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Piston Choice Key For High Performance Applications
By Jim Walbolt
Piston selection, like all other elements that go into a high performance motor these days, is a critical ingredient with many choices. Although 80 percent of racers can get by with off-the-shelf pistons for their applications, shops that are always looking for that extra edge for their customers and do lots of research and development will most likely have their own ideas and designs. In fact, it is not uncommon for a piston manufacturer to manufacture pistons for particular shops to a proprietary design.
If you’re new to the performance business, it’s best to start off with the recommendations of the piston manufacturer. As you gain experience, you can begin experimenting.
A quality piston begins with a quality forging. Although a cast piston could be used for a street engine, for a true performance engine, a forged piston is the only way to go. You will hear a lot of talk about "hypereutectic" pistons for performance engines, but although they are better than OEM, a hypereutectic piston is primarily a cast piston with increased silicon content.
Forged pistons will generally come from one of two aluminum alloys: SAE 4032 and 2618. A 4032 alloy is high silicon and usually used in naturally aspirated engines, while a 2618 alloy is a low silicon material designed for the abuse of marine, supercharged and nitrous applications. A 2618 alloy is about 3 percent heavier and doesn’t have as good lubricity qualities as the 4032 because of the lower silicon content. A 2618 alloy will also expand more than 4032 because of the denser material. Because of this difference in expansion, the manufacturer will design the necessary clearances into the piston; the engine builder will not normally need to change recommended running clearances.
The 4032 composition will be what you find in virtually all circle track-type applications as well as street applications in which a forged piston is desired. The 4032 can be made as strong as 2618; it would just need to be thicker. The melting point of these alloys is about the same, so that wouldn’t be a consideration.
In piston design, aside from the forging design issue, the factors that designers pay the most attention to are the thickness of the crown and skirt and the ring lands.
When machining a piston into a finished product, the two keys are piston cam and piston taper. Because of the structure of the piston, the crown sees the most heat and has the most expansion, but because of the design of the skirts there needs to be a certain amount of cam from side-to-side of the piston.
Manufacturers will not give engine builders a choice of taper with most off-the-shelf pistons, but some top makers will give high-end racing customers taper options. Generally, you will find builders in NHRA Pro Stock, NHRA Pro Stock Trucks, and NASCAR Winston Cup Series competition specifying a particular taper. A lot of R&D has gone into piston design by these top teams.
Piston taper will generally run in the .050˝ range from top to bottom because the piston will expand more at the top than the bottom. For instance, a piston for a 4.030˝ bore size will measure 4.026˝ at the bottom of the piston but the top land diameter will be 3.976˝. You will find maximum temperatures at the top, but as you move down the piston, heat begins to bleed off through the rings and skirt of the piston into the cylinder walls to the point that the bottom of the piston will be no hotter than the oil temperature. With the proper taper, the entire piston will maintain an equal piston-to-wall clearance.
The most important factor in piston choice is application. What do you need the piston to do? Street and drag race applications are easy on pistons, circle track and road race applications are somewhat tougher, while marine, supercharged and nitrous engine applications will produce the toughest environments.
A related factor to consider is how often do you want to inspect and/or change the pistons? Do you want the pistons to last a full season or even several seasons? Or do you want the lightest pistons possible and plan to change them after every few races?
When fitting a piston to an engine combination, bore would be your first consideration for obvious reasons. The second factor, though, would be compression height. Compression heights must vary with any given engine design because of the different blocks, strokes and rod combinations. Compression height is the distance from the center of the wrist pin to the deck of the piston (not the dome). When adding half the stroke plus rod center-to-center length plus compression height, the goal is to have a piston with adequate piston-to-head clearance without being excessive.
Piston-to-head clearance recommendations will vary with rod material, piston mass and piston speed. Steel rods in a big block usually require .045˝, while steel rods in a small block might require .036˝. There are specialty head designs out there that actually allow the piston to hit the head — a so-called “pinched head” design — but such a design is best left to the experts. As hard as it is to believe, however, it does work well.
Your target compression ratio can also have an effect. As a general rule, a flat-top piston will be the most efficient, but if you’re looking for a 16.5:1 compression ratio it will be much easier to do with a domed piston. Keep in mind, however, if you are running more than 14:1 and got that increased compression ratio from an increase in dome height, you are actually making less power because a domed piston is less efficient.
The current trend by Pro Stock and Winston Cup engine builders is to run much smaller combustion chambers and to use 12, 14, or 18-degree heads. They use the shortest dome that gives them the compression they are looking for.
Most top builders these days want to use the shortest piston possible that still gives the necessary crown thickness and ring land thickness. Rod-to-stroke ratio is also important. A good rule of thumb for a beginning engine builder would be 1.75-1.81 rod:stroke ratio.
Some builders believe that a lot of dwell at the top of the stroke equates to better power and a more complete burn and would probably be good for restrictor plate motors and maybe antique tractor pulling motors. However, the longer the piston stays at top dead center (TDC) the slower it pulls air in on the intake stroke. Many engine builders say using the biggest bore with a shorter stroke puts the engine at its best power potential for a given cubic-inch limit.
We’ve all heard the debate regarding "Hemi" and "wedge" style heads. Each style has its place. In a high compression engine, a wedge design is nearly always superior because the dome rise needed to produce high compression in the hemi is detrimental because of much lower combustion efficiency. In normally aspirated classes, a wedge-style engine will always be better.
However, that superiority begins to disappear when you get into turbocharged or supercharged classes. Turbo and blower motors generally will have a much lower compression ratio and therefore a much more efficient piston design can be used, generally a flattop or even a reverse dome piston.
Piston Skirt Design
There are three main skirt designs found in today’s performance pistons: full-round, slipper skirt and strutted design. The full-round is used for extremely heavy-duty applications and is the longest wearing. The slipper design is lighter, but nearly as strong as the full round. The strutted design has the best strength-to-weight ratio.
You’ll want the lightest piston for your particular application in most cases. However, again, you need to balance the life of the piston with its weight and design. If you want the pistons to last a full season or two, then you’ll want something much different than if you don’t mind replacing the pistons several times per season, or even after each run.
A lighter piston won’t necessarily mean more horsepower, however. It simply helps get the power to the track faster or getting up to the optimum rpm range quicker, just as a lighter crank, rods or flywheel will help to accomplish the same thing.
Piston choice for maximum performance requires much more than just pulling a boxed set off the shelf. If you’re just beginning to build race motors and do little R&D, it’s probably best to use "off-the-shelf" pistons per recommendations from your preferred piston manufacturer for a particular application.
But, if your shop does a lot of R&D and has a lot of experience in engine building and design, you can do much more experimenting with piston design and can probably find advantages for your customers. In either case, the key is to find a knowledgeable manufacturer to work with and let them guide your way.
Thanks to Wiseco for sharing their technical expertise in putting this column together.