Maximum Squish Velocity
The higher your engine revs the faster you want the combustion to happen and the higher the squish velocity, the faster the burn* except that too much velocity actually slows down the combustion time by overtly dispersing the flame kernel. .
Blair recommended a max of 20m/sec at peak power RPM which to him was a safe high which wouldn’t contribute to detonation.
But there are many people who have used much higher with no detonation and so it’s obvious that detonation usually don’t happen just from a high velocity, it has to be a combination of things;
- lean jetting (people jet the main correctly but not the needle and since they are mid throttle often then that can cause detonation if the needle is lean. see www.dragonfly75.com/moto/carbtuning.html)
- ignition timing too advanced (typical normal is 10 degrees BTDC @ end of pipe powerband but be aware of lower RPM timing)
- too hot an engine (contributed to by summer heat, air cooling, insufficient fin area, too hot a spark plug)
- sharp corners at the inner edge of the squish band that can get too hot
Also you can have the same velocity from a wide squish band with more squish clearance as with a narrow squish band with small squish clearance. But the second one will be more prone to detonation because more head and piston area is exposed to the main combustion area. So I will add this 5th cause of detonation:
- too narrow a squish band
In a research paper investigating different squish velocities it was found that from 20m/sec to 29m/sec at peak power RPM is the range of safe velocities that produce the best power. Above 29m/sec produces too high a rate of combustion pressure rise (over 30psi per crank degree) which can cause knocking and power loss.
I have heard from people of their squish velocities as high as 48m/sec with no perceived detonation but according to this paper they were experiencing power loss as a result of too high a squish velocity. At this page I’ve put the essential info from that research paper: dragonfly75.com/moto/SquishVelocity.html
- Blair had wrote: “Squish velocity has a very pronounced effect on the rate of burning and heat release in two-stroke engines. High squish velocities lead to rapid burning characteristics and that rapid burning approaches the thermodynamic ideal of constant volume combustion. There is a price to be paid for this, evidenced by more rapid rates of pressure rise which can lead to an engine with more vibration and noise emanating from the combustion process. Further, if the burning is too rapid, too early, this can lead to high rates of NOx formation and slow and inefficient burning in the latter stages of combustion. “