Squish Velocity Calculator results show maximum squish velocity and the peak crank angle, calculated from bore, stroke, rod length, squish clearance, and the engine’s own peak RPM.
The Squish Velocity Calculator Sweeps the Full Approach to TDC
The Squish Velocity Calculator doesn’t just compute one number at TDC; it sweeps crank angle from 90 degrees out to top dead center to find exactly where and how fast the squish band jet peaks. Two-stroke and high-squish four-stroke builders use it to set ignition timing and judge detonation risk from chamber geometry alone.
Why the Peak Isn’t Found at TDC
Piston speed drops to zero at TDC, and so does the squish gap, so their ratio actually peaks somewhere before top dead center rather than at it. Gordon Blair’s combustion research documents this squish-velocity relationship as piston speed divided by the instantaneous gap, scaled by a geometric factor tied to how much of the bore area sits under the squish band.
$$v_{squish} = \frac{r_c^2 – r_i^2}{2r_i} \times \frac{v_{piston}}{gap}$$
With this calculator’s own default geometry, a 3.5 in. bore and stroke with a 0.040 in. cold clearance and a 35 percent squish area peaks at 34.9 m/s around 10 to 11 degrees before TDC, not at zero degrees. Entering squish band width in millimeters or inches where the tool actually wants an area percentage is an easy way to get a nonsensical result, since the two describe the geometry very differently.
Gordon Blair’s own published target is 15-20 m/s of cold squish velocity at peak power RPM, and a 1973 SAE paper studying four different two-stroke combustion chambers (SAE 730186) found velocities in the 15-19 m/s range at their respective RPMs gave the best power, broadly consistent with Blair’s number despite coming from separate combustion chamber tests.
This calculator’s velocity is a cold, static figure. At real operating RPM, crank flex and connecting rod stretch can lift the piston measurably closer to the head, tightening the actual hot gap below what the cold measurement assumed, which is exactly why Blair’s own recommendation is specified as a cold value in the first place.
How Much Bore Area the Squish Band Should Cover
Squish area percentage isn’t the same as squish band width, and the two don’t scale the way intuition suggests: 50 percent of a bore’s area is a noticeably narrower ring than half its radius, since area grows with the square of radius. Design writeups on this exact point put narrow, high-RPM race chambers around 15 percent of bore area, wider torque-oriented chambers at 45 to 55 percent, and roughly 50 percent as the most commonly used figure historically.
A separate, frequently repeated rule of thumb keeps cold squish clearance itself between about 1 and 1.8 percent of stroke for race applications, tighter than that risking piston-to-head contact as components heat and expand.
Applying motorcycle- or kart-derived velocity thresholds unchanged to a very different bore-to-stroke ratio, such as a chainsaw’s notably over-square design, is a documented mismatch; builders working those combustion chambers have reported the productive upper limit for squish velocity running higher than the roughly 30 m/s figure that’s standard guidance for motorcycle and sled engines.
A Few Ways the Inputs Get Mixed Up
Reading squish clearance off a warm, assembled engine instead of measuring it cold on the bench feeds this calculator a hot number where it expects the cold baseline Blair’s targets are built around.
Entering redline instead of the engine’s actual peak-power RPM shifts the result away from the RPM Blair’s target range was meant to be checked against.
Guessing squish area percentage from a rough look at the piston crown, rather than actually measuring bore diameter and the machined squish step, tends to understate how narrow a real high-percentage squish band actually is.
Common Questions About Squish Velocity
What is squish velocity?
It’s the speed at which the fuel-air charge is forced out of the narrow squish clearance toward the center of the chamber as the piston nears TDC, expressed in meters per second and often shortened to MSV.
What squish velocity does Gordon Blair recommend?
15 to 20 m/s, measured cold, at the engine’s actual peak power RPM rather than redline.
What squish velocity is too high?
Research on two-stroke combustion chambers associates roughly 20 to 29 m/s with strong power, while velocities climbing well past that raise pressure-rise rate enough to risk knock and, per that same research, can start reducing power rather than adding it.
How wide should the squish band be?
Narrow, high-RPM designs commonly run around 15 percent of bore area, wider torque-focused chambers run 45 to 55 percent, and roughly 50 percent has historically been the most common figure.
Does squish velocity change once the engine is hot and running?
Yes. Crank and rod flex at speed can close the gap below its cold measured value, which is part of why cold clearance is specified as the baseline rather than a running measurement.
What’s a safe squish clearance?
Roughly 1 to 1.8 percent of stroke is a commonly cited range for race applications, with less risking contact as the engine heats up.