Meanwhile, gexcon shows propane/air transitioning in about 11 meters in a 4" pipe.
That is an interesting read...
Note that these numbers are from experimental data.
A 4" ID chamber would need to be nearly 36' long in order to propagate a transition from deflagration to detonation (DDT) in a 4.2% propane/air mix at 1 atm. In other words: A simple, yet massively long combustion cannon with metered propane, no chamber fan and one point of ignition at the furthest point from the barrel.
Smaller diameter pipes reach DDT in shorter distances.
At the point of DDT, pressure spikes of 50 times the initial pressure have been measured.
This being said, if you want that huge boost of pressure, make your combustion cannon to the above specs and make sure you make the DDT site of your canon extra strong.
"The run-up distance, i.e. the distance from ignition to transition to detonation in pipes is an experimental value giving some indication of the likelihood of transition to detonation."
Factors influencing location of DDT:
initial pressure: increase this and the run-up distance decreases
initial temp: decrease this and the run-up distance decreases
turbulance: increase this and the run-up distance decreases.
Therefore, what to do so your cannon isn't 1/4 the length of a football field:
Use a chamber fan to create more turbulance.
Do not increase the starting temp of your fuel/air mix. (This is counter-intuitive to me)
Increase the intital pressure. ie. hybrids.
Decrease the diameter of the chamber to reach DDT pressures in a shorter distance ie. 16" ID chamber needs to be 104' long... not a significant factor.
This article does not take into account the use of an oxidizer, which will also significantly decrease the run-up distance.