Tune Your Engine
Secrets for finding your engine's
"Sweet Spot"

Finding the perfect ignition point    Managing head temperature    
Tuning for cold weather
Tuning for high altitude

by Dave Gierke
with illustrations by David Baker

Have you ever swung a baseball bat or golf club hard …and hit the
ball perfectly? You would know it right away—the ball travels long
and straight with an effortless feeling at the instant of contact.
These are examples of finding the sweet spot. Glow ignition
engines also have a sweet spot. When found, a smooth, powerful,
long-lived performance is the reward.


Combustion within a piston engine is not an instantaneous,
explosive process. Burning takes time. Maximum engine
performance is realized when the air-fuel mixture is ignited before
the piston reaches top dead center (TDC), while the peak cylinder
pressure occurs slightly after TDC—the operating cycle's sweet
spot location. The exact location of the sweet spot depends on
many factors including the engine's design. By adjusting the
ignition point timing, the operator can experimentally search for the
engine's sweet spot … defined as the highest attainable rpm for a
given propeller size.

This represents a normal pressure curve; notice how the ignition
point occurs before TDC,
while the cylinder pressure peaks after TDC.

Using a tachometer, set the primary needle valve 100-200 rpm
below peak. Note the position of the operator behind the engine;
also note the eye and hearing protection.

After disconnecting the start-up battery, ignition of the air-fuel
mixture within the glow engine depends on catalytic action,
compressive heating, and the ability to retain a portion of the plug's
heat from cycle to cycle. These factors will be explained later. The
resulting temperature of the Platinum alloy wire within the glow plug
ignites the mixture, producing a flame-front that burns its way
through the remainder of the charge. Burning is rapid enough to
allow the engine to operate at the highest practical shaft speeds.


Ignition occurs toward the end of the engine's compression
operation; when ignition begins, compression is said to end. There
are some variables that cause the ignition point to occur earlier on
the compression operation—producing an advanced ignition point;
others cause the ignition point to occur later on the compression
operation—producing a retarded ignition point (e.g., a lean air-fuel
mixture ignites earlier than a rich mixture, therefore it advances the
ignition point timing). By advancing or retarding the glow engine's
ignition point timing, its performance sweet spot may be determined.

If the ignition point moves away from TDC, the timing is said to
If the ignition point moves toward TDC, the timing is said to retard.


Unlike spark ignition engines, determining a glow engine's exact
ignition point is difficult without using sophisticated and expensive
instruments. Fortunately, there is a roundabout method available.
By using a tachometer and a glorified thermometer, the engine
tuner has the necessary tools to find the illusive sweet spot. These
variables affect the ignition point timing of a given engine and are
listed in the approximate order of importance:

Needle valve setting
Compression ratio
Nitromethane content (fuel)
Propeller load
Glow plug heat range
Oil type and percentage (fuel)
Setting the needle valve. The most important adjustment an
operator makes is setting the engine's high-speed needle valve.
When backed-off (rich) 100 to 200 rpm from its peak wide-open
throttle (WOT) setting, the maximum cylinder pressure will locate at
the sweet spot … if all the other variables are adjusted correctly.
However, set the needle valve a bit too lean and the combustion
temperature will rise, causing the ignition point timing to advance.
Overheating, possible combustion defects, and a missed sweet
spot are the consequences.


High compression ratios are directly related to detonation—a nasty
engine damaging combustion defect that must be avoided.
Detonation occurs when the advancing flame front pressurizes and
heats the unburned mixture ahead of it, causing spontaneous
combustion. The mixture's abnormal combustion produces very
high flame speed resulting in a local temperature and pressure
spike. This causes a "knocking", "pinging", or "frying egg" sound—
a sure indicator that mechanical damage is occurring to the piston
crown and/or cylinder head.

