Let me try to do some math/estimation...
the question would be what to measure and then use that in a calculation model.
I will try to do a first calculation , based on engine vacuum, that we all can verify/measure/use in a convenient way.
we take the vacuum between throttle valve and the engine as the value needed to calculate the degree of "filling" the engine with useable gas ( engine suction efficiency )
Since full vacuum does not exist, i assume the value at total closed throttle valve, to be 0,9 bar and the wide open throttle at 0,0 vacuum.
Further i take the max RPM , under load, at 3000 RPM WOT ( wide open throttle )
At WOT, the engine gets a max Efficiency of 0,7 (standard assumption ) and i will keep that value overall the RPM even as it is lower at higher rpm ( depending the engine )
The idlle rpm 1000 RPM
As engine we take the dakota 318 CI, equals in metric 5,2 liters
To calculate the turn down ratio, i don't convert it into gas/air mixture because the ratio will be the same.
now at idlle rpm the engine will get 5,2 Liters / 2 * 1000 Rpm *0,7 efficiency * 0,1 ( rest from the 0,9 vacuum )
= 182 liters
At 3000 rpm WOT, the engine will get 5,2 Liters / 2 * 3000 Rpm * 0,7 efficiency * 1 ( from 0,0 vacuum)
= 5460 liters
Turn down ratio then would be 5460 to 182 or 30 to 1
Now, the above is based on assumptions and need to tested with real values, but if you where able to run your engine WOT on 3000 Rpm in perfect AFR then you would have some real power
But most likely the engine would run 3000 Rpm with max half throttle and then gets to lean.
All depending the builds from the gasifiers
If i get some data from the others, i can make a calculation model, that we all could use for our needs.