Lean mixture, excessive spark
advance, high compression, low capacity cooling system, detonation and
high HP per cubic inch all combine to increase an engine's heat load.
Most new generation pistons incorporate the top compression ring high on
the piston. The high ring location cools the piston top more
effectively, reduces detonation and smog, and increases horsepower. If
detonation or other excess heat situations develop, a top ring end gap
set to the close side will quickly butt, with piston and cylinder damage
to follow immediately. High location rings require extra end gap because
they stop at a higher temperature portion of the cylinder at
top-dead-center and they have less shielding from the heat of
combustion. At top-dead-center the ring is above the cylinder water
If a ring end gap is measured on
the high side, you improve detonation tolerance in two ways. One, the
engine will run longer under detonation before rings butt. Two, some
leak down appears to benefit oil control by clearing the oil rings of
oil build up. Clean, open oil rings are necessary to prevent from
reaching the combustion chamber, which is also why we do not like
gapless rings. A very small amount of chamber oil will cause detonation
and produce significant horsepower loss. Top ring gaps can be increased
50% with hypereutectic pistons.
Ring Options of
1/16" or stock 5/64" are offered on most performance pistons.
The 1/16" option reduces friction slightly and seals better above
6,500 RPM, while being considerably more expensive. Stock, (usually
5/64" compression rings), work well and help with the budget.
Piston to Bore Clearance for
hypereutectic pistons were Dyno tested at wide open throttle with
.0015", .0020", .0035" and .0045" piston to bore
clearance. After 7-1/2 hours the pistons were examined and they all
looked as new, except the tops had normal deposit color. Even with 320
degrees Fahrenheit oil temperature, the inside of the piston remained
shiny silver and completely clean. Excessive clearance has been shown to
be safe with hypereutectic pistons. Loose Hypereutectic pistons over
.0020" do make noise. As they get up to temperature they still make
noise because they have very restricted expansion rate and do not swell
up in the bore. The Hypereutectic alloy not only expands 15% less, it
insulates the skirts from combustion chamber heat. If the skirt stays
cool piston expansion is drastically reduced. Running close clearances
is beneficial to piston ring seal and ring life. A small short term HP
improvement can be had by running additional piston clearance because
friction is reduced. To obtain actual piston diameter, measure the
piston from skirt to skirt level with the balance pad.
Pin Oiling should
be done at pin installation, whether it is pressed or full floating,
prelube the piston pin hole with oil or liquid prelube, never use a
grease. If you are using a pressed pin rod be sure to discard spiral pin
retainers. A smooth honed pin bored surface with a reliable oil supply
is necessary to control piston expansion. A dry pin bore will add heat
to the piston rather than remove heat. Pistons are designed to run with
a hot top surface, and cool skirts and pin bores. High temperature at
the pin bore will quickly cause a piston to grow to the point of seizure
in the cylinder.
require an extra .001"-.003" clearance because of the possible
combination of high load operation and cold water to the block. A cold
block with hot pistons is what dictates the need for extra marine
Ratio" as a term sounds very descriptive. However,
compression ratio by itself is like torque without RPM or tire diameter
without a tread with. Compression ratio is only useful when other
factors accompany it. Compression pressure is what the engine actually
sees. High compression pressure increases the tendency toward
detonation, while low compression pressure reduces performance and
economy. Compression pressure varies in an engine every time the
throttle is moved. Valve size, engine RPM, cylinder head, manifold and
cam design, carburetor size, altitude, fuel, engine and air temperature
and compression ratio all combine to determine compression pressure.
Supercharging and turbo-charging can drastically alter compression
The goal of most performance
engine designs is to utilize the highest possible compression pressure
without causing detonation or a detonation related failure. A full
understanding of the interrelationship between compression ratio,
compression pressure, and detonation is essential if engine performance
is to be optimized. Understanding compression pressure is especially
important to the engine builder that builds to a rule book that
specifies a fixed compression ratio. The rule book engine may be
restricted to a 9:1 ratio but is usually not restricted to a specific
compression pressure. Optimized air flow and cam timing can make a 9:1
ratio but is usually not restricted to a specific compression pressure.
