timing and valve clearance recommendations

[Derek UK]

Note that some have found that reducing the advance when on a dyno at over 4000 rpm, has increased the power. Confusing isn't it!

[142 Guy]

Originally Posted by Derek UK

Note that some have found that reducing the advance when on a dyno at over 4000 rpm, has increased the power. Confusing isn't it!

Not really.

As a rather simplistic rule of thumb, for maximum torque (and horsepower is just Torque x RPM) you want to achieve peak combustion pressure at around 10- 20 degrees after top dead center. So, you need to time your ignition point such that the burn has completed consistent with achieving peak cylinder pressure at 10-20 deg ATDC. At high engine load the fuel mixture density is greater and burns faster so you may not need as much time to achieve that optimum burn completion point.

This is the purpose of vacuum advance. At 4000 RPM with engine light load it increase the advance to allow for longer burn time. It is effectively a function that retards ignition as engine load increases independent of RPM.

Because combustion time is 'sort of' fixed, as engine RPM increases the piston is moving faster so to keep the time constant you need to ad more degrees of advance. At some point the increasing mixture densities become a bigger factor in combustion speed which is why there appears to be no upside to further increases in advance after about 3000 - 4000 RPM. On a performance engine, if you have increased the cylinder volumetric efficiency you are presumably dealing with higher mixture densities in the cylinder which may burn faster in which case it might be beneficial to retard the ignition slightly so that your peak cylinder pressure still occurs at the most beneficial point.

On an engine that is detonation resistant, hypothetically you can advance the ignition setting to the point that peak cylinder pressure occurs before TDC which means that cylinder pressure is retarding the piston movement which reduces torque output. At any RPM and engine load there is an optimum advance for maximizing torque. More advance is not necessarily more torque.

On a B20 B engine, Phil Singher made the observation that optimum maximum advance was around 32 - 35 degress around 3000 RPM. More advance did not increase torque output.