Are there any major differences between engines when factory equipped with a throttle vs a fixed venturi? Other than the obvious, do they incorporate changes to port timing, compression etc? Bottom line.... Can you just swap a throttle for a venturi and spraybar on an OS LA 40 and be good to go?
Thanks

Excellent, thank you very much.

Oddly enough, one well respected tuner prefers to use the RC version of the ST 51, when modifying it for stunt! You most certainly modify any RC engine to take a venturi and NVA. It will certainly run, but whether it will make a GOOD control line engine for stunt, is another matter.

Andrew.

You can view it this way: when you use an engine that's purpose-designed for CL, you're getting one that's best for CL in the opinion of the manufacturer.  Said manufacturer may have different opinions than you, time has marched on with respect to what engine runs work best,

 Absolutely. Most of them, Fox in particular, but ST as well, seem to be stuck in the days of 4-2 break engines, so they make "CL Stunt" engines that are intended to chug around at relatively low revs. The Stalker 40RE was the best example I saw, 3-4 different engines all were completely incapable of keeping up with a Fox, and wouldn't fly a Nobler with any sort of authority at all. 5.4 second laps, gutted out lean, with a 10-6. A Fox could get that below 5 sec/lap, easily, and flies the airplane with relative authority in a 4-stroke, even on 5% nitro. 100,000 guys in the 60's couldn't all be wrong.

  Moreover, they don't actually listen when they solicit input.

 The best stunt engines I have are set up like RC pattern or racing engines - 20FP, 40VF, RO-Jett 61 "Brett" version. The latter has the same crank timing as the Jett QM40 engine. They started with a narrower duration "stunt" version, but didn't want to make a special type, so they tried the QM40 crank, and it worked better anyway. Same with the ports, the 144 degree exhaust works better than the 136 degree exhaust duration. Same with the VF, and the best VF I had was also the one with the biggest exhaust duration and the biggest blowdown. The 25LA is a pretty darn good stunt motor, and it is literally identical to the RC version aside from the venturi.

   Compare this to the Head Gasket Patrol type of modifiers, they are doing the same thing as the major manufacturers in many cases to an even greater degree, detuning the engines until your 46LA runs weaker than a Fox 35. It's almost as if there is a race to get the weakest and slowest engine possible. No one has need that for close to 25 years now.

  They key seems to be that outside of a very few custom suppliers, people have absolutely no idea what is needed to make a modern stunt motor run properly.

    Brett

 

While some folks like OS NV Assy's, there are many more brands available in .156" dia., so if you have a problem at the field or a contest, I think it makes sense to go with a brand that's .156". Randy Smith's is easily the best, in the opinion of most, but ST, ST Clones, K&B, Brodak, 2Star and others will work. Not as well, but you might save a dollar or two. Further, OS NV Assy's can be damaged (crashes), to where they look pretty ok, but don't work worth a darn. I'd much rather have it look broken when it's broken. The OS'ns are pretty 'spensive, too...about the price of Randy's.  Steve