RE: norvel engines
If you remove the glowhead from your Big Mig 061's, and look down into the cylinder, with the piston at bottom dead centre you will be able to see five transfer ports feeding the cylinder-plus the single exhaust port on the right hand side. As you look from above the five will appear in an arc from the front to the rear on the left side of the cylinder. The AME version has only three transfers-two large ones at the front and rear of the cylinder, and a smaller one directly opposite the exhaust. The Big Mig has all 5 transfers the same size. The speed difference on the same prop ranges from about 1000 rpm on larger props to 2000 rpm on smaller props-with the AME version being the fastest. There used to be a section on the Norvel website which not only explained the differences in detail but also gave an rpm table for different fuels and prop for both versions. If the original plan was for two 020s then you will have no trouble with two 061s-other than perhaps TOO MUCH power-the 061s produce about 4 times the power of a TD 020. Be aware that ducted fans require a very close fit of the fan to the duct to operate efficiently-and your workmanship and alignment within the ducts will have to be spot on.
For ducted fan use, it would have been better if you had purchased the higher revving AME versions, as on larger ducted fans, the engines are ported to run a lot faster with fans, than with props-typically around 23,000-25,000 rpm even with 90 sized engines. They (the larger DF units) tend to be hard on plugs, and like plenty of nitro as well.
'ffkiwi'