The bung is so that the air-fuel ratio can be monitored by using a wide band oxygen sensor. The wide band oxygen sensor will be installed in the threaded bung that they weld in the exhaust system. After the dyno, they will install a threaded plug in the bung.
Dynojet has a probe that can be inserted in the end of the exhaust that measures the air-fuel ratio. There are different opinions on the accuracy of measuring at the end of the exhaust pipe. If the car has convertors, then the tail pipe probe would be downstream of the convertors where the welded-in bung would be upstream.
Considering the installation of the bung, that price sounds reasonable. Typical rates I have seem for dyno's is $50-$75 for two pulls without air-fuel monitoring.
It's hard to say whether or not a single trip to the dyno is sufficient for a baseline. Dyno numbers can change depending on the ambient conditions. The software calculates a correction factor, but other issues can change the numbers.
Whatever you end up doing, try to do the same thing everytime you dyno so that you can get the best comparisons. Some variables to try to watch may be same tire pressure, same tension on the tie-down straps, similar engine coolant & oil temperatures, similar transmission and rear end temperatures, etc...
Dynojet has a probe that can be inserted in the end of the exhaust that measures the air-fuel ratio. There are different opinions on the accuracy of measuring at the end of the exhaust pipe. If the car has convertors, then the tail pipe probe would be downstream of the convertors where the welded-in bung would be upstream.
Considering the installation of the bung, that price sounds reasonable. Typical rates I have seem for dyno's is $50-$75 for two pulls without air-fuel monitoring.
It's hard to say whether or not a single trip to the dyno is sufficient for a baseline. Dyno numbers can change depending on the ambient conditions. The software calculates a correction factor, but other issues can change the numbers.
Whatever you end up doing, try to do the same thing everytime you dyno so that you can get the best comparisons. Some variables to try to watch may be same tire pressure, same tension on the tie-down straps, similar engine coolant & oil temperatures, similar transmission and rear end temperatures, etc...
