Custom Pipe

[overdosed]

Ok guys, will this concept work using a DIY alum pipe thats around 2-3mm thick?
http://img444.imageshack.us/img444/7118/12cs8.jpg

PS. NOT MY IDEA!

[Killerush]

Sure, it'll work for a little while, until you blow your engine. You have to have a tuned chamber for nitro engines, that's how they maintain compression.

[F-150man94]

Thats a little...well....nevermind.

[overdosed]

my mate says he has studied engine dynamics and that it will work and that he has also set it up so the fuel line is still pressurised

[awnelson]

Sounds like you need to start mating different people. JK

The engine won't build backpressure, which is necessary on all motors except Jet Engines. Engine Dynamics must have been a real easy class that he was able to sleep through if he actually thinks that will work.

BTW, I've seen better fencing materials used on an Arkansas ranch.

[F-150man94]

Funny, I've never seen any professional racer use acut up Pepsi can for a pipe and win a race with it....Hmmmm..There has to be a reason why....Maybe because there is no way that could possibly work. If it does, I would like a full explanination on how it has enough backpressure to keep the motor running. Furthermore, it would run so lean it would fry itself. This is the complete opposite design as a tuned pipe.

[maxxmustang]

guys he sid this was a CONCEPT! not the real thing. i think hes saying can he put the pipe on lk this

[overdosed]

as i said before:

Ok guys, will this concept work using a DIY alum pipe thats around 2-3mm thick?

i dunno, he also said something about making it a bit bigger than the exhaust port bit where the manifold goes so that it has no restrictions on power.

[1stGenCRXer]

Alright, taking an engine dynamics course is one thing...

I've written a couple two-stroke tuning and exhaust theory features for karting sites and publications, so maybe I can jump in here and not be flamed by the "experts" that have already replied on this thread.

Pay close attention, as some of this doesn't translate well from engineerese. :teacher:

Two stroke engines, or any engine for that matter don't need backpressure in any size, shape, or form. There is not an engineered exhaust in existence that should take backpressure into consideration during it's design.

Back pressure is a fact of life, like death and taxes, that just simply can't be avoided. Every pipe, tube or hose that carries any kind of fluid or gas is going to have some form of "backpressure" for the simple reasons that an object [or fluid] at rest tends to stay at rest, and there is always going to be some interaction between the fluid and the vessel transporting it in the way of friction.

What engines do need is exhaust velocity, because and object [or fluid] in motion tends to stay in motion, and nature abhors a vacuum, so the gas moving at high velocity is going to try to maintain an equal pressure, pulling more exhaust with it, making room for a fresh fuel-air mixture in the cylinder. Backpressure is a side effect of having the proper exhaust velocity in place, and is not a determining factor in whether the exhaust is properly formed or sized.

Now, 2 stroke engines have a special need to make peak power, and that is found in the exhaust. The reason for this is because two strokes do not rely solely on the scavenging effect of evacuating exhaust the way 4 strokes can, due to the exhaust valves they possess. Instead, two strokes rely on sound energy to pull burnt exhaust gases out, without allowing the fresh fuel-air mix to enter the exhaust.

This control of the sound the combustion process makes is possible through the use of three key features in a 2 stroke tuned pipe. They are the divergence cone, belly, and convergence cone of the exhaust. The divergence cone allows the sound wave to slow down across the belly, and the exhaust pressure to decrease for exiting through the stinger or cutout without over-scavenging. That sound pulse continues to travel accross the belly of the pipe to the convergence cone which reflects it back toward the engine.

The key to all of this in order to make more power, is timing. If everything is timed correctly, the sound wave will reach the piston JUST as the piston is closing the exhaust port. This is important because the incoming fuel-air mixture is denser, and is more easily affected by the sound wave, without pushing the less dense exhaust back into the combustion chamber. At the same time, the sound wave bounces off the side of the piston at the exhaust port, and bounces back along the path it started on. There are countless sound waves in action in our little RC engines at the crazy high RPM they turn!

Something else to keep in mind with 2 stroke engines is the stinger or outlet size of the pipe. Larger outlets allow more exhaust to exit per pulse, but they also allow more sound energy to escape. At low RPM this leads to excessive scavenging, and an unstable idle, but at higher RPM, the engine may be able to "find" the timing it's looking for from the sound energy and make more power with less restriction. Or it may continue to be overscavenged without a strong enough sound pulse to keep the highest concentration of fresh air-fuel mix in the cylinder, and run dangerously lean.

On the flipside, too small of an outlet on the pipe will have a great idle, but be too restrictive to make good power at higher RPM, as the pipe simply can not get rid of enough exhaust quick enough, and the pressure in the pipe builds far too high compared to the combustion pressure.

So, the likelyhood of the above exhaust concept working to make more power, is very slim.

One more thing: I know we use exhaust pressure as a free fuel pump on our models, but please don't confuse this for being a requirement. If retuned with proper fuel tank placement, your engine will run just fine with no pressure line to the fuel tank from the exhaust at all!

Man are my fingers tired...

