Squish Velocity Calculator


This squish velocity calculator is an Excel spreadsheet based on Gordon Blairs formulas. It shows the velocity from 30 to 0 degrees BTDC. The peak velocity will usually be at 10 or 15 degrees. All linear dimensions are in millimeters. An inch to millimeter converter is at the bottom left of the page. All entry boxes are in light blue. Just click onto the entry box, type in the correct value, and then hit the Enter key. Don’t enter any values in the white boxes. Enter the cylinder bore (diameter) at B1 and the piston stroke at B2. Go to the hot rods site if you need to look up the length (center to center of its holes) of your connecting rod for its entry at B3. If you've seen a horsepower/torque graph for your engine then enter at B4 the RPM that's 1000 more than the peak power RPM. Otherwise enter the RPM that's the max RPM (at the end of the powerband). The engine compression ratio at B5 is “trapped” which means it is the ratio of engine volume above the top of the exhaust port to the volume when the piston is at TDC. Next item down at B6 is the exhaust port height from the top of the exhaust port to the piston top edge at TDC. (example: there is 51mm from top of cylinder to the top edge of exhaust port. The piston at TDC is .8mm below the top of the cylinder. Port height would then be 51-.8 which equals 50.2mm.) At D2 you will enter the value of the cold squish band clearance (from piston to squish band at TDC) at the outskirt of the bore, and at D3 the cold clearance at the inner part of the squish band. These values reduce when the engine is hot and is shown at F2 and F3. That heat expansion amount (at F4) is based on the expansion factors for steel (9.6) and aluminum (13) and the rod length and bore diameter (because the pistons pin to top distance is usually around half that value). My 55cc engine has a cold 18m/s squish velocity which is 22.3m/s when hot which is a significant change. But still everyone refers to the cold squish velocities so this calculator shows the cold values. The value of squish band “inner diameter” is to be entered at D4. That is the juncture of the squish band and the head dome. Measure it right before it starts to curve upwards towards the dome. The squish ratio at D5 is the ratio of the squish area to the cylinder bore. The value of .4 means the squish surface area is 40% of the bore area. This is determined from the squish inner diameter value listed above it. The column starting at D26 shows the squish velocities at 1000 RPM less than the top RPM which approximates the peak power RPM. The column starting at E26 shows the squish velocities at top RPM. Those columns are both the "cold" values.

Gordon Blair recommended 15-20m/s squish velocity at the peak power RPM. (I think that was “cold” because there's never been a calculator showing the hot values). To be able to stick to his recommendations this calculator figures the velocity both for top RPM and for 1000 less (for peak power RPM). As a test case we can look at the '89 Honda CR500 which had a detonation problem from too high a squish velocity. I was able to gather all the details but I had to assume a 2 degree squish angle. At 1000 RPM less than its peak 7700 its cold velocity with a 2 degree angle was 22.1m/s and with Eric Gorrs fix of lessening the squish band from 18mm to 15mm the velocity became 18.1m/s. Since 2 degrees squish angle is the normal max I'm going to agree that Blairs 20m/s is the max before detonation. Click here to read up on this topic.

Here's a report from a user of my squish velocity calculator:
Chris H wrote: "The spreadsheet was very straight forward to use and understand. The main reason I needed it was to check what MSV we are currently running, as the engine is suffering from extreme detonation, even after retarding the ignition timing and richening up the jetting. This lead me to believe the MSV we are running may be to high. Your calculator told me the MSV was indeed around 36m/s hence we are in the process of reducing the squish band width and increasing the squish angle."

Click here for on-line usage instructions. Click here to see my video on the subject. You can email me at 19jaguar75 @ gmail.com with any questions or just to chat.

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