Optimized air flow and cam timing can make a 9:1 engine act like a 10:1
engine. Restrictor plate or limited size carburetor engines can often
run compression ratios impractical for unlimited engines. A 15:1 engine
breathing through a restrictor plate may see less compression pressure
than an 11:1 unrestricted engine. The restrictor plate reduces the air
to the cylinder and limits the compression pressure and lowers the
octane requirements of the engine significantly.
At one time compression pressure
above a true 8:1 was considered impractical. The heat of compression,
plus residual cylinder head and piston heat, initiated detonation when
8:1 was exceeded. Some of the 60's 11:1 factory compression ratio
engines were 11:1 in ratio but only 8:1 in compression pressure. The
pressure was reduced by closing the intake valve late. The late closing,
long duration intake caused the engine to back pump the air/fuel mix
into the intake manifold at speeds below 4500 RPM. The long intake
duration prevented excess compression up to 4500 RPM and improved high
RPM operation. Above 4500 RPM detonation was not a serious problem
because the air/fuel mix entering the cylinder was in a high state of
activity and the high RPM limited cylinder pressure due to the short
time available for cylinder filling.
Before continuing with theory, a
little practical compression information is in order. If you have a 10:1
engine with a proper .040" assembled quench and then add an extra
.040" gasket to give 9.5:1 and .080" quench you will usually
experience more ping at the new 9.5:1 ratio than you had at 10:1. Non
quench engines are the exception to this rule. Some racers make the
effort to convert non-quench engines to quench type engines, as with our
Mopar Squish Deck Heads. Compression ratios that work are as follows:
8.5:1- Non-quench head road use
standard sedan, without knock sensor.
8.5:1- Quench head engine for tow
service, motorhome and truck.
9.0:1- Street engine with proper
.040" quench, 200° @ .050" lift cam, iron head, sea level
9.5:1- Same as 9:1 except aluminum
Light vehicle and no towing.
10:1- Used and built as the 9.5:1
engine with more than 220° @ .050" lift cam. A knock sensor retard
is recommended with 10:1engines.
12.5:1- Is the highest compression
ratio suggested with unrestricted race gas engines.
15.5:1- Is the highest compression
ratio suggested for unrestricted alcohol fuel engines.
Satisfactory use of 14:1 - 17:1
compression engines can be made when restrictor plate or small
carburetor use is mandated by the race sanctioning. High altitude
reduces cylinder pressure so if you only drive at high (above 4500 feet
altitude) a 10:1 engine can be substituted for a 9:1 compression engine.
General compression rules can be violated but is usually a very special
case such as a 600 HP normally aspirated engine in a 1500 lb. street car
with a 12:1 compression ratio. The radical cam timing necessary for this
level of performance keeps low and medium RPM cylinder pressure fairly
low. At high RPM detonation is less of a problem due to chamber
turbulence, reduced cylinder fill time, and the fact that you just can't
leave the above combination turned on very long without serious
non-engine related consequences.
Piston temperature and horsepower
are interrelated. High horsepower per cubic inch engines not only make
more horsepower but they make more heat. How the excess heat is handled
has a significant effect on total engine power and longevity.
Various piston, cam, valve,
chamber and port configurations have been and are currently being tested
to optimize engine internal temperatures. Some engines have ceramic
exhaust port insulation coatings that allow cooler cylinder head
operation while keeping exhaust temperatures elevated for efficient
catalytic converter operation. The same ceramic type insulation on a
piston top has been quite successful. Ideal piston temperatures in an
operating engine would suggest refrigeration during the intake and
compression stroke, and incandescence during the combustion and exhaust
stroke. The advantage of a hot piston on the power stroke is that less
combustion energy is going to be absorbed by the piston. So far, it is
not practical to heat and refrigerate a piston 6000 times a minute.
