Just came across this while trying to find again how to upgrade electrical engines.
Reminded me how to do it and made me check all possible combinations (was going to do that anyway) to see the best options.
So, 2 bronze and an iron give the best results?
If you are going purely for a one engine and the most power you can get while still having some efficiency, then yes.
2 Bronze + 1 Iron = 10 MJ for 35 EU : ratio of 1 to 3.5
But, you can get the exact same from this:
1 Bronze + 2 Tin = 10 MJ for 35 EU : ratio of 1 to 3.5
If you could care less about efficiency and are willing to toss all the EU it can take, then the best bet is:
2 Bronze + 1 Tin = 12 MJ for 43 EU : ratio of 1 to 3.583333...
Looking for the best ratio?
1 Iron = 2 MJ for 5 EU : ratio 1 to 2.5
What does this mean?
For one engine, best power is 2 bronze and a tin.
Slight energy savings is 2 bronze and an iron... or 2 tin and a bronze.
But, if you are having more trouble with producing EU and less trouble producing electrical engines... just use one iron and make multiple engines.
Compare:
1 engine, 2 bronze tubes and a tin tube : 12 MJ per 43 EU
or
6 engines, 1 iron tube each : 12 MJ per 30 EU
Personally, I already have 20 engines going... 40 MJ per 120 EU.
If I iron tube them all, 40 MJ per 100 EU... or I can use four engines with either of the combos listed above to get 10 MJ each, making that same 40 MJ per 140 EU.
Considering I am not producing enough EU to cover the 120 EU, why increase the cost?