i'm putting a long ranger fuse on my GPH. two questions...how large of a cooling hole do i need in the bottom for the engine. just the size of the cooling shroud exit, or as large as the entire bottom surface area of the engine? also, how long of a exhaust extension/deflector can i put on the muffler without causing back pressure problems? is about 4-5 inches too long?
thanks,
gregg

Nice to see some scalers out there.Questions you need to ask yourself...

1. How cool did it run before you put it in the fuse?
2. How much air is available in the fuse? Are the windows open?
3.How heavy is the all up weight of the chopper?
4. What is the climate where you live?

I have built a couple of award winning scale choppers and the only one I had overheating problems with was marginal without the fuse..My error. All the above questions will help you answer yours. If it ran cool all the time before and the fuse is light and you have open windows in the fuse you should be fine.I usually run my scalers a little rich for the added cooling as well,I always wanted good running a lot more than the most power in my scalers. If I can be of any help ,feel free to e-mail me at [email protected] and I will try to help if I can.
Good luck.

As far as the back-pressure issue goes, the amount of back pressure is really decided by any sections of reduced area in the muffler. An extra 4 or 5 inches of straight pipe shouldn't be a problem. I haven't experienced this on an exhaust but am just figuring rom engineering principles.

These are two stroke engines though, any extra length in the muffler section is going to create additional back pressure because some air does go in through the muffler during the stroke. Any additional length means just that much more air to move and added resistance to exhaust and air flow. You might want to try to use over sized tubing for the extended part of the exhaust, that'll help the problem somewhat.

thanks for the help everyone.

The volume of air moving through the muffler is the same for a certain throttle position, given by the amount of combustion. i.e. it takes a fixed amount of air to combust a given amount of fuel. The force required to move the air through the muffler is directly proportional to the pressure drop across the muffler.

An extended muffler will increase the friction loss from the muffler sides, but there should be no increase in the volume of air that the engine is "pushing".

Try this experiment: blow air through a straw, feel how much resistance is there. Attach another straw on the end, it shouldn't be much harder at all to blow through both straws. Now partially block the end with your finger and blow. It should now be more difficult.

One way yes. But some air does flow through a muffler in the opposite direction of exhaust flow during the 2nd half of the exhaust phase of a 2 stroke, it's just the way they work. A steady stream of air has a resistance relative to the friction of the muffler, but when the air alternates many times per second the volume of air (length of the muffler/pipe) begins to affect how the engine breaths. This is the entire theory behind how tuned pipes work. They're designed to cause just enough back pressure at a certain frequency to help blow-back the tail end of the exhaust; which contains highly mixed fuel and air giving a torque boost at the frequency the pipe is tuned for. It's analogous to how a capacitor works in electronics. 4 inches of half inch wide tubing doesn't contain a massive amount of air, but it's still air that has to be accelerated. Constant streams of air aren't subject to this because only resistance of the muffler walls affects drag. Acceleration or deceleration of a mass ALWAYS takes energy. Take the same straw and do the same thing trying to fill the straw with water and then blowing it back out as fast as you can, double the length of the straw and do it again. A human has to use water, because the frequency of air in a 2 stroke is much much higher than a human can reproduce. Keep in mind this is just physics, I don't know how much it affects the actual power output of the engine I just know it does.

I agree with your analysis now, I thought you were only considering the exhaust and not the re-mixing of air on the other half of the stroke.