Radiator NACA Duct

[P Johnson]

I'm fitting the cowling around my Rotax 912is engine, the firewall forward plans call out for the NACA cut out to be 9 inches long, I remember someone saying that they had cooling problems on the ground with the 912is engine and was advised to open up the NACA cut to 11 inches which solved the problem.
Has anyone else had cooling problems with the 912is as I would rather make this enlarged cut now?

[Kitfox Pilot]

The IS engines run really lean on the ground at low rpms and this is where the cooling problem comes from normally. I would open mine up to 11 inches for any IS engine if I was working on the cowl.

[Geek]

When I cut mine I was lucky enough to talk to Brandon on something else and mentioned what I was doing. He had me cut mine to 11 inches as well. Haven't flown it yet so can't tell you if that's for better or worse but it was Brandon who told me. That's pretty much gold in my book

Gary

[rocketman2tm]

I would cut the duct back 11" as well. I think mine is probably at 10" and it does pretty good.

Something to keep in mind when you're flying. The water temp may still get hot on the ground. Especially if you're sitting there for long periods. For example, Oshkosh departure on one of the busy departure days. Before I started taxing, I made sure to turn the heat on to help keep the water temp under control. This work great. I think I saw it get up to 185-190 and not go any higher than that.

[P Johnson]

Gentlemen, thank you of the replies I will open the duct up to 11 inches.