MBT Timing Theory

What is MBT?

MBT (Minimum advance for Best Torque) is the spark timing that achieves the

combustion phasing producing maximum brake torque. It occurs when 50% of the fuel

charge has burned (MFB50) at approximately 8–12° after TDC.

The MBT Formula

θ_MBT = MFB50_target + θ_burn × BurnShapeFactor + θ_delay

Symbol	Meaning	Typical
`θ_MBT`	Required spark advance [°BTDC]	28–42°
`MFB50_target`	Desired 50% burn point [°ATDC]	8–10°
`θ_burn`	Total 0–100% burn duration [°]	30–60°
`BurnShapeFactor`	Fraction that is 0→50% MFB	0.42–0.56
`θ_delay`	Ignition delay [°crank]	5–15°

Burn Duration Model

The burn duration in crank degrees uses a two-term empirical model calibrated

to Heywood Fig 9-23 and SAE 850345 measured MBT data:

θ_burn = [K1×(1000/RPM)^p + K2×(RPM/1000)^q]
       × (Patm/MAP)^0.35 × (SL_ref/SL_cell)^0.5 × BurnShape

The low-RPM term produces high advance at idle (long time, few degrees/ms).

The high-RPM term captures the rise in required advance above the timing minimum.

The timing minimum RPM is derived from stroke geometry:

RPM_min = 10 m/s × 60 / (2 × stroke_m)

*Sources: Heywood (1988) Ch.9; SAE 850345 (Beretta et al.); SAE 902135.*

Knock-Limited Timing

if KnockMargin >= KnockZoneK:
    θ_final = θ_MBT

if 0 < KnockMargin < KnockZoneK:
    θ_final = MaxRetard + (KnockMargin/KnockZoneK) × (θ_MBT − MaxRetard)

if KnockMargin <= 0:
    θ_final = MaxRetard