Engine Idle Vacuum Estimator

Quick, tunable model that predicts manifold vacuum at hot idle using cam specs at .050", advertised (.006"), LSA, compression ratio, displacement, idle RPM, and altitude. Built for sanity‑checking combos and customer calls.

Engine Displacement (CID)

Cylinders

Compression Ratio

Idle RPM

Altitude (feet)

Units

LSA (°)

ICL ATDC (°) optional

Advance (°) derived

Intake Duration @ .050" (°)

Exhaust Duration @ .050" (°)

Overlap @ .050" (°) computed

Intake Adv Dur @ .006" (°)

Exhaust Adv Dur @ .006" (°)

Seat Overlap (.006) (°) computed

Model notes & equations

Overlap (general): Overlap = max(0, (Dur_int + Dur_exh)/2 − 2×LSA) at the chosen checking height (.050" or .006"). Overlap is not changed by simple cam advance/retard if LSA is fixed.
Intake Closing ABDC (seat): IC_ABDC = ICL + Dur_int(.006)/2 − 180. Earlier closing usually improves idle vacuum.
Sea‑level base vacuum: Starts from ~19.5 inHg at 750 rpm, 9.0:1 CR, 350 CID V8. Adjusted by size, CR, RPM.
Penalties: Vacuum drop scales with both seat and .050" overlap. Aggressive lobes (smaller adv−.050 spreads) slightly help.
Altitude: Scales by P_alt / 29.92 using the standard‑atmosphere baro model. Vacuum readings are relative to ambient, so big altitude reduces the max number any gauge can show.

Predicted Hot Idle

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Idle Quality

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Key Angles

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Tuning knobs this model respects

Displacement per cylinder (more tends to raise idle vacuum all else equal).
Compression ratio (higher increases vacuum modestly).
Overlap at .006 and .050 (both hurt vacuum; .050 overlap hurts ~5–6× more per degree than seat overlap).
Intake closing ABDC (earlier helps, later hurts).
Idle RPM (slightly higher raises vacuum).
Altitude (simply reduces the gauge number available).

This is a practical estimator for tech calls and planning. Real engines vary with ring seal, throttle blade angle, PCV plumbing, cam asymmetry, exhaust reversion, EFI idle control, leakdown, and more.