What's a 12 mm Nelson?  Do you mean 12 mm venturi?

I was running a Nelson on the bench and got it over 30,000 on a 4 mm venturi.  I went bigger on the venturi and barely moved the tach.  I keep the diameter under 6 inches to guard against supersonic tip speed and use the pitch that gets the revs.  This is for an event with 10% nitro.

In F2D they mandated a 4 mm venturi in an effort to slow down. All they proved was that the wide open venturis weren't helping.

Motorman

I've found the Nelson and the GP Rossi's are similar in that to get the power necessary to run in the 14's they really need to rev. From experience I can say that you will want to run 29k on the ground mininum. If you have a strong engine, you should be able to turn around a 6.25" x 4.6" at that rpm. That will get you close. I would start with a 6.25" x 4.75" and go from there.
 If your not getting at least 29k then you can drop the diameter down to 6" if need be and and the pitch down to 4.5" and not lose any airspeed (provided you have a clean model).
You don't really need a large disc area on an AMA Goodyear model. My best prop for my Gillott Rossi was a carbon Gillott Nitro Goodyear prop cut down to 5.75"x5". It gave a consistent 14:99 airspeed, with amazing acceleration off the start.

The thing to remember is that these engines hate to be lugged. They are timed to make peak HP at 30+K which is about what they should turn in the air, so try and run them at that.

Les

I didn't know there was a selection of crankshafts.  As of this year Mr. Nelson is still working on engines but not making new ones.

MM

The Nova Rossi is a heavy engine, physically big too. The 3 port version is the one you want if wish to go down that road. Henry should be able to fit a piston to your liner provided it is still round. He might be able to update with an AAC P/L set if he has any left.
In either case, have him set up the P/L fit tight enough for restarts. Most of his customers for the .15 engines have been the Free Flight types of late and he has fit the AAC P/L's so they run fast with little break in. Mine was good for one contest, it's too loose now!

Les

MM

A 1oz. tank won't get you very far. You need at least 2.25 oz. and I would go with 2.5 if you have the space.

At last years Nat's, I ran my "Stinger" 1oz. Goodyear model with Moki 15 and a carbon prop in the AMA Scale Race event. I had to detune with an APC prop so I could get 35 laps on 60' lines to one stop the heat. Strangely enough, I was competitive with the other entries. I guess that goes to show how much the performance level of AMA Scale Race has slid the past couple of years.