'Back when' we ran 2 tubes from the fuel tank, one went to the carb and the other was left open.
Now we have that 2nd tube going to the muffler pressure tap.
The Medallion engines had a tap you could drill out on the side of the crankcase, but I haven't ever seen Medallions run using this tap. I also have an old backplate for TD/Medallion with a presure tap in it. Is there a difference between using the crankcase tap or the backplate tap?

Is using crankcase pressure on a sport engine mainly for better idle?

If there's an article on line about this, I'd like to read it.

Beeza

I've never used the crankshaft tap. For one thing it is in the way of my usual engine mount. I run TD's exclusively on backplate pressure or bladder type pressure tank. I fly CL stunt with them so don't know what to tell you about idle.

Hello Beeza, Yes there is a difference in using the tap on the front part of the case (by the crankshaft) as compared to a backplate tap.

Pressure in the case varies. When the piston is decending then positive pressure builds in the case. When the piston is accending you get negative pressure. The negative pressure allows the engine to take in air through the intake and the positive pressure helps force the air/fuel mixture in the case into the cylinder.

With a backplate tap you end up with relatively low pressure to the tank because you are mixing both (alternating) levels of pressure. But with the tap by the shaft you only get "timed pressure" that occurs when the intake in the shaft lines up with the pressure tap and when the piston is decending and is higher pressure.

The first VA .049 RCs I got came with a backplate tap. I could not get them to idle satisfactorily. Switching to muffler pressure solved the problem.

I'm thinking there is little reason to use case pressure on a sport RC engine, unless muffler pressure is not available.

Quite apart from the difficulties associated with using high, 'timed' pressure from the crankcase pressure nipple, and getting a very sensitive needle as a result, there is also the problem of getting the pressure line to actually stay put on the plastic nipple-which is smooth, too short, and relatively flimsy. In general a real pain! Go with a backplate pressure tap if you're going to use pressure-or go the bladder/pacifier route! [this presumes that you're not using a muffler-otherwise you have the muffler pressure tap option as well.]
'ffkiwi'

In addition to assisting fuel draw at idle, pressurization also allows you to ream the venturi larger to increase power. The engine's instructions should have covered this but Cox people actually recommended not to do so (in my case a .010) for stable run. For those who still want to try the recommened bit size is too large (for .010) and will damage the carb.

Clement

Actually the opposite. R/C carbs are not set up for crankcase pressure, and at anything but WOT the mixture gets totally messed up. Muffler pressure is far less in comparion, and gives only a very light amount of pressure. It may be that with a tiny orifice in the crankcase or backplate tap you "might" get okay throttling, but everything I've run into that addresses this subject says crankcase pressure is for venturis only. On larger engines I suppose you could use a Cline or equivalent demand regulator at the fuel inlet and take advantage of crankcase pressure, but since those weigh an ounce themselves they hardly apply to 1/2A applications.

On my VA .049 the muffler pressure tap is somewhere around #78-#80 drill, and that's just muffler pressure being moderated there.

Timed pressure was for speed engines. But even the speed flyers soon realized that a bladder was a better source of pressure.

.........and bladders can only supply enough consistent pressure on demand to a certain point, then you go full circle back to what the C/L SPEED guys use, which is either pressure off the pipe or off the crankcase. The problem with pipe pressure coming off 1/2A engines [with the clipped on muffler] is that it isn't a sealed system, so the pressure will be inconsistent. Back to the original post, with no pressure except for atmospheric, the engine is more sensitive to the models' pitch, attitude and G forces. A model engine with muffler pressure cares less which way the nose of the model is pointed.