Hi all,
         what difference does lengthening the venturi tube above the spraybar make?
I have seen many diesels and four strokes with quite long entrances and many glows with unbelievably short ones, so they all seem to work but why the differences?

Thanks.

Thanks for the reply Dave, all that I have is that an intake runner, either below the jet or above tends to curb flow reversion or 'spitback.'

Nice snorkel on that engine mate!

There's theories about tuned length induction for you to research Chris, me I'd rather bolt it into a plane & go fly it.

As for the Enya "swirl" venturi, is it oriented for northern or southern hemisphere coriolis effect?

Right now it is oriented on my engine shelf!  This is in a package marked .61 CX.  I'd like to try it in a RO Jett but haven't tried to fit it.  I have a couple Enya .60 III R/C conversion and I hope it will work in one of those.  One day I plan to design a competitive sidewinder design for these many muffled 60s out there.  Always many to be had cheap on da bay.  A good 'big plane' for those not wanting to make the big block/ pipe investment,  and still prefer IC over electric.

Dave

With quick thinking, I think the main effect is increasing the suction, not by making flow faster in narrowest point, but creating turbulence friction before it. A bit like adding an air filter. I think those vanes in Enya's venturi are far too close to the mouth of venturi to work as advertised. The airflow right after prop is quite chaotic.
But in general, I think a longer venturi shroud is a good thing, it makes the suction less affected by the direction of cooling airflow.
Also in rear intake, it keeps engine compartment cleaner and accepts a more restrictive airfilter.
In (racing)classes with venturi size rules, it's good to have a plenum chamber before the point of fuel atomization, and the narrowest point well before it. For this reason the rules also usually state a maximum volume for the space between narrowest point and crankshaft/intake valve. But of course, it also depends on wether a pressure tank or venturi suction is used.
I use rear intake and with my plug-in venturi's the distance from mouth to fuel posts is restricted to about 1/2" by exhaust header. For this reason I make an extension from silicone tubing, it makes the engine run clearly more constant, even without a filter mesh.

Lauri

Another effect that it may have -- and this view is almost completely unfounded but for one data point -- is that it may help with fuel economy, and affect (one way or another) how well the thing holds a mixture.

My one data point for this is that the typical OS <something>LA engine (FP, too) with short venturi spits a lot of raw fuel out the front of the intake, not all of which gets sucked back into the engine.  I verified this on my own planes with a case of Powermaster fuel from the batch where they way over-dyed it and everyone's planes were streaked with purple.

A longer stack on the venturi might not give the fuel time to be thrown out the front, thus it would go into the engine where it would get burnt up instead of just splattering the plane.

Hi Tim,
          it has been well proven that a longer tube above the jet does indeed improve fuel economy as it contains an otherwise lost charge - but with stunt its a case of 'who cares?'

Hi Tim,
          it has been well proven that a longer tube above the jet does indeed improve fuel economy as it contains an otherwise lost charge - but with stunt its a case of 'who cares?'

   Right, that part doesn't matter much. What *does* matter, and what is one of the most important issues, it that the long intake acts as a flow straightener. That makes it less influenced by the changes in flow due to maneuvering. That's also the goal of the panty hose diffuser, which accomplishes a similar function without requiring a very long intake.

      Brett

   Right, that part doesn't matter much. What *does* matter, and what is one of the most important issues, it that the long intake acts as a flow straightener. That makes it less influenced by the changes in flow due to maneuvering. That's also the goal of the panty hose diffuser, which accomplishes a similar function without requiring a very long intake.

      Brett

The Bru-line did the same thing, as well as the  RC car intake filters I am running now, plus the helped keep some of the fuel from being ejected out the top

Randy

Agreed, I have learn't a bit here, thanks guys!

Another allied use of extended intakes is in combat, a flexible length of pipe acts as restriction, crushing upon impact and prevents dirt ingress.