How far back did you folks move the firewall ? With the firewall moved back is there sufficient clearance for the OS 91 FX std muffler ? Would mounting the engine sideways help with the muffler clearance ( also makes for nicer side profile ) ? I notice that BTE website now offers a cowl for the Vortex, I wonder whether this would fit if I moved the firewall back . TKs

I would side mount the engine as the large surface area of the top of the wing will catch a lot of exhaust reside from an up right engine[:'(]... I had a 91 FX on mine and it was fine, but a strong 61 would also work... There is enough ground clearance of props much larger than 13"... My BTE DV ARF weighed right at 8 lbs sans fuel...

Here's the URL for a site dedicated to the Delta Vortex. You can probably ask them all these questions.
[link=http://www.deltavortex.com/]Delta Vortex[/link]

Bruce also answers questions.

I mounted my O.S. 91 FX in side mounting. I trimmed the leadind edge of the right wing a bit to fit the cylinder head. I did not move the engine mount. If you do, the CG will be way off. I had to put a few ounces of lead in the trailing edge to get the CG correct. My DV had many hundreds of flights and was a wonderful plane. I hope you enjoy it.

www.cardiacengineering/radiocontrol has a few pictures.