Have a question on basic venturi design. And specifically on my EVO 36 NT engine (profile CL application).

The EVO 36 came with 3 venturis that use a spigot type spray:

-Short one with 0.280" venturi throat diameter
-Medium height one with 0.236" venturi throat diameter
-Tall one with same 0.236" venturi throat diameter

Most have said the 'small' venturi works best on the EVO 36. But which one is considered the 'small' one in this case? Is the throat diameter all that matters? And how does the height of the venturi play into engine operation?

I was using the shortest one (largest throat area) thinking it was the 'small' venturi. The engine did run OK, but from time to time I have noticed erratic operation. Seemingly going lean sometimes in flight. Maybe one of the taller venturis (smaller throat) is the proper one to use.

Thanks in advance helping me better understand venturi design.

Spigot venturi is akin to chaos. Get a spray bar for the long one.

Edit: I just read your second post, I guess you took my advise before I wrote it lol.


MM

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

Thanks to all for the information, and that the venturi throat effective area is the critical parameter. I was originally using the large venturi on my EVO 36 which was a bad idea.

Concerning recommended fuel, the EVO 36 owner's manual actually says that Cool Power Omega or Powermaster 10-15% nitro fuels are acceptable. I have been running Powermasrter 10% nitro/18% oil on the EVO with no noticeable issues. Engine has run rock-solid from takeoff to landing.

   I note that you are not using muffler pressure, that would just exacerbate any fuel draw issues with the original venturis.

  As above - you have a working system to go out and fly it, you can do experiments later.

    Brett

I'd stay away from the Cool Power unless you doctor it up with some castor oil.

I would typically agree with you however the Evo seems to be allergic to castor for some reason.  There are a few of those.  I have acquired a couple OS .32F's that are like that.  In the 'old days' cool power was what you ruined engines with in my experience.  Never enough oil for normal stunt engines.  I think I was told they put in oil by weight instead of volume which should have made the content even less by normal standards.  What it is today I do not know.

Dave

The primary issue with the operation of my EVO 36 ended up being my incorrect use of the large venturi that the engine came with. But now that I am using a properly sized venturi (venturi/NVA from Lee Machine) the engine runs great.

Concerning fuel, I have only used VP 10% AIR fuel on my EVO which is 18% oil (20% castor/80% syn blend). I have not noticed any issues with the VP fuel on the EVO 36. The engine owner's manual says Coolpower Omega or VP Powermaster are OK to use.

BTW, Morgan Omega is 17% oil (30% castor/70% syn). I'm not sure if the Omega fuel components are blended by weight or volume.             

I'm playing with one now that was given to me attached to a Hangar 9  PT-19.  I don't know which venturi is in it(but I will measure it). I use my home brew fuel and after drilling the spigot a bit larger it runs much better.

Perry

The EVO 36 NT I have came with 3 blue color venturis. (I included a pic of them farther up in this post topic. The tallest one has the smallest diameter. The short one is the largest diameter. For stunt, the tallest one with the smallest throat diameter offers the best chance of success. My opinion is that even the smallest stock EVO venturi is still a bit too big. I am now using a venturi and conventional front NVA that I got from Lee Machine that works great. It is even a bit smaller than the small EVO one. My 36 now runs great on my Brodak P40 ARF.

Here are the published* timing numbers for the Evolution .36NT:

34 ABDC / 48 ATDC intake (194 total)

74 BBDC / 74 ABDC exhaust (148 total)


* Bruce Smith, Scepterflight

Hi Dave,

These timings are for the RC version. I wonder if the CL version has the same or lower timing.
Best regards,
Martin

    148 degree exhaust duration certainly sounds OK for stunt.

     Brett

    148 degree exhaust duration certainly sounds OK for stunt.

     Brett

yes absolutely, my Enya 40XZ is 149o and worked very well with my Pathfinder.

Martin

  My experience with the two examples of the .36 were less than encouraging!! The first one was just a wild animal!! If going strictly by the instructions, it was unusable and i should have sent it back to them. I could barely get it to behave on the break in stand, and the needle system would not allow the engine to run less than redline RPM !! As it came, it was supposed to have been set at the factory with a limiter on how far you could adjust it. Even after removing that, I could not get anything resembling an acceptable run. I eventually drilled the smallest venturi for a spray bar, and even then it was still barely manageable. I tried the recommended fuel and everything else I could find. I finally pulled the engine off the PT-19 I got it with and substituted an FP.40. I eventually traded the engine for a ready to fly Sakitumi with an LA.40 it to a guy that just had to have it. I flew the heck out of the Sakitumi with great runs from the LA.40, and the guy I traded with put the Evil.36 out of it's misery in some sort of spectacular crash. I acquired another one and decided to try it again. I figured my first one has some sort of defective case that allowed air in somewhere that I couldn't find, and thought it would be worth revisiting.
   This second one was a bit more compliant when breaking it in. I used recommended fuel and such and after the required break in runs, ran it on the SIG 10% Champion that I use in just about everything else I fly. It didn't run any different on the test stand. I put it in the PT-19 and on the first flight, the covering on the bottom of the outboard wing decided to depart the airplane!! I managed to land it, but have yet to recover the wing still. I may get back to that sometime this year as I don't remember how it was running once in the air, since the covering came off in the first few laps. I had a .60 control line version also, but never ran it, and sold that off, since I wasn't seeing much feedback on the engine from others. Jim Lee was the only one regularly using one, and he put his on a machine shop diet to shed a bunch of un-needed weight, and I think he has set it aside for now. They were both a heavy engine, not to mention butt ugly!! and they did not produce either one for very long, and I think that is very telling on what the engine is like.
  Type at you later,
  Dan McEntee