However, if the incoming air is not heated by the piston and the piston
reflects the heat of combustion, you start to approach ideal conditions.
A polished hypereutectic piston will reflect combustion heat back into
the combustion process. This reflection, combined with the insulating
qualities of the hypereutectic alloy, keeps the heat in the cylinder
during the power stroke. A smooth polished piston runs cooler than a
non-polished piston, even after combustion deposits have turned both
pistons black. A cool, smooth piston will transmit a minimum of heat to
the incoming fuel air mix.
The Hypereutectic piston gives the
racer a real out of the box advantage with smooth diamond turned piston
heads. A polish is relatively easy to achieve and does improve the
already excellent reflectivity of the hypereutectic piston. If a buffing
wheel is used, you will note a gray cast to the finished piston. The
gray results from the exposure of the Silicon particles that are
dispersed through the piston.
Experimental work to reduce piston
heating of the incoming fuel mix has been very limited but, in theory, a
thin ceramic coating may prove to be beneficial. A thin, smooth coating
over a polished piston should still reflect combustion heat while
reducing carbon buildup and protecting the piston polish. It is easier
for a thin film to change temperature with each engine cycle than it is
for the whole piston to do the same. A thin film can be cooled by the
first small percentage of inlet fuel mix, allowing the main quantity of
fuel mix to remain relatively cool. Tests have shown that polishing the
combustion chamber, valves and piston top can increase horsepower and
fuel economy by 6%.
All this polishing probably sounds
counter to the practice of cimpling the combustion chamber. Dimpling has
been show to put wet flow back into the air flow and improve combustion.
We do not recommend dimpling, but do suggest cutting a small
discontinuity close to the valve seat to turbulate wet flow. Some bench
flowed cylinder heads encourage fuel separation at the inlet pot. If a
small step is added at the valve seat to force the wet flow over the
resulting sharp edge, fuel will reenter the air stream and give you the
same affect as dimpling only without losing the benefit of a completely
polished chamber. As you reduce wet flow you will improve combustion and
most likely need to install leaner carburetor jets. Leaner jets
compensate for the excess fuel that is available when wet flow is put
back into the air/fuel mix. Significant additional horsepower gains can
be had with careful attention to cylinder-to-cylinder fuel distribution
by allowing all cylinders to be set "just right".
Combustion chamber design work has
increased steadily the last ten years. Some of the work is mandated by
the EPA and some is the result of race engine development. Some of the
smog work has actually enhanced race engine development. Combustion
chamber science is now more concerned with the effects of swirl,
tumbling, shrouding of the valve, quench, flame travel, wet flow and
spark location. A combustion chamber shaped dished piston can improve
the flame travel in the combustion chamber. A dish allows the flame to
travel further and expand more before it is stopped by a metal surface.
This rapid flame travel makes it unnecessary to run big spark advance
numbers. Ideally, we would like to be able to initiate ignition at top
dead center since this would reduce negative torque in the engine that
is now cause by spark advance. We are some time away from a practical
spark ignition system that will make optimum power with a TDS setting.
Some day it will happen. Don't go out and buy dished pistons for your
open chamber 454. The advantage in flame travel is more than offset by
the low compression ratio this combination yields. Small combustion
chambers respond well to dished pistons, especially reversed dome or
"D" cups. A 400 small block Chevy can use a 22CC D Cup piston
and still have 10.4:1 compression. The trend in modern engine design
seems to be smaller combustion chambers with recessed piston tops for
more HP per cubic inch.
Ignition timing on most
installations should be 34 degrees total with full mechanical advance
dialed in. More advance may feel better off the line but the engine lays
down as the combustion chamber components come up to temperature. At the
drag strip set timing for maximum MPH not best ET. Too much spark
advance will shorten the life of any performance engine, sometimes
Nitrous oxide can double the
horsepower of most engines with less effort and money being spent than
any other modification. Even the "smog people" are usually
happy, as the nitrous is activated only during full throttle "open
A nitrous engine can be built as a
stock rebuild or it can be a dedicated effort to maximize the total
performance package. As more power is generated, more waste heat,
exhaust air flow and combustion pressures push the limits of engine
strength. Often more beef is needed in the drive train and tires.
