Max Squish Velocity Calculator

Reviewed By: Patrick Myers (BSME)

Enter the squish height (clearance), engine RPM, stroke length, bore, and squish-band width into the calculator to estimate the peak squish velocity (simplified model).

Max Squish Velocity

Mean Piston Speed

Enter the values below to estimate peak squish velocity. This is a simplified annular squish-band model evaluated at a fixed crank angle (15° BTDC).

Engine Preset (optional) Fuel Type (optional note) Bore (B)

Squish Band Width (W)

Squish Height at TDC (H)

Engine RPM Stroke Length (S)

Estimated Peak Squish Velocity

Max Squish Velocity Formula

The following equation is a simplified estimate of peak squish (radial) velocity using an annular squish-band model. It uses a simple harmonic approximation for piston motion and evaluates the flow at a fixed crank angle of 15° before TDC (BTDC). Real engines can differ due to chamber shape, squish angle, rod ratio, and detailed flow effects.

\begin{aligned} R&=\frac{B}{2},\quad r=R-W \\ A_s&=\pi(R^2-r^2) \\ \omega&=\frac{2\pi\cdot RPM}{60},\quad v_p(\theta)=\frac{S}{2}\,\omega\sin\theta \\ h_{\text{eff}}(\theta)&=H+\frac{S}{2}(1-\cos\theta) \\ V_s(\theta)&\approx\frac{A_s\,v_p(\theta)}{2\pi r\,h_{\text{eff}}(\theta)}\quad(\theta=15^\circ) \end{aligned}

Variables:

V_s 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 used for the estimate (15° in this calculator)

In words: compute piston velocity and an effective clearance at 15° BTDC, compute the annular squish-band area, then estimate radial velocity from (volume flow rate) ÷ (annular flow area at the bowl boundary).

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.

Determine the squish height at TDC (H).
Determine the engine RPM.
Determine the stroke length (S).
Determine the bore (B) and squish band width (W).
Use the equation shown above (with θ = 15° BTDC) to compute the estimated squish velocity.
Check your result with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge (θ = 15° BTDC, as used by the calculator).

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

Estimated Peak Squish Velocity ≈ 24.1 m/s

Follow-Up Calculations

If your max squish velocity is about 18 m/s on an 11.0:1 small-block, the chamber is creating a meaningful pressure rise as the piston approaches TDC. The pressure in bar gives you the pressure side of that same story, which is useful for judging detonation margin and fuel octane needs.

A turbo engine that shows 17-18 m/s of max squish velocity at 10.0:1 static compression behaves very differently once 8 psi of boost is added, because the effective compression ratio jumps into the low 12s. The effective compression under boost helps you see how much harder the chamber is being loaded, especially when you’re deciding how much timing to run.

If your max squish velocity is only about 14 m/s on a street 5.7L, the mixture may need a little more help from the tune to burn cleanly. The right AFR target lets you match that chamber motion with a mixture that makes sense for your fuel, boost, and load range.