Cooling. Air-cooled glow engines are also partially liquid cooled.
Vaporizing fuel in the crankcase and combustion chamber helps
keep engine temperature within an acceptable range. A very rich
needle valve setting ensures that the engine will run cool at the
expense of efficient burning of fuel. However, when the primary
needle valve is optimized for performance, airflow through the
cooling fins is required. If the temperature climbs beyond the
established limits, ignition and burning begins earlier (advances). If
the cylinder head temperature is too low, as often happens during
winter conditions, ignition and burning begins later (retards),
causing erratic combustion and premature wear in certain engine

Compression ratio. An engine's compression ratio (CR) is the
comparison of cylinder volumes prior to compression and after the
piston reaches TDC. As the compression ratio is increased, the air-
fuel mixture is squeezed into a smaller volume prior to ignition,
resulting in a greater heat release rate after ignition. Within limits,
high compression ratios produce enhanced cylinder pressure,
torque and power.

Raising an engine's compression ratio advances the ignition point
timing; lowering it retards the ignition point timing. Raising or
lowering the compression ratio by removing or adding head shims
(gaskets) is the most common method of achieving this change.  

Removing a head shim (gasket) increases the compression ratio;
adding a shim reduces the engine's compression.

Considered a radical adjustment, compression change is usually
reserved for significant changes in altitude, atmospheric
conditions, or nitromethane content in the fuel

Nitromethane content. Nitromethane is a powerful fuel ingredient.
Under ideal conditions, increasing the fuel's nitro content (% by
volume) will increase engine power. Because nitro is very slow
burning, the ignition point timing must be advanced to maintain the
correct pressure peak sweet spot after TDC. Raising the fuel's
nitro content often requires adjusting one or more of the other
variables to obtain satisfactory performance, as we will discuss.

Propeller load. Larger propeller diameter and/or pitch increases
the engine's load, compelling it to run slower at WOT. Running
slower allows more effective cylinder compression and increased
heat release from the air-fuel mixture. Decreasing propeller load
causes the engine to operate faster, effectively reducing its
compression ratio and heat release.

Increasing the propeller size (load) advances the ignition point
timing; decreasing propeller size retards the ignition point timing.
When an engine is close to its sweet spot, seasoned operators
experiment with minute changes in propeller diameter.

Glow plug heat range. Glow plugs have two functions—get the
engine started and keep it running. By passing a low voltage
electric current through the plug's wire element, a high temperature
orange-white glow is produced; this allows the engine to be started
when the proper mixture of fuel and air is present within the
combustion chamber. After starting, the source of the electric
current is removed from the plug … but it continues to glow,
keeping the engine running.

Removing a head shim (gasket) increases the compression ratio;
adding a shim reduces the engine's compression.

A glow plug works by a combination of catalytic action, compressive
heating, and the ability to retain a portion of its heat from cycle to
cycle. First, the platinum alloy coil of wire within the plug's cavity
heats-up as it comes in contact with the fuel's methyl alcohol vapor;
this is a heat releasing catalytic action. Next, the temperature of the
plug's wire element is further increased by the engine's
compression of the air-fuel mixture. Manufacturers produce glow
plugs in various heat ranges: hot, medium, and cold. Many factors
determine the heat range, but it's important to note that hot plugs
advance the ignition point timing, while cold plugs retard it.

Fuel-oil type and percentage. We know that the fuel's nitromethane
content plays an important role in locating the engine's sweet spot.
However, there's another fuel-related factor to consider—the
percentage and type of lubricating oil.

For most sport flying applications, the percentage of lubricating oil
used in modern R/C aircraft engines ranges from 15-22% by
volume. Since oil must be heated along with the fuel's other
ingredients, the ignition point timing is affected—fuel with higher oil
percentages require more time to heat, which retards the timing. Of
course, lower oil percentages produce the opposite effect,
advancing the ignition point timing.

The type of oil also affects the ignition point timing. In general,
synthetic lubes are easier to heat than bean oils, such as castor.
For fuels with equal oil percentages, synthetics advance the
ignition timing more than blends containing castor oil.
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