All stock factory engines are
built with a safety factor when it comes to RPM, HP produced, cylinder
pressure, engine cooling, etc. If you are only going to use a 100 HP
nitrous setup on a 300 cubic inch or larger engine, built in factory
safety factors are probably sufficient. As power output levels are
raised engine modifications are usually prudent.
The most common mistake made when
using nitrous oxide injection concerns ignition timing. A normally
aspirated engine makes its best power when peak cylinder pressures occur
between 14 and 18 degrees after TDC. Pistons usually require 34 degrees
BTDC ignition timing at full mechanical advance to achieve proper ATDC
peak cylinder pressure. The total time from spark flash to the point of
peak pressure is typically 48 to 52 degrees. If an engine is producing
30% of its power from nitrous, the maximum cylinder pressure will occur
too close to TDC to avoid run away to detonation. If ignition does not
get retarded, good-bye horsepower and head gaskets. The key to getting
max HP from a max nitrous engine is to shift the maximum cylinder
pressure event progressively further after TDC.
Cylinder pressure of 1000 PSI at
TDC, (FIG. 1), can drop to 500 PSI with less than 3/8" of piston
travel, (FIG. 2). If you can manage to get 1000 PSI in the same engine
after the 3/8" travel, (FIG. 3), the pistons will have to travel an
additional 3/4" to lower the cylinder pressure to 500 PSI, (FIG.
4). Work is defined as a force times distance. An average pressure, (750
PSI X 12-1/2 sq. in.), times distance in feet, (3/8" divided by
12), equals 293 foot pounds of work. Our second example, because it has
twice the chamber volume above the piston location, must move twice as
far to lower the cylinder pressure by 1/2. Since all the other numbers,
by our own definition are the same, the force multiplied by a distance
twice that of the first example will equal twice the work done, 586 foot
pounds of work. There is no free lunch in horsepower equations because
to get 1000 PSI above the piston in the second example takes twice as
much fuel and energy as the 1000 PSI in the first example. What this
offsetting of the peak pressure does is allow us to use the extra fuel
mix available to a nitrous engine without breaking and melting things.
The system that allows us to postpone maximum cylinder pressure is
ignition timing retard. To a lessor extent short rod ratios, lower
compression ratios, high RPM, aluminum heads, a tight quench, a rich
fuel mixture, a small carburetor and hotter cams tend to delay maximum
Understand that, in our quest to
delay cylinder pressure's peak time, more is not necessarily better.
Instead, consider that the ideal cylinder pressure would be just short
of detonation pressure and this pressure would be maintained from top
dead center, and as long as possible after TDC. If timing is really
late, you won't build enough cylinder pressure to start the car, let
alone drive it. The 1000 PSI pressure in the example is not the maximum
allowable combustion pressure but, rather, a comfortable pressure for
illustration of the work principle.
Some nitrous manufacturers
recommend, "retard the timing two degrees for each fifty horse
power of nitrous". Other nitrous kits have the flame speed
artificially slowed by the intentional use of a rich fuel to nitrous
ratio. The maximum performance engine with a heavy nitrous load must
achieve peak cylinder pressures, with the combustion chamber size being
drastically increased due to the piston being on its way toward bottom
dead center. The strongest engines have less compression ratio, less
spark advance, and more nitrous.
Many people just don't like the
idea of any retard. They say, "retard timing and exhaust heat goes
up". It usually does in a stock non-nitrous engine because lower
peak cylinder pressure slows the burning. If the timing is retarded in a
non-nitrous engine, the exhaust opens before the fuel mix is finished
burning and exhaust temperatures go up. Piston temperatures usually go
down and exhaust valve temperature goes up. In the nitrous engine,
exhaust temperature goes up for several reasons. The first is that the
power output has gone up considerably. More power usually produces more
waste heat. Second, the need to keep maximum cylinder pressures within
reason has dictated that the biggest part of the fire happens closer to
the exhaust valve opening time. There just isn't enough piston travel to
extract all the energy out of the charge before the exhaust valve opens.
