Yes. The second ring gap being larger then the top gap was an upgrade and
improvement to a select number of our 4-Stroke ring packages. As the piston
moves down on the power stroke, combustion pressure accumulates in the land
area of the piston between the top ring and the second ring. This accumulation
of pressure can cause the top ring to unseat from its sealing area at the
bottom of the ring groove. This is addressed by increasing the gap area of the
second ring. This affords a controlled release of the inter-land pressure, and
keeps the top ring seated on its lower sealing surface for improved
performance. Please remember this gap difference is not present on all of our
ring packages, but just a select number of 4-Stroke applications.
Quality begins with a forging. Castings are porous and weak, Hypereutectic
pistons are merely a cast piston with an increased silicon content. Although
slightly better, they are not nearly as strong as forgings-which are pressed
into shape with a 2000 ton press. Wiseco starts with certified extruded bar
stock and forges our pistons in-house with the world's most advanced forging
presses. Most aftermarket piston companies do not make their own forgings and
therefore cannot achieve the level of quality control that Wiseco demands.
After thorough inspection, the parts go through an extensive heat-treat and
finishing process. This creates an excellent forging with maximum strength and
minimum variance. Another benefit to making our own forgings is the ease of
making the finished piston design dedicated to your specific engine
combination. Most companies buy one-size-fits-all forgings that require massive
amounts of milling to create a useable part. These parts are heavy and are not
nearly as strong as Wiseco Piston. Wiseco can forge a variety of advanced
materials including S.A.E. 4032 and 2618 alloys. High silicon 4032 is a durable
and lighter material usually used in naturally aspirated engines. 2618 Alloy is
designed for the rigors of blown, marine, and nitrous applications.
The factors that affect this are cylinder wall thickness, whether the block is
filled, the overall compression height of the piston, piston material and
thickness, and whether a marine engine is to see fresh-water cooling. Most
small blocks get .004 piston to wall clearance and most big blocks get .005 due
to the use of our 2618 high-strength alloy. For heavy blower and nitrous
applications, Wiseco recommends adding .001 to the standard clearance. Special
note: Clearance numbers are obtained from measuring the largest diameter of the
piston, typically at the bottom of the skirt. All measurements should be taken
90 degrees from the pin centerline.
There are many important factors, but the first two are bore size and
compression height. Bore size is important for obvious reasons. Compression
heights must vary due to the different block, stroke, and rod combinations.
Compression height is the distance from the center of the wrist pin to the deck
of the piston (not dome). When adding 1/2 the stroke + rod center to center
length + Compression height, our goal is to have the piston with adequate
piston to head clearance without being excessive (which causes a loss of
compression and quench).
Rod material, the mass of the piston, and piston-speed are the factors that
determine this. Steel rods in a big block usually require .045. Steel rods in
small blocks require at least .036. Most imports can get by with as little as
.030. Aluminum Rods generally require .010 more clearance than steel rods.
*Remember, the compressed gasket thickness can vary from .025 in steel shim
applications to .040 for composite and up to .100 for some copper gaskets.
The most important issue is to have a piston that is built for the right
application. Material and thickness are the major factors. Street and drag race
applications are fairly easy on pistons if ignition and fuel curves are
correct. Circle track and road-race pistons see much on-off throttle usage and
see more laps as well. Blower and Marine usage are equally as tough as marine
engines can be under extreme load for extended periods. Finally nitrous
applications are about the toughest environment a piston will ever see.
Wiseco makes super-light pistons for those people that are out to set records
and are willing to check and replace pistons on a regular basis if needed.
Wiseco's standard weight pistons are intended for use in endurance and other
more abusive environments. If a blower or 300 horsepower of nitrous is to be
used, please call Wiseco technical for specific recommendations. Always-Always
use a timing control computer and have an adequate fuel supply when using
Compression ratio and Dome/Dish volume determine combustion efficiency and
resistance to detonation. Detonation can and will destroy any piston in short
order. Lean conditions will melt any piston as any aluminum alloy melts within
a few degrees of each other. Most of a piston's heat is dissipated through its
contact with the cylinder wall and oil splash. Very short pistons and excessive
clearance will melt a piston sooner. Static compression varies more than most
people would believe. A piston running .020 down in the cylinder at tdc as
opposed to "zero-deck" on a 64cc combustion chamber headed 350 Chevy changes
the compression ratio from 10.25:1 TO 9.8:1.
