Engine Tuning Principles and
often ask us for tuning advice. While we cannot make specific recommendations, we
have compiled information from a variety of technical resources including
engineering textbooks and industry reference manuals.
The most common mistake that we
encounter is excessively high compression ratio. Besides serious starting
difficulties, the problem usually manifests itself as extreme detonation
(spark knock). Spark timing must then be retarded, far back from the maximum
brake torque (MBT) value, resulting in a loss of torque. For any given
compression ratio, there exists a minimum gasoline octane requirement, as
shown in the chart below (ref: Advanced Engine Technology by H. Heisler, pp.
177). The values in the chart are conservative. With good cylinder head design
that promotes high swirl and fast flame front propagation, a slightly higher
compression ratio is possible. However, the practical limit for 93 octane pump
gasoline is about 10.5:1. If you try to use a higher value, you will have to
retard the spark timing to the point where the engine will actually generate
less torque than one with a lower compression ratio.
Whereas excessively high
compression ratio is the most common engine building mistake, the most common
engine builder misconception is that increasing the compression ratio has a
significant effect on power (or torque). The chart below shows that this is
not the case (ref: Internal Combustion Engine Fundamentals by J. Heywood, pp.
843). A useful rule of thumb is that raising the compression ratio one point
(i.e. from 10:1 to 11:1) increases power by about 3%. However this potential
power increase is only available if the gasoline octane allows running the
engine at the MBT timing value without detonation.
We often receive inquiries
about optimizing ignition timing during dyno tuning. Very sensitive and
precise dyno tests are required to determine MBT timing. The chart below shows
why (ref: The Internal Combustion Engine in Theory and Practice by C. F.
Taylor, pp. 443). The engine torque curve is very flat near the MBT timing
value. A useful rule of thumb is that advancing or retarding the timing
5 degrees from the MBT value reduces torque about 1%. You cannot
reliably measure a 1% torque change on a chassis dyno. We have witnessed
many dyno test sessions where attempts to optimize ignition timing generated
strange results that were probably caused by measurement error.
In most engines (assuming
compression ratio and other factors, such as air fuel/ratio, are within
reasonable limits), the MBT timing value is a few degrees below the
detonation limit. If you select a wide open throttle (WOT) timing advance
curve that is retarded about 3 degrees from the point where detonation is
detected, you should be close to MBT ignition timing.
The table below lists
recommended maximum ignition advance at WOT for various V-twin engine
applications. Twin Tec ignition and fuel injection controllers allow setup of
ignition advance tables that meet these
Advance at WOT
Compression (less than 10:1 using premium gasoline)
deg BTDC at 2500-3000 RPM
deg BTDC at 5000 RPM
Displacement (>120 CID or bore approaching 4 inch)
deg BTDC at 5000 RPM
Twin Tec Models 1005-1007 allow
a maximum advance adjustment from 30-35 degrees using the switch settings. For
use with high displacement Evo style V-twin engines, we have provided a custom
advance table file that can be uploaded to the Twin Tec ignition with PC Link
Evo software. When using this table, you can set the timing using a dial back
timing light and the TDC timing mark.
Displacement Engine Advance Table
The advance table is saved as
ZIP archive file that you can download by clicking on the link above. You will
require PKZIP to unzip the archive. If you do not have PKZIP installed on your
computer, you can download it from the
Air/Fuel Ratio (AFR)
Higher AFR values correspond to
a leaner (less fuel) condition. The practical operating range for most engines
using gasoline fuel is from approximately 11.5 to 14.7 AFR. Combustion of a
stoichiometric mixture (exactly enough air to burn all the fuel) results in
14.7 AFR indication. Automotive engines with catalytic converters operate near
14.7 AFR during cruise and idle. Air-cooled motorcycle and automotive race
engines require a richer mixture to limit cylinder head temperature and
prevent detonation. The table below lists recommended AFR values for engines
without emission controls.
Start (first 30 sec)
(values down to 11.5 may be used to reduce detonation)
Where do these values come from
and what is the effect of AFR on engine torque? The chart below provides some
answers (ref: Automotive Handbook 2nd edition by Bosch GmbH, pp. 439). In the
absence of other limiting factors, maximum engine torque occurs at about 13.5 AFR.
