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 shown at the right: (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.
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Octane Requirement Chart
Increasing Power Output Is The Primary Goal Of All Engine Tuning - Here's The Formula:
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Where do these values come from and what is the effect of AFR on engine torque? The chart shown below will provide some answers. (ref: Automotive Handbook 2nd Edition by Bosch GmbH, pp. 439)
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We often receive inquiries about optimizing engine ignition timing during dyno tuning. Very sensitive and precise dyno tests are required to determine MBT timing. The chart at the right 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.
Effect Of Engine Timing Chart
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 at the right 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.
Basic Understanding Of The Equation Which Determines Engine Power Output Is Helpful
Engine Air/Fuel Ratio Reference Table
Higher Air/Fuel Ratio (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 at the right lists recommended AFR values for engines without emission controls.
Compression Ratio Effect Chart
While We Cannot Make Specific Recommendations - We Have Compiled A List Of 'Basic Guidelines' From A Wide Variety Of Our Technical Resources Including World-Class Engineering Textbooks And Aftermarket Industry Reference Manuals - Please Utilize All Of The Information Provided Below
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 shown here lists recommended maximum ignition advance at WOT for various applications. Daytona Sensors™
Engine Controls allow setup of ignition advance tables that meet these recommendations.
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.