Guys,

I've seen this subject somewhat discussed before, but I wanted to run it up the mast again a little differently.  I've accumulated a pretty good stock of FP and LA .40's.  I also have a bunch of OS venturies, large and small, for these motors.  The large one's bore measures between 0.285" and 0.289".  The small ones are more consistant and around 0.265" to 0.266".  The question is, among you guys who run these engines, which size venturi works best for you or have you tried both? 

I've run both on the stand, but not in the air, and can't tell much difference in the way they act.  What are your feelings?

Herman Green

Thanks Dave,

That's a great tip to allow using both sizes.  I'll probably take advantage of that.  Understanding the differences in the FP's and LA's, do they seem to like the same size venturi?

Herman

Thanks Dave,

That's a great tip to allow using both sizes.  I'll probably take advantage of that.  Understanding the differences in the FP's and LA's, do they seem to like the same size venturi?

Herman

The fp/la engines are from a power pov interchangeable. The la's have a couple mods that make them easier to handle and more funtional for PA's in "out of the box" condition. I had a great discussion with GMA once on venturi size. I believe that the venturi is just a fixed throttle. It controls how much air an engine can be filled with. If you look at venturi sizes across some pretty large displacement deltas you will find that you can use a 0.281 dia with a ST needle on engines from .29 through .60 with good results. The .29 is filling near it's max (high bmep) while the .60 is more restricted (lower bmep but larger displacement) so it works. This issue is "how much power do you need." When were really workin the fp's for my nephew Arch we used vent dia to control power. He built a ........ ahhhhhh........ corpulent stunter and we needed to get some more out of the fp40 so we opened the vent to .312 and it allowed us to swing a 12" three blade carbon prop. It was a lot of thunder from a .40 and the engine didn't last as long as it would have at a lower bmep. A better choice would have been a .46fp but they didn't exist. Soooo....... The point is that the vent dia is a part of the total system of "tunables" that allow you to maximize your individual needs. Thanks for your great question.

In Philly we run FP40s with the large diameter (.283) venturi, stock needlevalve, free flowing tongue muffler, an extra head gasket or two extra Tower 40 head gaskets, an Enya #3 plug, a 101/2 by 41/2 apc prop, 5%, 22% (50/50) Powermaster fuel. We use the larger venturi and a free flowing tongue muffler in order to improve the cooling of the engine. We've found this setup to be powerful with 2x2 break.

The OS head gaskets measure .016 while the Tower gaskets measure .010, that's why we use two of them to get the needed head clearance. Leonard Newman sells excellent .010 head gaskets. Stock up if you plan to run these engines. The FP40 is sometimes more finicky than the LA. However, outside of purpose built stunt 40s, it's tops in 40 sized power.

Set the engine on the ground, waiting for it to warm up for twenty seconds or so. Try to get that back and forth stunt idle. With an FP it'll sound pretty nasty with the muffler opened up enough to work. See how it does in the air. The setting on the ground may be different than the resulting flight setting. The trick is to set it on the ground so that it works right in the air.

Profiles sometimes cause weird vibrations that can make it rough. (See my posts concerning an ARF Primary Force that took a while to tune in.) Build the nose as rigid as possible using the usual tricks.

It helps to have someone around with gifted ears, who can set the engine in the range where everything works sweet. Cold weather condition, high humidity, hot weather etc. may need to be accounted for with slightly different needle settings.

Avoid uniflo initially. If you get a solid consistent run gradually leaning up during the flight, you might want give uniflo a shot. We've found uniflo to often be a problem, creating extra air bubbles, hard to regulate.

Break in time tends to help the engines run better after a while. But usually we fly them as we set them, even new.