Under wide open throttle (WOT) conditions, a richer mixture (12.5 to 12.8 AFR)
is generally required to reduce cylinder head temperatures and avoid
detonation. While the torque curve appears relatively flat from 12 to 14.7 AFR,
the effect on cylinder head temperature is more pronounced. Please remember
that the chart is based on lab experiments under carefully controlled
conditions and with gasoline octane high enough to avoid limiting effects from
Engines with race camshafts
exhibit large cyclical variations at idle - necessitating a relatively rich
idle to prevent stalling.
Increasing power output is the
primary goal of all engine tuning. Understanding the equation that determines
engine power output is very helpful.
nc nm nv pi Vd (N/X)
F Q (ref: Engines An Introduction by J. Lumley, pp. 26)
brake power output
inlet air density
rotational speed (RPM)
revolutions per power stroke (always 2 for a four stroke engine)
value of fuel
The engine builder determines
some of these factors (an obvious one is Vd). Other factors can be
optimized by the engine tuner. Let's examine each factor in more detail.
indicated efficiency is based on the pressure-volume relationships and heat
transfer (losses) during the engine cycle. Compression ratio is a primary
determinant. Octane requirements and the effect of compression ratio
have already been discussed above. Heat losses are difficult to optimize and
beyond the scope of the engine builder or tuner. AFR has an affect on gas
properties (specific heats at constant volume and pressure) and thus a minor
influence on ni that contributes to the shape of the torque
versus AFR curve shown above.
combustion efficiency is the percent of fuel that is burned. The value is
usually close to 1.00 (100%). Good mixture distribution between cylinders,
high swirl, and optimum ignition timing influence nc.
mechanical efficiency includes all friction losses in the engine (and drive
train for the case of chassis dyno measurements). "Blueprinting"
an engine can reduce friction losses. Lower viscosity synthetic oils reduce
viscous friction loses (the effect can be several horsepower). Windage and
pumping losses associated with the crankcase and oil systems should also be
considered. Gear drive camshafts have lower friction losses than chain drive
arrangements. Evo style V-Twin engines have inefficient charging systems.
Changing to a series regulator reduces high RPM alternator losses. Careful
attention to all areas that influence nm can result in gains of a
few horsepower with the additional benefit of reduced heat generation and
volumetric efficiency is the actual volume of air inducted into the cylinder
divided by the cylinder displacement. The value of nv can exceed
1.00 (100%) in a well tuned race engine by means of inertial (ram) and
resonant effects within the intake and exhaust systems. Head design,
including valve size and port shape, and camshaft characteristics greatly
affect nv. Modifications that affect nv are the
traditional realm of the experienced engine tuner and can result in
significant power gains.
inlet air density is a function of pressure and temperature. A restrictive
air cleaner or throttle body reduces inlet air pressure. Excessive heat
transfer from the intake system increases inlet air temperature and harms
performance. While so called "cold air intakes" are commonplace in
the sport compact marketplace, inlet air temperature effects are usually
overlooked by V-twin engine tuners. The effect is significant as a 20 degree
C (36 degree F) rise in inlet air temperature results in a 6% power
displacement volume is the most basic engine parameter and has a direct
effect on engine power.
fuel/air ratio is the inverse of air/fuel ratio (AFR). The effect on power
is somewhat more complicated than the equation suggests. The effect is only
linear for lean mixtures (above 14.7 AFR) where excess air remains and all
the fuel is burned. The lean part of the torque versus AFR curve shown above
is where the equation applies. For performance applications, engines are
always run rich. Additional fuel cannot be burned due to lack of air and the
torque curve levels off and then starts to drop at very rich AFR values.
value of fuel, i.e. the energy released when a given mass of fuel is burned.
Q is varies slightly with different grades of gasoline (regular 87 octane is
about 42.7 MJ/kg, premium is about 43.5 MJ/kg, and fuels blended with high
levels of MTBE and/or ethanol have lower heating values).
From a practical standpoint,
engine power output is most readily increased by increasing the displacement
volume and volumetric efficiency. Additional increases can be obtained by
selecting the maximum compression ratio (about 10.5:1 for 93 octane gasoline)
and careful attention to details that increase mechanical efficiency and inlet
Engine control systems can only
affect ignition timing and AFR. These parameters are optimized by setting
ignition timing to a value near the detonation limit and AFR within the range
of 12.5-12.8 at WOT. Further tuning efforts involving small changes in
ignition timing and AFR generally fail to yield measurable