People running compression ratios higher than 14:1 are not making more power if
it means an increase in dome rise. More dome rise only hurts combustion
efficiency which will loose more power than is gained by increasing
compression. The current trend in Pro Stock, Winston cup, and others with
big-budgets for engine development is to make the combustion chamber smaller
and go to 12, 14, or 18 degree heads to make that happen. They are going for
the shortest dome rise that gives them the compression they are looking for.
The trend is to also use a bigger bore with a shorter stroke to put an engine
at its best power potential for a given cubic-inch limit. One point to remember
is a pro engine builder will never trade ring-seal for cubic inches from making
a cylinder wall too thin.
It is different for every different valve-train configuration, but depends more
on cam timing and valve-train mass more than anything. Cam duration is the key;
actual lift doesn't really come into effect, as the piston is half-way down the
cylinder by the time the valve is at max-lift. The usual rule of thumb for an
engine using rockers is .080 on the intake and .100 on the exhaust. The most
common problem with broken valves comes not from lack of vertical clearance,
but in fact a lack of radial clearance. Because of manufacturing variance in
the cylinder heads, a piston must to be built with a valve pocket that is
larger than is needed. Always clay or use other steps to measure clearance
around the edge of the valve. Most of the time measuring one cylinder is not
enough as a problem can start at one end of the engine and get progressively
worse down the length of the engine.
This scenario is interesting because it brings up almost all of the unusual
problems that can occur when building a stroker motor. A flat top of minimal
crown thickness would be needed to accommodate the thick "small-end" of an
aluminum rod. This compression ratio requires a 55cc dish-which is not
possible. In addition, ring land thickness would be well below Wiseco minimum
standards. Always call your Wiseco representative first before designing a
large stroker combination that may be impossible to build a piston for.
Whenever it is necessary to put the wrist pin bore into the oil-ring groove, it
is necessary to keep the oil ring end gaps from rotating and falling into the
open pin bore. Wiseco developed and uses a dimpled spacer. The spacer is
installed first with the dimple down in the pin bore to keep it from rotating.
This method is far superior to buttons in every aspect.
The Anti-detonation grooves prevent carbon-buildup from locking up the top ring.
They also help keep the air and fuel in suspension. The accumulation of gases
that get by the top ring can unseat it. A pressure groove delays this action.
This is why today's recommendation is to keep the 2nd ring end-gap as large or
larger than the top ring end-gap.
Compression ratio is the volume of the cylinder when the piston is at the bottom
of the cylinder compared to where it is when the piston is at TDC. A 100 cubic
inch cylinder would have its volume squeezed into 10 cubic inches with a 10:1
ratio. The easiest way to keep track of it is to think of every thing as
volumes that are stacked on top of one another. The factors that stack up are
the displacement of the cylinder (bore and stroke), the volume of the deck
clearance (getting back to the zero-deck issue from before), the volume of the
gasket (which is basically a short-round cylinder), and the volume of the
combustion chamber. The net combustion chamber volume means you must subtract
dome volume add the dish volume. Use the formula (bore x bore x stroke x .7854
x 16.4) to get the volume for a cylinder in cc's. Stack up the cylinder + the
deck volume + the gasket volume + the net chamber volume---take this number and
call it A…Stack up the deck, the gasket, and the net chamber volume and call it
B….. Take the big number A and divide it by the small number B and it will give
you the compression ratio.
Ring packages almost always consist of 3 rings these days. If ring life is not a
concern, the .043-.043-3mm is common. For people looking for better performance
with longer life, run the Wiseco GFX Ring package. The GFX rings have a
stainless steel Gas nitrided top ring with a Napier hook second ring and
nitrided oil ring rails. The GFX rings are a .047-.047-3mm size. The most
common long life ring package for 4-inch and over bore is a 1/16, 1/16, 3/16.
Dikes rings are currently only popular in blown alcohol or Top Fuel
applications. Gas ports should be added when using the extra-thin ring
packages. Important note: It is the customer's responsibility, when using a
.043 ring-pack, to specify when a back-cut (Pro) ring is used versus a standard
Vertical gas ports are primarily used in drag race applications. Lateral gas
ports are more often used in circle-track and road-race applications where
carbon build up can occur. When gas ports are used, pressure is directed to
build up behind the compression ring and seal it against the cylinder wall.
This helps prevent ring flutter and extends the power curve upward in the RPM
range. Vertical gas ports have the holes drilled from the deck of the piston
into the top ring groove and 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
A full FAQ page for our EFI Controllers can be found in PDF form on our installation manual page or by clicking HERE.