One more factor that will help you consume all your oxygen inside the cylinder - the burnt rate at various a/f ratios
(ref: Internal Combustion Engine Fundamentals - John B.Heywood. ISBN 0-07-028637-X )
Ignition timing had to be close to MBT (maximum brake Torque) timing to achieve maximum cylinder pressure (torque or power per RPM)
Figure left shown Cylinder pressure versus crank angle for overadvanced spark timing (50deg), MBT timing (30deg), and retarded timing. Your a/f ratio will greatly affect the MBT timing due to slowing down of burnt rate. Figure right shown the effect of spark advance on brake torque at constant speed and (A/F), at wide-open throttle. MBT is maximum brake torque timing.