Is it possible that the lack of an exhaust pipe cut down on the scavenging effect of the exhaust gases? Valve overlap in engines allows cylinders to help each other get those exhaust gases out. The exhaust system diameter, length of tubing (longer tubing provides a kind of hallway for those gases to get running to provide momentum to the gas flow to keep sucking the fumes out) is all engineered to produce an amount of torque at low rpm to help with low end acceleration, since most of us don't use these vehicles for racing and they are heavy. That is why that low end torque is so important, and that is one of the advantages of a torque convertor-it allows the engine to spin up into a sweet spot of producing some torque for acceleration, kind of like having a granny gear. So yeah, maybe a tail pipe would help.