[avablast]

Another reason and one of the main reasons why 2 stroke exhausts are desgined the way they are, is because when the piston moves down to open the exhaust port the inlet ports are also opened at the same time just for a couple split seconds and the sound waves the above guy above me described push the fresh fuel back into the chamber, instead of losing fresh charge out with the exhaust.

[overdosed]

wait til my mate sees this!

how about a tube 100mm long x 5.2mm in diameter with a stinger on the end and a sorta like cone shape on the other end which gets smaller and then fits onto the manifold bit?

[1stGenCRXer]

The length between the divergence cone and convergence cone will determine your sound wave timing, so you can't just start with some arbitrary length measurement and diameter and expect it to work.

The diameter of the belly, angle of the divergent and convergent cone, location and size of the stinger, all these factors need to be taken into account.

If you want to make a functional, higher performance pipe for your application, I'd suggest you start by measuring an off-the shelf pipe for your application and make small changes with testing in between.

Chances are, if you're thinking of doing something that you've never seen done before on a 2-stroke exhaust [as in radically different], it probably won't have the desired effect.

[Budget racer]

Sorry i don't mean to hijack the thread

1stGen, what about cleaninng up and polishing intake and exhaust ports? Will that change the sound wave? Is it a good or bad thing?

[overdosed]

ah well. on another note (a question by me):

How would I make the exhuast sound louder without killing the engine?

[1stGenCRXer]

1stGen, what about cleaninng up and polishing intake and exhaust ports? Will that change the sound wave? Is it a good or bad thing?
Polishing the ports will affect the sound dynamics, but usually not enough to cause a drop in performance. As long as you don't go crazy and totally change the shape of the ports, you'll typically just smooth out the airflow at the ports, which, if nothing else, would tend to help your fuel mileage/runtime even if you don't gain a noticable performance bump. Just don't let grit or filings get into the engine internals, or wash out the stripped down engine very well before trying to run it.ah well. on another note (a question by me):

How would I make the exhuast sound louder without killing the engine?
Easiest way is to go with a single chamber pipe designed for your application. Most tuned pipes are dual chamber, where the convergence cone is actually covered by a chamber that contains the stinger to direct exhaust out where you want it. A single chamber pipe might look the same on the outside, but the outer cover is what actually functions as the convergence cone. Fewer obstructions for the sound to get out of the pipe equals louder engine.

[overdosed]

but will i lose much performance?

[Budget racer]

1stGen,
I ran Friday night on a indoor track. After break-in i started to tune for perfomance and yes it did make a diffrence in peformance. Very nice top end and good punch out of the hole. I didn't go crazy just clean up of the ports and polished up. Even with a rich low end setting idle was very strong and never shut off. I am very happy with the perfomance of this .12.

Thanks for the insight.

[Secretsg]

2fast2furious

[1stGenCRXer]

but will i lose much performance?
Depends on where you want the performance at.

A single chamber pipe is usually quite good for top-end power, but typically lacks in low-end torque.

A dual chamber pipe will have good torque [usually], but may not have quite as much top-end power as a single chamber pipe.

Any time you start playing with different pipe combinations, you might as well be ready to experiment.

[Adanmtxt1]

I guess the money question here concerns 4-stroke engines: will max power and efficiency be attained using straight pipes with minimal returning pressure waves?

[1stGenCRXer]

4 strokes are a "whole 'nother ballgame".

With 4 stroke engines, you already have an exhaust valve to keep the fresh fuel air mix from being lost.

So, the balance now is between exhaust velocity, and restriction. As I said before, backpressure is a fact of life, and NOT a determining factor in what kind of exhaust is going to be best suited for your application. So the trade off is going be between exhaust velocity you need to get the best scavenging of the cylinder and the least amount of restriction [in this case the restriction will show up in the form of backpressure].

There are two ways to achieve this balance, by way of exhaust diameter, and length.

Since 4 strokes have a mechanical means of gaining efficiency, you don't need an expansion camber, divergent or convergent cone like a 2 stroke needs.

The easiest way to fine-tune a 4-stroke exhaust is usually in it's length. The reason being is that you normally want to start with an exhaust that has the same diameter as your exhaust port. There isn't a huge advantage to be had by going drastically larger or smaller than the size of your exhaust port. In this application, just have some faith that the engine designers made the exhaust port the optimum size to begin with and go with it.

When tuning the length, there are some things to keep in mind. The longer the exhaust length, the more consistent the exhaust velocity tends to be, giving a more stable idle, and more torque off-idle. The downside of a long exhaust is more restriction, which impedes the exhaust velocity at higher RPM, and limits power up-top.

The shorter the exhaust length, the easier it is for the exhaust velocity to change due to less restriction throughout the RPM range. Since the exhaust velocity changes easier, it also means that at lower RPM, the exhaust velocity can slow to the point of weak scavenging, and can make idle quality suffer. If the exhaust length is too short, there is even a chance of inadvertently sucking in fresh air through the exhaust port if the cam profile has a lot of overlap, leading to a lean running condition.

For trial-and-error fine tuning, it's usually best to start long and trim back the exhaust length about a 1/4" at a time until you quit seeing a gain in power at higher RPM.