One thing I forgot to mention. I recently acquired a venturi and front NVA from Lee Machine for my EVO 36. The engine is now running great, better than it ever has.
Here are some more comparison numbers on the venturi effective throat areas after accounting for the spray bar restriction.
EVO stock shortest venturi: 0.0486 sqin effective area
EVO stock medium height venturi: 0.0325 sqin effective area
EVO stock tallest venturi: 0.0325 sqin effective area
Lee Machine 0.280" venturi w/ 4mm diameter spray bar: 0.0183 sqin effective area
After looking at these numbers, it just seems to me the EVO 36 stock venturis are all just way too big for a good engine run.
The choke area is the critical parameter. The stock areas are all too large *
for conventional stunt fuel* and Jim's, not surprisingly, is right in the ballpark, on the low end of the range, but plenty forgiving.
Note well the italicized part. When the Evo 35 came out, absolutely everyone decided, for some good reasons, that the venturis were "too big". That appears to be the case for conventional stunt fuel. The solution was exactly a you have done, get a conventional venturi or a smaller one, and that works.
However, if you have read the instructions, they actually recommend Cool Power. The chief characteristic of Cool Power is that it has extremely low oil content and mostly or all synthetic oil. It's far less viscous than, say Powermaster GMA. So it's easier to pull through the fuel system and needle, meaning it also *requires less fuel suction*, meaning your "too big" venturi would more-or-less work.
This has far-reaching implications, among them. that the less viscous your fuel or the less restrictive your fuel supply system, the bigger the venturi you can use, or the smoother it runs with a particular venturi. David F. has used the latter theory for years now, after my observations of Evo runners, and our comparison of the PA VS RO-Jett spraybar and venturi.
So, it will probably work OK with the stock parts and the recommended Cool Power fuel, or something equivalent.
As to the spigot/fuel post venturi, that has proven to be a nearly miraculous improvement for venturis that would otherwise use a flush inlet. The big advantage is injecting the fuel in the middle of the opening and keeping it from dribbling down the side. This has solved some otherwise completely intractable run symmetry issues, particulalry on the PA61. I figure the only advantage it might have over a conventional "through" venturi is that it sucks fuel better and maybe run smoother or allow a larger venturi for the same smoothness. But engines come with either one system or another so you use what you have. You example is one of the rare cases where you actually could compare a through spraybar to a spigot. But I don't recommend that - get it working and go fly, leave the experiments for later or someone else.
Brett
Topic: Evolution 36 NT Venturi Design (Read 965 times)
