I picked up a ST35 and ST 46 at a swap meet for $5 each....yes, you read that correctly....anyway they were both equipped with R/C carbs. I oiled them up and both have tons of compression and were not owned by some ham fisted guy. So where do I start with the venturi sizes ? Both engines will be in planes used to fly the complete pattern. 4 oz tank for the 35 ,5 oz for the 46......Thanks, PhillySkip
I think you are going to want more than 5 ounces for the 46. I would suggest this setup (from the mists of time, that is, 1986):
BOX-STOCK ST G21/46 -do not "fix" head, do not add head gaskets (replace with stock gasket if it begins acting funny as gasket compresses), do not replace bearings unless rusted, do not look for "porkchop" crank, do not replace with trick conrod, etc. "Red Box" version with full muffler lugs preferred (only because they had, usually, good rings).
Standard "long" venturi bored to .173
10 or 15% SIG
K&B R/C plug.
SST Tongue muffler with 16 3/32 holes
Rev-up 12-6 Pro II or Special Pro III, greatly thinned with 5.75" basic pitch, increasing to 6.2 over the last 1" (good overall)
OR
Rev-Up 13-5 cut to 12", greatly thinned, with 4.9 helical (windy weather).
Uniflow with NO PRESSURE I would also suggest getting a stock .157 venturi, and a .165, in addition to the .173. Mine were all stock, then drilled out. DO NOT drill out the case to put a "through the middle" spraybar (why is it that people figure they can improve engines by drilling holes in them). It will run very nicely with the stock venturi and will lose a bit of power, but very smooth and reliable. I would also forget 5% nitro with the .173, the needle was too touchy for larger props, and there's no reason to give up power.
Check the compression forwards and backwards. If it is wildly different depending on which direction you flip it, you can certainly run it, but it won't be putting out much power. Then start looking for someone with a good track record to make you a replacement ring, because you will need it pretty soon. A good ring will have lots of compression in both directions (although not as much as a typical ABC or AAC engine).
DO NOT run straight castor oil, run at least a 50/50 blend of castor and synthetic. It doesn't matter what any so-called engine expert says, if you run straight castor at stunt speeds you will lose compression as the ring glues itself into the groove. It responds very well to more nitro, as much as you can carry (hence you want the 6 ounces+) tank. You usually can leave the compression alone when changing nitro. If you go over the top, you will notice it very quickly, it will start backfiring, quitting, and all sorts of other things.
Mostly, you get a good ring and leave it alone. There are many very silly "modifications" that people want to make, and have made, to the engine. Some were legitimate attempts to find a solution to the one problem the engine has (getting a good ring), but all the rest were the 70's version of people carving up 40FPs with Dremel tools "for lucky best stunt run", i.e. making them even wimpier than stock. The guys who knew what they were doing were trying to pump them up to the gills, because if you got a few percent better power with no added problems, it was a huge advantage. Unlike now, where anything you might want to stick in the nose has so much more capability than you could possibly use that you don't even think about it much.
For the ST35, it depends on which one. I had moderate luck with the one version of the STG21/35, with everything stock including the venturi and an 11-6 Top Flite, but that was a long time ago when I really didn't know any better. Expect a bunch of combat guys to make snide comments about hitching a thoroughbred to a plow.
Brett
Topic: Venturi Size for ST 46 and ST 35 (Read 1500 times)