Video summary

Dual Plenum Design: Air Speed and Taper Ratio Rules for 4 and 6 Cylinder Engines

Main summary

Key takeaways

Technology

Technological concepts & design rules (dual plenum)

The video focuses on dual plenum design for multi-cylinder engines, specifically how to set:

  • Plenum taper/ratios
  • Air-speed targets

…for a layout described as a pre-chamber + secondary chamber style arrangement (with common usage examples attributed to Cosworth and Lemond).

Primary design goal: Maintain a stable velocity gradient and improve cylinder-to-cylinder air distribution, which helps reduce EGT spread (more equal exhaust gas temperatures).

Plenum geometry approach

Key geometric choices include:

  • Using a second plenum with a parallel roof
    • This does not taper like a typical “single plenum top” shape.
  • Adding a transitional slot (transition/feeding slot) that connects the first plenum to the secondary plenum
    • The slot is intended to stabilize flow and improve distribution.

Airspeed targets and ratio rules

Baseline airspeed target

  • Throttle body target airspeed: 150 ft/s at peak (used as the baseline).

Transition slot sizing

  • Make the transition slot 100% bigger
    • This effectively halves feeding velocity to about 75 ft/s into the secondary plenum.

Taper computed by area/ratio (not angle)

The speaker emphasizes that taper should be computed using area ratios rather than taper angles.

  • The argument: angles (e.g., 5–7 degrees) are inferior because the governing factor is air speed.
  • Therefore, taper is determined by making the back area match the required fraction relative to the throttle-body area.

Cylinder-based rules

  • 4-cylinder rule:
    • Taper so the back area = 1/4 of throttle-body area
  • 6-cylinder rule:
    • Taper so the back area = 1/6 of throttle-body area

Reasoning provided: the flow is divided among cylinders progressively (described as 4 → 3 → 2 → 1 cylinder feed states along the taper).

CFD guidance

  • Put the design into CFD to get close immediately, then fine-tune based on results.

Example using component area

If the throttle body area is 4,500 mm², then the transition slot area should be approximately:

  • 9,000 mm² (about double)

Practical benefits / operational reasoning

  • The goal is to keep cylinders equidistant to feeding points, improving uniform air charge.
  • Better uniform charge supports EGT consistency.

Additional flow/exhaust dynamics notes:

  • As RPM increases, overlap time increases (linked to higher engine frequency), which affects distribution.
  • The speaker connects higher RPM needs to larger “pinches” in merge collectors, while keeping the plenum taper rules as the foundation.

Additional guidance notes

  • The same taper logic can be applied even if the plenum design steps mention a “D-cell area” used in shotgun manifolds.
  • Throttle body placement: In a dual plenum design, the throttle body/throttle plate can be closer than in a single-top-plenum style, because the transitional slot helps stabilize flow.

Quick recipe recap (as stated)

  • Target throttle body airspeed: 150 ft/s
  • Feed into secondary plenum via slot: 75 ft/s
    • Slot area ≈ 2×
  • Taper ratio:
    • 1/4 for 4-cylinder
    • 1/6 for 6-cylinder

Reviews / guides / tutorials emphasized

This is presented as a quick rule-based guide for designing dual plenums efficiently:

  • Use airspeed targets
  • Use area ratios (e.g., 1/4, 1/6) rather than taper angles
  • Validate via CFD, then fine-tune

Main speakers / sources

  • Main speaker: an unnamed technical presenter (referencing their own experience and prior discussions).
  • Referenced examples/companies/designers: Cosworth, Lemond.

Original video