Engine Tuning Principles and Guidelines

Our customers 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.  

Compression Ratio  

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. 

 

 

Ignition Timing  

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 recommendations.    

Engine Application

Maximum Advance at WOT 

Stock Compression (less than 10:1 using premium gasoline)

35 deg BTDC at 2500-3000 RPM

High Compression (10.5:1)

30 deg BTDC at 5000 RPM

High Displacement (>120 CID or bore approaching 4 inch)

28 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. 

High 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 PKWARE Inc. website.

 

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.

Operating Mode

Recommended AFR

Cold Start (first 30 sec)

11.5-12.5

Idle

12.8-13.5

Part Throttle Cruise

13.0-14.0

Wide Open Throttle

12.5-12.8 (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 detonation.

Engines with race camshafts exhibit large cyclical variations at idle - necessitating a relatively rich idle to prevent stalling.  

 

 

Power  Output

Increasing power output is the primary goal of all engine tuning. Understanding the equation that determines engine power output is very helpful.

Pb= ni nc nm nv pi Vd (N/X) F Q   (ref: Engines An Introduction by J. Lumley, pp. 26)

where:

Pb    brake power output

ni      indicated efficiency

nc    combustion efficiency

nm   mechanical efficiency

nv    volumetric efficiency

pi     inlet air density

Vd   displacement volume

N     rotational speed (RPM)

X     revolutions per power stroke (always 2 for a four stroke engine)

F     fuel/air ratio

Q    heating 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.

ni      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. 

nc    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.

nm   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 fuel consumption.

nv    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.

pi   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 reduction.

Vd   displacement volume is the most basic engine parameter and has a direct effect on engine power.

F     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.

Q    heating 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 air density.

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 improvements.