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Discussion Starter · #1 ·
Squish Velocity
It’s purpose is to spread the combustion flame faster so that peak pressure happens close to 12.5 degrees ATDC. The higher the max RPM, the quicker that needs to happen. Increased squish turbulence increases the burning rate and causes the needed optimal spark timing to retard as the degree of turbulence increases. Gordon Blair recommended 15-20m/s cold squish velocity at the peak power RPM (but he didn't say for what RPM). (I think that was “cold” because there's never been a calculator showing the hot values). 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 7% less than its peak 7700 RPM (7200) its cold velocity with a 2 degree angle was 23.6m/s and with Eric Gorrs fix of lessening the squish band from 18mm to 15mm the velocity became 19.3m/s. Since 2 degrees squish angle is the normal max I'm going to agree that Blairs 20m/s is the max before detonation for a big engine. Click here to read up on this topic.

General Max Velocity Guide
Since Blair said that velocity should be a max of 20m/sec (most likely for big engines), and the research paper "Squish Velocity in the Combustion Chamber of a 2-Stroke Cycle Engine" showed the 15.2m.s velocity @ 6000RPM (18.7 @ 7200) to give the best power, and the sample CR500 needed 19.3m/s at 7200, I decided to make a linear graph utilizing 19m/sec for 7200 just as a general guide, keeping in mind that compression and carb size also are factors contributing to combustion speed. This may be a good starting point when setting up an engine. Keep in mind though that different calculators vary in results so it's best to use those that agree with Blair’s calculations when going by this graph. Here’s more info on this topic. Here's the resultant graph:
 

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Discussion Starter · #2 ·
Here’s an interesting display showing the varying squish velocity in relation to the combustion pressure graph. After TDC the direction reverses but I doubt it causes a reverse swirl* which means the main swirl happens the last 10 degrees before TDC.
* (Air/fuel squished out as the piston rises has direction and inertia which keeps it from greatly dispersing after it clears the squish zone. But the suction caused by the descending piston readily pulls in air/fuel from every direction which greatly minimizes any potential reverse swirl. So the 2nd half of that velocity chart isn’t realistic.)
 

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