The volumes are not matched, you will need to increase the volume of the chamber to match the increased volume of the 25 engine and I am fairly certain that you could modify the E-2030 muffler to suit - if you could spare the time to do so.
The volume is not the key parameter. The key parameter is the distance to the reflecting surface.
The problem with the 25 seems to me to be (and bear in mind, this is just my guess) that it had enough additional power to rev past the tuning valley. The regulation works by depressing the power as the rpm rises. It's not a brick wall, even with conventional stunt pipe that works on the pretty strong first mode - you still get some power running at the bottom of the valley. If you ever get it unloaded enough, even the residual torque at that RPM is enough to permit it to move past the valley - at which point it becomes unstable, and the torque starts to *rise* with RPM again, as you jump to the next reflecting surface or node.
The muffler is particularly prone to this because you are operating on the (as I recall) the 4th mode, which has both a weak tuning effect, and it's not that far to the 3rd mode. The 20, set up per recommendations, can't seem to get past the valley around 12000 rpm. The 25, set up similarly, has A LOT more power and even with the pipe 4th mode fighting you it can still get past, on to the next node, and sustain that. The result is it tends to "home in" on an rpm past where you want it to run with a 4" pitch prop, and run around the unstable point - it gets unloaded a little, runs past the valley, and then continues to rev until it runs out of poop. Then it gets loaded, gets dragged down *below* the valley, at which point it gets regulated at the normal RPM again. So you get a kind of conditionally stable system that will run great for a while, then too fast, then right. Or, it takes off, and then homes in a just a little too fast a speed.
That's why the 25 also needs sometimes needs a smaller venturi* - you need something else to kill the torque off as the RPM rises to keep the torque at, say, 12500 to a low enough level to it can't get past the valley. That also turns it into a pretty conventional motor, losing some of the advantage.
That's not to say it doesn't work. All of the small engines like this worked pretty well if you *ran them stock* and *in a two-stroke*, with appropriate props. They all were more than satisfactory for replacing Fox 35s on small profiles, which was the original goal. But the 20 was much nicer in terms of run quality and much more consistently reliable due to the tuning. Other than being teeny-tiny, the run quality ranks up there with the gold standard of stunt engines, the OS40VF.
Brett
p.s. *BTW, you can screw up the 20 run pretty easily by doing the OPPOSITE. If, you say, ream out the muffler outlet excessively and run a lot of nitro to "get more power", you can create enough power at the tuning valley to allow it, too, to run past on the unstable side of the valley and be on the *rising* section of the power curve instead of stable *falling* section - and then it has the same sorts of problems as the 25! You have to work at it, but you can screw up the 20 run - just like all those people that make a living screwing up 40FP runs by using 40-year-old "common knowledge" . When someone calls or comes up to me with a 20FP problem, I almost always find that they have done something intended to either "get more power", "save weight", "get a real stunt run", or "swing some real lumber". I am not saying you might not be able to make it work by deviating from the plan but the solution to any run problem is to *put all the stock parts back on the way it came from Tower*.
Topic: What is magic about the BBTU FP-20? (Read 8024 times)