Now, we could and sometimes do, open the exhaust valve later so more
combustion pressure energy can be used to turn the crank. The trade off
is negative torque on the exhaust stroke. If we still have significant
cylinder pressure in the cylinder as the piston moves from BDC to TDC on
the exhaust stroke, your net HP falls drastically. A real problem at
You can improve maximum power
stroke efficiency and minimize exhaust pumping losses by running the
engine at lower RPM and/or improving the exhaust valve size, lift and
port design. A big nitrous engine likes everything about the exhaust to
be big. If it flows good enough the cylinder will blow down by bottom
dead center, even at high RPM with relatively mild exhaust valve timing.
There are many variables in the design and development of an all out
nitrous engine. A mistake will cause the melt down of any piston. The
high strength of the hypereutectic piston will withstand detonation and
severe abuse. Unfortunately, all pistons, even forged will melt and when
cylinder pressure limits are exceeded, run away detonation can occur.
The excess detonation heat makes the plugs, valves and pistons so hot
the ignition system alone cannot be used to shut the engine down.
Continued operation worsens the situation to the point of a total melt
down. Designing a maximum performance nitrous engine is more of an
exercise in heat management than it is in engine building. Serious
nitrous users should review our list of ceramic coatings.
A lack of a sufficient fuel supply
is probably the most common killer of the nitrous engine. If you add a
300 HP kit to your present 300 HP engine, your fuel requirements roughly
double and a shortage doesn't just slow you down, it melts things. An
electric fuel pump and fuel line devoted entirely to the nitrous
equipment is recommended. If you are using a diaphragm mechanical pump
to supply fuel to the carburetor, it is worth while to increase the fuel
line I.D. If the carburetor goes lean while the nitrous is on, the
pistons can melt even with a rich fuel line trick (1/2" dia.) only
makes a major improvement in the operation of diaphragm mechanical pump
is not recommended and does not do well at high engine RPM. A large size
line is effective with a mechanical pump, even if you use smaller
fittings at the tank, fuel pump and carburetor. The advantage of the
1/2" large line is not related to the steady state flow rate of the
The advantage relates to the
acceleration time and displacement of the pulsating flow common to the
High compression ratios can be
used with nitrous but shifting the maximum pressure after top dead
center becomes more and more difficult. I prefer to use street
compression ratios and then just work with adding more nitrous to get
desired horsepower levels.
We are currently testing some
pistons specifically designed for Nitrous use. Current "off the
shelf" pistons have been successfully run with a 500 HP nitrous kit
combined with a nitrous control system. Most of our effort has been to
develop new ideas that will push the limit of nitrous technology. More
testing is planned with a piston especially coated to reduce detonation.
When choosing piston rings for an
engine the most important factor is the intended use of the vehicle. A
piston ring set that delivers excellent street performance may not be
correct for an engine that will see competitive action, or for one that
will be used with nitrous oxide.
Piston rings serve two purposes -
to contain the cylinder pressure, and to prevent oil from getting into
the combustion chamber. Sealing against pressure leakage, or "blow
by", is the responsibility of the top ring. The keys to good ring
sealing are cylinder wall finish and piston ring groove condition. If
pressure gets past the top ring it is already "lost". Any such
leakage will not be ignited by the spark plug, and is unlikely to
produce any significant power, even if captured between the first and
second ring. The second ring is primarily an oil control device. If the
top ring is doing the job, the second ring will see fairly limited
combustion pressure. Some companies sell second rings that use complex
or fragile designs for sealing. These are prone to premature wear and
have unpredictable behavior at high RPM levels. Cylinder leakage test
percentages are only useful for comparison to data generated when an
engine was fresh. Unfortunately this kind of information can be
misrepresented to show very low leakage numbers by folks trying to sell
"trick" parts. Leakage tests are steady state - they do not
account for time, piston movement, or true operating pressures.
