As Dan pointed out, the efficiency is the same for a specific amount of gas and air. Where the forced air burner becomes more efficient is at the extremes. You can not run a venturi burner at a low pressure, or the venturi will collapse.At lower pressures ( ie. lower temperatures), the burner is very inefficient or won't work at all. The same goes for the high end. You can only force so much gas through the orifice, and the atmospheric pressure is a constant factor. A blown burner allows you to deliver any amount of gas you need ( no orifice) and the air supply is variable to match the gas delivery. Adjusting the chamber atmosphere on a venturi burner is difficult, whereas the adjustment on a blown unit is as simple as turning a dial. So a blown burner is far more versatile and more efficient in use.
Adding a PID control will make a forge run in the manner that produces the best results. It greatly reduces overheating,allows a balanced atmosphere at all temps, allows for longer soak times, and provides reproducible results.
I would suggest enhancing the forge first. A HT oven is a great thing ( and necessary for stainless HT), but without a good forge, it may not matter how good the HT oven is.
Another plus is that any upgrading of the forge is transferable. If you build a new forge, the same blower and PID control can be re-used. Many folks forget that there is no need to have multiple PID controllers ( unless you run multiple devices at the same time). All you need is a thermocouple for each device ( with a TC plug on the wires).
The cost to build a blown burner arm for your forge and add a PID control should be less than $150. You may even want to build an entire new forge, for only another $100-150. A 24", two burner forge will do a lot of work . Keep your venturi forge for hammer-ins and demos.
Stacy
