My R-985 came with a Peterson Auto-Fuel STC.

This surprised me a bit and I was wondering if anyone is still actually running Ethanol Free in a 985?

Why wouldn't they?
Low compression so don't need the higher octane,
and don't need all that lead either.
The main problem might be logistics of getting mogas to (usually) a floatplane,
but I understand that a lot of gasoline sold to boaters in marinas is e-zero.

R-985 was certified in 1929, 10 years or so before anything better than 80 octane was readily available. As long as it’s truly ethanol-free it should be no problem.

I know at least one crop duster that has auto gas / 100LL mix. However, because they are low compression they don’t get a free pass, they are supercharged. Is this a well instrumented engine, cht’s on every cylinder? If you can know that it doesn’t run appreciably hotter on mogas than on 100LL, imo, I’d consider using the STC. The type clubs for 985 equipped aircraft and Covington may have knowledge they can share.

Supercharger raises compression from 6:1 all the way to 8:1. Beaver is certified to run on 80/87.

Yes, you're correct, you don't need ALL that lead from 100LL, but you DO need some. Especially if your engine is fresh. I have what's left of a set of exhaust valve seats from one that was running on no-lead for less than 100 hours, fresh from overhaul. It wasn't intentional to be running without lead, but it happened. All 9 jugs had to come off.
I've overhauled over a hundred of those engines, and yes there were some that came in that weren't running leaded fuel, and we could tell almost instantly. The only word I can describe them with are "dry". Straight no-lead isn't a great idea. I think a 50/50 mix is great with 100LL. I wouldn't hesitate to run straight no-lead for a little while, or to get home. You're sure not going to hurt anything there...but I wouldn't ever give it a steady diet of it.
Just my opinion, and worth what you paid.
John

Thanks for the post!! I appreciate it very much.

It's a fresh 985 - approx 90 hours. - I'll consider a 50-50 mix every now and then!

Running a R-680 with soft seats. Peterson told me to run 25%100LL with auto fuel, this is what they did with their spray planes, this is equivalent to 80/87. We did this from break-in to present 300hrs, compression on all 9 mid 70s. No lead fouling.

hardtailjohn wrote:.... you don't need ALL that lead from 100LL, but you DO need some. Especially if your engine is fresh. ...... Straight no-lead isn't a great idea. I think a 50/50 mix is great with 100LL. I wouldn't hesitate to run straight no-lead for a little while, or to get home. You're sure not going to hurt anything there...but I wouldn't ever give it a steady diet of it.....

Petersen recommends a 75% mogas / 25% avgas mix for radials & Franklins.
I generally burn an 80/20 mix in my O470.
From an old Petersen brochure:

nickelb wrote:Thanks for the post!! I appreciate it very much.

It's a fresh 985 - approx 90 hours. - I'll consider a 50-50 mix every now and then!

I'd definitely have 100LL mix in there until you get a few hundred hours on it. If it's been run without so far, I'd suggest taking a look at valve settings asap. You might find them pretty tight.
John