"Blow-by" measurement will give a better picture of ring
condition, but on track performance is the only real measurement of
success. Our moly rings are intended for applications where cost is of
Engines being built for serious
competition will be far better off using Plasma Moly ring sets. These
feature an extremely durable ductile iron top ring with Plasma Moly
facing. This design allows the ring to seat quickly and to maintain its
sealing integrity under the severe stress of racing. The second ring is
a special low tension plain iron design. These taper faced rings are
specifically designed to break in quickly and to keep oil from migrating
into the combustion chamber. The SS50U stainless steel oil control rings
are the absolute best in the high performance industry. This ring
combustion gives dependable sealing and allows maximum power production.
Piston ring sets are available
with either standard or low tension oil rings. The standard tension
rings are recommended for street driven applications, and for race
vehicles which may see frequent open to closed throttle transitions in
use - such as road racing. They are also useful in engines that may
experience cylinder bore distortion during operation.
Low tension oil rings deliver
increased performance due to their reduction in cylinder wall drag.
These are highly recommended for many racing applications. Engines using
low tension rings should be built with particular attention to cylinder
concentricity, and often benefit from the use of a crankcase vacuum
RING END GAP CLEARANCE
The piston ring's end gap can have
a significant effect on an engine's horsepower output. Rings are
available both in standard gap sets, and in special "file fit"
sets. The file fit sets allows the engine builder to tailor the ring end
gaps to each individual cylinder. Ring gaps should be set differently
dependent upon the vehicles use, within the range of .003" (for the
2nd. ring) to .004" (for the top ring) per inch of cylinder
diameter. The more severe the use, the greater the required end gap
(assuming the use of similar fuels and induction systems). Engines
having low operating temperatures, such as those in marine applications
is too small. The chart below is a general guideline for cylinders with
a 4.00" bore, adjust the figures to match your engine's cylinder
Top Rings (ductile iron, 4"
Nitromethane .022 - .024"
Alcohol .018 - .020"
Gasoline .022 - .024"
Normally Aspirated -
Street, Moderate Performance .016
Drag Racing, Oval Track .018 -
Nitrous Oxide - Street .024 -
Nitrous Oxide - Drag .032 -
2nd Rings (plain iron, 4"
Nitromethane .014 - .016"
Alcohol .012 - .014"
Gasoline .012 - .014"
Normally Aspirated -
Street, Moderate Performance .010
Oval Track .012 - .014"
Pro Stock, Comp. .012 - .014"
Nitrous Oxide - Street .018 -
Nitrous Oxide - Drag .024 -
INSTALLATION NOTES -
CYLINDER WALL FINISH
When installing new rings, the
single greatest concern is the cylinder wall condition and finish. If
the cylinders are not properly prepared, the rings will not be able to
perform as designed. The use of a torque plate, head gasket, and
corresponding bolts are necessary to simulate the stress that the
cylinder head will put on the block. Main bearing caps should also be
torqued in place. The correct procedure has three steps. First the
cylinder is bored to approximately .003" less than the desired
final size. Next it is rough honed within .0005" of the final
diameter. Then a finer finish hone is used to produced the desired
"plateau" wall texture. Use a 280 - 400 grit stone to finish
cylinder walls for Plasma Moly rings.
Note - the "grit" number
we are referring to is a measurement of roughness, it is not the
manufacturers stone part number (a Sunnen CK-10 automatic hone stone set
#JHU-820 is 400 grit). The cylinder bores should be thoroughly scrubbed
with soap and hot water and then oiled before piston and ring
Piston ring grooves are also
sealing surfaces, and must be clean, smooth and free of defects. Ring
side clearance, measured between the ring and the top of the groove,
should be between, .001" and .004".