Does anybody know of a set of rules that I can use as a rule of thumb when setting up non piped engines?  Let me explain.
1) Adding more nitro cuases.......
2) Enlarging a venturi causes......
3) Adding head shims causes......
4) What causes the amount of break?
5) How can I set when the break occurs?
6) etc

I like a good hard break going up and on top of the circle and I'm looking at what can be done to achieve such a break.  No engine in particular, it could be a PA, ST, or an OS.

Thanks guys.

Getting more break entails getting the engine closer to the peak (or the point where it breaks).

To get the engine to break more or sooner you do:


1.  Add load (more prop diameter or pitch).  Most people do not realize that pitch can be relative.  I have done a lot of testing changing prop pitches at the field "on the fly".  If you raise the pitch, many times the engine will simply compensate by running at a lower RPM at the same needle setting.  Many times the speed does not change drastically, nor does the break change radically if you simply re-pitch one prop.
2.  Lower the nitro (this reduces the 4 stroke peak, which lowers the RPM point where the engine starts to break).
3.  Reduce the venturi and run the same load and RPM as you did with a larger venturi.  This forces you to turn the needle in to get the same RPM on the ground, setting the engine closer to the break.  This will also knock off some 2 cycle top end.  This the first thing I would do!!!! Cut up some squares of panty hose and add layers between runs until the engine is breaking the way you want.  This can be done in seconds at the field.
4.  Uniflow or a smaller breather inlet forces a harder break.  Suction venting does not "snap" to a break as fast as uniflow.
5.  Pressure reduces break, but barely.
6.  Raising the head forces more break and sooner break.  Most of the best running 4-2 break engines are lower compression.  A Big Jim ST 60 is literally designed to 4-2 break, and it is significantly decompressed.
7.  Use a colder plug.  This will cause you to go in on the needle to get the same RPM, making your ground setting closer to the break.
8.  A slightly lower oil content effectively lowers the compression, and lowers the break point.
9.  GET A SMALLER MOTOR!!!

To eliminate "break":


1.  Add nitro.
2.  Opening the venturi effectively opens the throttle which allows you to open the needle to achieve a given RPM, but I would do this last as an open venturi can cause all kinds of weird stuff to happen.
3.  Use a hotter plug.  This will let you open the needle.  The thunderbolt 4 cycle worked the best in the PA 75 as a hot plug.  You might try the Merlin 2004.
4.  Lower the head.  Up to a point compression can help keep the engine achieve a higher RPM and thereby stay in a 4 cycle.  Be wary, if this is combined with nitro.  You can get an engine that has lots of power and charges out of the bottoms (even in a 4 cycle).  Doug is going to jump in here and say I am an idiot BTW.
5.  Use pressure.  The easier it is for the engine to draw fuel uphill the less likely it is to break.
6.  Use suction venting over uniflow.  Best in combination with pressure.
7.  Slightly underload the engine.  Use a little prop as possible and the engine still stay hot.  You do not want the engine to be breathing hard.
8.  More oil (up to a point) especially castor oil, makes the engine more likely to stay in 4 cycle or shorten the duration of the break.


For the "breaking" PA 75 I recommend using the stock engine and a big prop (Randy's PA 75 three blade prop or Eather's PA 75 three blade prop) , 5% nitro/18% oil fuel (if you use Sig Champion Fuel drop the nitro or add one .005" head shim), uniflow venting to atmosphere, and use panty hose to reduce the venturi until the engine is breaking hard in the manuevers.

I would not mess with anything else.

I have an Eather PA 75 three blade and a 15" Eather Phelps style two blade if you want to try them.

The 14-5 Zinger Pro is not bad but will grab a lot of air in the wind.  Narrow blades are going to be better in the wind typically.  You might try the 14-6 Top Flite Powerpoint.  Doug said it was great on the Saito 72.


My advice is pick a PLUG, FUEL, NITRO and then SHIM YOUR TANK TO PERFECTION, THEN DO NOT CHANGE THESE VARIABLES MUCH AND ALWAYS RETURN TO THESE BASELINE VARIABLES.

Vary the panty hose and pitch for conditions.  For hot air go to more pitch and remove panty hose.  For cold air go the opposite.

I fly in different conditions than he does (5600 - 6000 feet above sea level) and have gotten different results on a couple of items. Biggest difference is in plug choice. I always run the hottest one I can. Also, compression...just adding head shims to lower compression may or may not improve the break.

There is no absolute "hottest" plug IMHO (if there is, please tell me what it is).  All plugs behave differently in different engines.  All of the plugs we typically use are HOT plugs, some behave hotter in some engines.  Going to one cooler plug typically means you will go in on the needle to get the same RPM you may have gotten from a hotter plug.  Anytime you go in on the needle you are CLOSER to the break (thins includes lowering the compression).

A real hot plug in hot weather can make the engine very jumpy (or very flat for that matter depending on the nitro).

For example, in PA's the plug behavior is as follows from hottest to coldest.  OS F, TB 4 cycle, TB RC idle bar, TB #3.  I would recommend the TB RC idle bar or #3 for a low nitro 4-2 break.  I would recommend the OS F for higher nitro and a solid 4 cycle run.

All are usable, but some work better for some things.

BTW I never said the break was IMPROVED by anything I said.