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Old 10-18-2016, 02:33 AM
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Originally Posted by Mr Cox View Post
Well, that doesn't really answer much though.

In previous discussions here the general consensus has been that the color comes from temperature of the combustion. Larger engines (above .15) then tend to have a darker residue compared to smaller engines. A larger prop also means a lower load (and less developed power) and these then have less colouring of the exhaust residue compared to a smaller prop on the same engine.

A 9x4 prop on a .15 engine sounds about right, although one can use a little larger prop than that too (eg. 9x5 or 9x6).
So if this gives a very dark, black, exhaust residue then I would still say that the fuel could be the problem. I mainly use engines below .15 and when these are propped for around 10krpm at full throttle, the exhaust is only lightly coloured. It will get darker on smaller props and higher rpms but not black.

I don't have the MMVS D7 engine myself, was that produced at the same time as the Modela series?
I regularly use .15 size diesels (R250, Parra, Fora, PAW GTS2, MVVS etc) on 7x6 and 7 x5 APC and JfX (Hobbyking) props. They usually run somewhere between 17K-19K. The exhaust is usually honey coloured. The MVVS do run a bit on the black side. Out friend Frank is a bit sparse on descriptions and pics of his actual engine setup. From his posts on other forums it seems that he's flying control line models. Perhaps he'd like to elaborate a little?