As load on the engine increases from adding more throttle opening (high inlet charge density / low manifold vacuum) the a/f ratio needs to be enriched to produce more power and reduce the drivability issues because of lean a/f ratios at high loads. The a/f ratio should be somewhere between the cruise and wot a/f ratio values, generally around 14.5 to 13.0:1 depending on load and speed.

Wide open throttle

All 4 cylinder petrol engines have around the same a/f ratio needs at WOT, where the goal is to produce the maximum torque / power from a given engine combination. The leanest a/f ration that produces maximum torque / power is referred to as "lean best torque" which is usually around 13.3:1 a/f ratio. The richest is a a/f ratio of 11.5:1 "rich best torque". The difference between "lean best torque" and "rich best torque" can be closer at high engine speeds, so the best target a/f ratio for wot usage is between 12.0 to 12.5:1 a/f ratio, this insures the best performance at wot power under all circumstances.

Spark advance requirements

The charge of the air / fuel is burned by a flame-front beginning at the spark plug. The flame starts a kernel with a rather slow rate of expansion, but once a small percentage of the charge is ignited, the combustion process accelerates at a faster rate. Due to the very slow initial reaction rates, ignition must occur "before top dead center" if maximum effect is to be utilized. This is the "advance" in ignition and is measured in degrees of crankshaft rotation. The best advance usually produces the best torque when maximum cylinder pressure is achieved at around 15 degrees "after top dead center". Depending on design and operating conditions, the spark advance can be from less than 5 degrees up to more than 35 degrees for a high performance / race engine, and 0 degrees up to more than 50 degrees for a modern stock production engine with EFI / engine management systems in place.

We strive to calibrate spark advance to values referred to as " minimum best torque", this is the minimum spark advance that will produce the maximum torque at a given operating condition of speed and load of a given engine combination. In most cases the spark advance curve can be advanced several additional degrees beyond "minimum best torque" before torque begins to drop off. If "knock" occurs before "minimum best torque" advance can be determined. The advance is referred to as "knock" limited. The fuel octane, camshaft profile and or the engine's static compression ratio will need to be addressed before maximum out-put can be achieved.

The variables that influence the spark advance requirement include the base engine design, the specific components of the particular engine ( camshaft, compression, piston and cylinder head configuration), the intended fuel to be used and the operating conditions (rpm / load / temperatures). The spark advance generally increases with engine rpm up to a point where it will "peak" and in some cases will decrease slightly with further increase of rpm. Advance requirements decreases with load and the minimum advance at any given engine speed is at WOT.

The advance requirements of an engine of the same design but different components are dictated more by camshaft profile, including compression. With a radical camshaft profile, the WOT advance curve can be very aggressive and reach maximum advance at a lower rpm because of the poor volumetric efficiency at low rpm, and a very slow combustion rate and its high resistance to "knock". Part throttle advance on engines with large cams can also be aggressive because of the reduced flame speed caused from a significant exhaust dilution of the inlet charge from a camshaft with a lot of overlap.

Testing of various configurations of 4 cyl engines on our Chassis Dynamometer has shown that there is a common approach to optimising the state of tune regardless of configuration. However it must be said that an air cooled engine performs best and operates in a most efficient temperature range if its window of operation is on the rich side of the line, lean running of an air-cooled engine leads to heat soak in the cylinder heads and barrels for which the available cooling effort is insufficient to maintain a stable operating temperature – that is to say more heat is being generated then can be taken away by the volume of cooling air and temperatures can increase to the point where exhaust valve, head and piston failure can quickly occur.