I have read that a long pipe will have the same effect and same peak HP as shorter pipe just at lower RPMs... what is your take on this? it doesn't at all make much sense... but waiting for you oaks expertise...

I see no one chimed in, pipe liength determines when the exhaust pulse will hit the end of the pipe and travel back to the exhaust outlet, shape, diameter and obviously liength play a role in the pipes "tune". Hence, the nick name called "tuned pipe", generally the reverb of back pressure pulse is to add back pressure but still allow the exhaust gasses to leave efficently. Consider it exhaust timing fine tuning, so if the exhaust takes longer to hit the end and travel back, it will let the engine flow better and increase RPM, too short will choke the engine and lead to plugs blowing and super sensative needle setting. The rule of thumb is buy a decent long pipe, start by getting a header and coupler that is long as well, now make marks on the coupler and pipe, run it long, and shorten it by 1/4-1/2" each flight till you see peak RPM's start to rise,...this is the base start point. Now keep trying shorter leingths till the needle get's unstable and sensative, back out the leingth a setting or two. Very short pipes produce more torque, this is why you see them on R/C cars.

Pipe length depends on what revs you want to run the engine at and what you're looking for. All a pipe does is ram more fresh mixture into the cylinder by first drawing some of it out of the cylinder through the transfer ports into the header pipe and then ramming it back through the exhaust port using a reflected positive pressure wave just before the exhaust port gets closed by the piston. This "supercharges" the engine and gives more torque which leads to increased HP. If you run the engine at the revs where it gives peak HP then this "supercharging" will give more HP allowing you to swing a bigger prop at the same revs (maybe higher pitch to get more airspeed from the model). This needs a pipe that's about as short as it can go, tuned for those peak revs.

But you can also use a pipe tuned for much lower revs to give a big boost in torque to swing a much bigger prop like they do in RC aerobatics where speed isn't wanted but great pulling power is needed. The pipe is then much longer so it's tuned for those lower revs. The HP it develops at those lower revs won't be as much as one tuned for peak revs but the torque developed will be far higher than an unpiped engine.

Another thing a long pipe does (when tuned for max torque) is it acts as a speed regulator. If the engine unloads in a dive the engine tries to speed up which puts the pipe out of tune and this drops the available torque quite dramatically (switches off the "supercharging" effect) so then there's not enough torque to turn the prop any faster. This effect is also used in CL stunt to keep the airspeed of the model constant through manoeuvres, giving heaps of power where needed (climbing vertically) but cutting back on power when diving vertically.

I didn't know it was that complicated... but I did get the answer, want a long pipe then to increase revs...

You're still thinking too simply. The length of the pipe determines the RPM where you get the benefit. So in piping an engine, you choose where you want the benefit to be, and then you pipe accordingly. Long pipes help low end grunt, and short pipes help high end speed.

but I want RPM cause thats where my power is at?

Let's put it simply in rough figures. At 16,000 revs the pipe may need to be 12" long from the glow plug to the first baffle inside the pipe. At 8000 revs the pipe would need to be 24" long to be tuned to those revs.

thats better to understand... I have had a 4stroker which was pretty simply cause it came with a pipe... the Rossi 65 I have now is way different and can supply loads of power if I pipe it correctly and choose correct prop...

Now to complicate matters even more, tuned pipes come in two styles, standard tuned pipe and muffled tuned pipe. I have a standard tuned pipe for my OS .10 FSR that is 10" long with no baffle. It expands as you move rearward, then constricts and has no baffle inside.

The other pipe I have is from my old Webra Speed .61 back on late 70's / early 80's pattern planes. This was a lower RPM engine. This pipe is 17" long with a baffle at 13", and a 1-1/2" long X 3/8" OD outlet at the end of the pipe for an overall length of 18-1/2". Measurements for both are for just the tuned pipe, not the header.

So you need to decide if you want the pipe to act as just a tuned pipe, or a muffler at the same time. I've always used the muffled tuned pipe, so I'm not sure on what effects this style pipe has on engine performance.

Another example is a Macs muffler. I put one on a Super Tiger GS-40 and gained over 1000 rpm, a noticeable increase in speed of the plane, not much gain in vertical penetration, but overall the engine runs (and throttles) a whole lot better since it's able to breath. I'm not sure if this would lean towards a short tuned pipe, but then anything has got to be better than a stock Super Tiger muffler.

Hogflyer

First you need to define the kind of plane you are using the pipe for. Is it a speed plane with a target speed, rpm and anticipated prop size? Or do you plan on using it for everyday sport flying in a lower speed plane? The plane dictates the pipe style. The guys at Macs' know their stuff and will sell you a header and pipe based on your application...just give them a call.

can you give me an idea of a speed plane? I want to go all out speed... 11x8 is the prop I selected for a Rossi R65... it will get me to 16K RPM with a 30cm pipe I have heard... target speed will be around 120 mph... I think its a fair result according to calculations I have seen on the net...

Some of the earlier .60 sized pattern designs were pretty fast, look for planes that feature thin wings and overall streamlined appearance. There are also semi scale P-51s and Reno Racer... modified P-51s that are naturally fast if built light. The .60 size planes really benefit from having retracts, plus warbirds look just plain stupid blasting around with fixed gear hanging down.

If you really want to go fast, there are a couple of purpose built speed planes built in Europe that are available on a limited basis. Contact ILJ at the speed forum for more information.
Angelrcdesign also custom builds speed planes, he might have something just right for your Rossi.

If it's about top end speed, then a short pipe (high rpm benefit) is the way to go.

i know, but I read at least 30cm... its not short... or what would you say? the length?

The length of the pipe is dependent on the engine size and rpm. The length is not random. Tuning is like tuning a musical instrament; it is either in tune or not and the principles are the same too. You also have to be ok with the engine being average when not on the pipe. Mouse can and big expansion mufflers, like the Tower style, give good upper end without the sacrifice of the rest of the range. If the engine you want to run has a factory pipe available get it. When tune pipes ruled pattern planes, the top guys would get a handful of pipes and test like a Nascar team.