It seems no one wants to take this on so I'll have a try .

Although I've never seen the 25VF I have several 40VF's, a 46VF and a 61VF. The 40/46 have very mild port timings, virtually identical to the CL version of the ST G51. The 61VF has somewhat more radical exhaust timing much the same as most RC sport engines. I would assume the 25VF also has quite mild port timings suited to lowish revs. Ducted fan engines are designed to run in the 25,000 rev area so have to be ported to get max HP around those revs. This is very similar to the revs most RC car engines develop max HP (and they're about the same size as the 25VF). Car engines have an exhaust timing much higher than sport RC engines so I'd suspect the main difference in the 25VF-DF is the exhaust timing.

As a guide, the 40/46 exhaust opens at 70 BBDC, the 61VF at 75 and my son's car engine at 83 BBDC.

Ralph,
In addition to the comments I posted on the other forum about the DF engines' rpm range and load requirements, the DF engines "usually" have a rather short shaft to accomodate the impellers, rather than a prop.

I don't know the shaft length of the particular DF engine you are asking about.........

Jim

Can someone tell me the differences between the OS .25VF and the .25VF-DF? Both are ABC, FIRE engines, and I know that the "DF" stands for "Ducted Fan", but what does that mean, performance-wise?

Thanks in advance . . .

Hi Ralph

On the 25 VF  and VF-DF the only difference was the timing and compression. Unlike the 46 VF which was completely differant. The 46 had a differant piston sleeve, head, crankshaft, front housing, carb, rear plate, bearings, and the DF was a rear intake rear ex. motor.

I setup and completely developed the 25VF many years ago for a T.P. system. we ran them on several airplane with very good results. I also sold about 12 to 15 of them to customers around the globe, and Werwage may still have one of mine in his Perroquet.
I still have headers and several other parts for them.

Regards
Randy

Thanks Randy, Jim and Brett.