Magazine Current Issue Past Issues.
Connect with us. Advertise Subscribe Contact Us. What has changed dramatically is the thickness of those rings, especially in Pro Stock and other non-endurance engines.
Through normal secondary motion piston rock , the top ring tends to become momentarily unsettled as it rapidly changes direction through TDC. As this occurs near peak firing, combustion pressure tends to make its way past the top ring until it re-settles.
Combustion pressure also gets through at the end gap of the ring. Less weight is always desirable but using a smaller or thinner wrist pin may compromise strength and engine durability. Again, more design changes to reduce weight and friction but these efforts are usually dictated and restricted by the engine architecture, such as length of the cylinder sleeves and the stroke.
Some engine builders want to reflect the heat away from the piston, preferring that the valves and cylinder head dissipate the heat to the coolant instead of the pistons and rings through the cylinder wall.
Thermal coatings are designed to repel heat from the piston crown, and in some cases the combustion chamber as best as possible. A distant second are the bearings. On racing pistons, top teams use coatings as an insurance policy against overheating. In OEM and daily driver conditions, they are used to protect against dry starts and other situations where oil on the cylinder wall is limited.
This step ensures that the crown has the necessary strength and heat resistance for competition with the least amount of weight. The actual crown design and valve relief dimensions will be dictated by the combustion chamber and valve geometry. Another racing piston cue is that the engine builder will send a mold of the combustion chamber to the piston manufacturer so that the dome design exactly follows the profile of the chamber.
Finally, some engine builders call for very slight adjustments in the overall piston shape and dimensions to suit their needs.
0コメント