Estimate the peak of an ideal squish-flow model from bore, band width, clearance, RPM and stroke, with the 15° estimate and mean piston speed for comparison.

Ideal flat annular band and harmonic piston motion. Real-engine peak flow and safe clearance require engine-specific analysis.

Radial width; smaller than half the bore.

Illustrative engine examples

Max Squish Velocity Formula

The following equation is a simplified estimate of radial squish velocity using a flat annular squish-band model, constant density and harmonic piston motion. The calculator maximizes this ideal model and also reports its value at 15° before TDC (BTDC). This is a geometric displacement-flow estimate, not a validated real-engine peak prediction. Real engines can differ due to chamber shape, squish angle, rod ratio, and detailed flow effects.

R& = (B) / (2), r = R - W \ Aₛ& = π(R² - r²) \ ω& = (2π × RPM) / (60), vₚ(θ) = (S) / (2) ωsinθ \ heff(θ)& = H + (S) / (2)(1 - cosθ) \ Vₛ(θ)& ≈ (Aₛ vₚ(θ)) / (2π r heff(θ)) (cosθₘₐₓ = (S / 2) / (H + S / 2))

Variables:

  • Vs is the estimated squish (radial) velocity (m/s)
  • H is the squish height (minimum clearance at TDC)
  • RPM is the engine speed (revolutions per minute)
  • S is the stroke length
  • B is the bore
  • W is the squish band width (radial width of the squish band)
  • θ is the crank angle before TDC; the model peak angle is calculated, and 15° is retained as a comparison

In words: compute piston velocity and effective clearance, then divide displaced volume flow by the annular flow area at the bowl boundary. The ideal maximum occurs where cos θ = (S/2)/(H + S/2). This omits compression, bowl volume, rod ratio and detailed flow; it does not establish a safe tuning limit.

What is Max Squish Velocity?

Max squish velocity refers to the peak speed of the air-fuel mixture as it is forced out of the squish region and into the main combustion chamber near top dead center (TDC). This “squish” motion increases turbulence and mixing, which can improve combustion. The actual peak squish velocity depends strongly on combustion-chamber and squish-band geometry (not just RPM and stroke), so simplified calculations should be treated as estimates.

How to Calculate Max Squish Velocity?

The following steps outline how to estimate peak squish velocity using the simplified model above.

  1. Determine the squish height at TDC (H).
  2. Determine the engine RPM.
  3. Determine the stroke length (S).
  4. Determine the bore (B) and squish band width (W).
  5. Use the equation above at its calculated peak angle; compare the result at 15° BTDC if needed.
  6. Check your result with the calculator above.

Example Problem : 

Use the following variables to compare the model peak with its 15° BTDC estimate.

Bore (B) = 0.066 m

Squish Band Width (W) = 0.006 m

Squish Height (H) = 0.0012 m

Engine RPM = 9000

Stroke Length (S) = 0.072 m

Ideal model peak ≈ 24.1344 m/s at 14.5926° BTDC; the 15° estimate is ≈ 24.1250 m/s.