For those not aware, the Burnt Latke webpage is the standard by which combustion spud gun simulation methodologies are tested. This webpage documents one man's investigations into combustion spud guns with a particular emphasis on the effects that barrel length has on performance.
My first attempts at this comparison were rather dismal, but then it dawned on me that frictional forces were likely a major player in this particular scenario. Tennis balls are physically larger than the stated barrel (2.5" sch 40 PVC). Further, when pressure is exerted on a tennis ball it expands laterally (which would wedge a ball even tighter into a barrel). Evidence of friction as a major player does appear on the Latke website: the "fuzz" on the tennis ball is burned by frictional forces where ever it touches the barrel!
Thus, I admittedly played around with frictional forces until the simulated muzzle velocity for the longest barrel (144") very nearly matched test data. At that point I "locked in" frictional forces and worked my way backwards accepting whatever curve I arrived at.
I'm pretty damned pleased with the result.
HGDT file for above scenario
Further note: Latke mentions firing a 1.5" wooden slug (same as used in the L1 tests) out of the L3 with a 1.1:1 C:B ratio. He chronoed it at 616 fps. On the first "try" HGDT simulated that scenario at 610 fps.
Note: If you came here via a direct link from somewhere else, odds are you'll want to click here.