Abstract
The “right” ignition advance is not one number, or even one curve — it’s a surface that bends with cam overlap, fuel chemistry, and the local operating point. Once you understand why MBT moves where it does, you can place timing deliberately in every cell instead of chasing a dyno number you can’t see on the street.
The core idea
The goal of timing is to put CA50 — the crank angle where half the charge has burned — at the point of maximum mechanical advantage on the crank. That’s MBT (maximum brake torque). Everything else is a correction to land CA50 in the right place as the things that govern burn rate change underneath you.
Three levers move MBT, and they interact:
- Cam overlap. More duration/overlap means more residual gas in the charge. A diluted charge burns slower, so it needs the spark earlier. On a turbo engine this is load-dependent: at light load exhaust pressure exceeds intake pressure, so overlap adds residual dilution (MBT → more advance); under boost the fresh charge scavenges residuals out (MBT → less advance).
- Fuel. Octane sets knock resistance; the fuel’s burn rate sets how much timing you need. Ethanol burns in a way that wants ~3–8° more advance to reach MBT, and its charge-cooling makes it very hard to knock — so on E60 the constraint flips from “don’t detonate” to “don’t make so much torque you break parts.”
- Operating point. Across MAP you lose roughly 2° per 30 kPa (peaking near 20 kPa); across RPM you add timing as the piston starts to outrun the flame front. MBT traces a U-shape versus load.
What this looks like in practice
- Idle (cammed): more overlap → diluted charge → needs more advance. Rough guide: pump/stock 10–12°, ethanol/big-cam 19–22°. But on a 264° cam the engine barely responds to timing at idle — so it’s run flat (~16°) and airflow owns RPM, with the ignition PID as a fast ±4° trim only.
- Cruise (40–80 kPa): MBT is typically 30–36° on pump, 34–40° on E60. You find it with an EGT probe (lower EGT = closer to MBT) and confirm with MAP rising at constant pedal. Then you flatten timing into a shallow plateau across the cruise band so small MAP wobbles don’t become torque wobbles.
- Boost: substantially less advance — each ~4 psi wants ~2–3° less on a given fuel. Low-20s°BTDC is typical for a 4-valve inline-six near MBT under boost.
- Flex fuel: two ignition tables (E0 conservative, E100 aggressive) blended by ethanol %. The blend curve is front-loaded (most knock-resistance gain is in the first 50% ethanol). You tune timing by moving the Table 2 values, never by bending the blend curve — the blend just multiplies your delta.
VVT coupling
Cam advance and the ignition table are coupled. Moving the cam changes residual fraction and effective compression, which moves MBT at that cell — so any time you retune cam advance you have to re-verify ignition there. The repo carries the full mechanism breakdown plus theoretical MBT surfaces (238°/264°/272° cams, E0 and E100) to use for shape, not as safe-to-run values.
Notes
- notes/timing.md — full principles, cam/fuel timing tables, theoretical MBT surfaces, flex-fuel blend, cruise plateau method
- notes/vvti.md — how intake cam advance shifts MBT (overlap vs. effective compression)
- supra/notes/timing_targets.md — build-specific E60 targets by boost
- On-site: street-tuning VVT without a dyno, reading & editing the ignition tables