Advantages of retiming an engine

The experts have not jumped in here yet. That may be their subtle way of saying, "If you have to ask, don't try it!" Let me say enough stupid stuff that they can't resist jumping in.

4 cycle engines go "suck, squeeze, pop, phooey" and the timing of each cycle is controlled by a camshaft. Each cycle is more independent of the others than is possible in a 2 cycle. The cam can open a valve and close it some different number of degrees of crankshaft rotation. Hot cams do some things we can't quite duplicate in a 2 cycle.

2 cycle engines go "phooey-suck" then "squeeze-pop." Timing the phooey-suck cycle means changing the relative heights of the exhaust port and intake port. If the exhaust port is raised, there is more time for phooey. If the intake port is raised, there is more time for suck. That ratio is critical. You can't change that without machining the liner.

Changing the entire ratio of phooey-suck time to squeeze-pop time is a matter of raising or lowering the entire liner. You can raise the liner without the aid of a machinist, but you will change the compression ratio. Lowering the liner requires a machinist.

You probably don't want to try this. Engine modifications are quite sophisticated today, and playing around with a jeweler's file and gaskets is akin to using a Cub Scout book as a guide to removing your appendix at home.

If you are planning to do anywhere near serious precision aerobatics, DEFINITELY get your OS 40 reworked by one of the reputable engine rework artists. It will be money WELL SPENT.
I rework my own Fox 35s but sent my OS 40 FP to Lew Woolard (Silver Foxx). Leonard Newman of this board is another very good one. I would recommend either one.
If you ever decide to try it yourself, be prepared to totally ruin several perfectly good engines in the name of experience. Talk to several different engine specialists, decide just WHAT you want the engine to do, and in my opinion the CHEAPEST way to find out just what to do, is send a new engine to one of the better engine rework specialists that can rework the engine to perform what you wish for it to do. NOW comes the PAINFUL part.
You WILL RUIN the engine, but tear it down and look at, and measure EVERYTHING that was done. Now you will have a ruined new engine, but some idea of what was done. You will not know EXACTLY how much machining, grinding, and fileing was done because you didn't know what dimensions and fits were from the factory (they have factory tolerances you know) but you will know what things are AFTER you got the engine back. AGAIN, I reiterate!!! You WILL ruin the engine when you do this.
One last thing that despite ALL the expertice and skill in machining that anyone can have, you must develop the FEEL of a well fitting engine. There is a certain feel that many of us cannot develop that the good engine rework specialists have, and that is essential for consistant rework performance. That is a feel of how things fit. I envy those with that talent.
If you ever do tear down a reworked engine, DON'T return it to the guy who did the work in the first place and complain because it don't run right. Or complain to others that it don't run right with out telling them that YOU tore it down.
Even still, To get good at doing your own rework, be prepared to destroy several engines. BTDT
Bigiron

The effect of changing the timing is to alter the torque the engine puts out as a function of RPM. In stunt, it's typically used to reduce the torque at high rpm. This can allow the engine to be stable at the rpm range one wants.

To be stable, any stunt engine has to be on the downslope of the torque curve in level flight. If the engine is loaded by slowing down the airplane (by maneuvering), the engine torque increases. If the props becomes unloaded, the rpm increases, and the torque falls. This tends to regulate the airpseed.

It's my opinion that any sucessful stunt engine has this characteristic. For the Fox, it a matter of the internal timing and lack of high speed breathing. For a piped OS40VF, it's the pipe being tuned to a lower rpm than the operating point. For the OS20FP, it a matter of picking a prop that's small enough and low pitched enough that the operating point is past the torque peak.

Yu have already found what happens if it isn't that way. The OS40FP is notorious for running away. The problem is that the peak of the torque curve is past the desired operating point. You can get it to sound great on the ground, but it's operating below the torque peak. As soon as something happens to unload the engine, it rides up the torque curve, which makes it spin faster,which runs it further up the torque curve, which makes it spin faster, on and on until it does hit the peak and stop. One possible solution to this is to let this happen and use less pitch to slow down the airplane. Unfortunately, for the average stock 40FP on a Twister-sized model, you need something like a 10-2.5 prop. Go down to "Hobby World" ask them for a 10-2.5 prop, and see what they say!

The result is that the 40FP was the basis of an entire cottage industry of modifying the timing to chop the torque curve off at lower revs. Our host is a good example of a successful employment of this technique. There are a lot of others who aren't - I've seen some amazing hack jobs performed on 40FPs.

It should also be noted that the speed the airplane fly is related to the power (torquexRPM), not the torque by itself. An airplane flying a particular speed requires a particular amount of power. A Nobler flying level at 5.2 seconds a lap is receiving the same amount of power whether it's from a 20FP or an ST60. The 60 may be putting out more power at the shaft, but the power going into the airframe is the same. The difference is the efficiency of the propellor.

It is my personal opinion that too many people give up too quickly on the stock motors. The same factors at work in the 40FP are also in work in the 20FP. But while the 40FP is putting out 2-3 times the power you need to fly a Twister, the 20FP straight out of the box gives you just a little more power than you need to fly the airplane at stunt speeds. Killing the small amount of extra power is a lot easier than killing a factor of three.

Having said all that, I've spent enough time and seen enough bad results to think that the success rate with stock 40FP's on the typical "35-sized" airplane is too low to make it worth trying. Get one modified from Leonard or Randy.