Expansion chambers-racing pipes

[trappermike]

Expansion chambers can seem mystical,how they are designed and work,but can almost double the horsepower of a motor if they are designed and tuned right. And they sure are exciting and ear-splitting when they run.
There's no way I can explain how they work in a short thread,but they basically work on sonic(speed of sound) and pressure waves. The divergent cone helps draw exhaust out of the cylinder and fresh fuel into the cylinder ,and the convergent cone reflects a pressure wave back at the cylinder and stops fresh fuel charge from escaping the cylinder at exactly the right moment(tuned length),much like supercharging. Everything depends on the tuned length of the pipe and the angle of the cones. You can pick the rpm you want max. power at,and adjustments can affect how wide or narrow(and intense) the power band is. You gotta be damn good at math(for the specs) and even better at geometry to actually make one. But if you can pull it off the rewards are big. The sonic and mechanical vibrations in them are incredible,and can defy your efforts to try to keep one on the engine for long. Of course they are only good for short competition runs,unless you can spring-mount and rubber mount them...

 

[trappermike]

If your porting and pipe specs meet properly somehow the motor explodes so violently it can be hard to control. Of course as a young guy I had airplane fuel,acetone,nitro etc. in the tank and it's a miracle I'm still here. At 8,000 rpm the motor just exploded with power and was very hard to control,would wind to 11,000 rpm,I would just hang on for dear life and hang on as hard as I could.
. But it was the pipes that did that,in concert with the porting. I played with the pipe specs,everyone said i'd die before 20,but I'm still here,with some big scars.
But if you want to go in competition you need a pipe or you don't have a chance.

 

[CR888]

Have your pipes doubled the H.P on a chainsaw before? Not a bike saw but regular chainsaw?

 

[trappermike]

I've never confirmed it on the dyno,but compared to the small restrictive little box that a muffler is I'll say yes,a saw muffler does nothing to aid power. Even a stock motorcycle or snowmobile muffler is far better,some are close to a pipe in design except they lack the convergent cone and stinger. Modify a saw motor and then put a good pipe on,it's a world of difference. A stock saw muffler has a flat wall directly in front of the ex. port reflecting a powerful return wave directly back at it which tries to force ex. back into the cylinder immediately,seriously hampering power.

 

[trappermike]

The "Tuned Length" of the pipe basically gives you the rpm it has max. power at.
To calculate that it is Tl (tuned length)= ex. x vel.,divided by rpm
ex= exhaust duration in degrees
vel.= sonic speed,use 1690
rpm= what rpm you want peak power at
So say we have an ex. port with 170 degrees duration and we want peak power at 12,000rpm,the result is 23.94". After you build the pipe you can alter it's power peak,making it 1" shorter raises power peak 500 rpm,making it 1" longer lowers power peak about 500 rpm.

 

[Ted Jenkins]

Tuned exhaust do not match in any way an application for chain saws. As far as performance spectrums are concerned chain saws are the lowest power producers. They are designed to be reliable lite and with minimum maintenance. This is why any application of tuned exhaust will be less than stellar. Small to major exhaust modifications will produce performance gains, but at a price of a clumsy piece of hot metal sicking out where no one wants it. Older 2 stroke snowmobile and 2 stroke road racing motors were designed from the ground up to produce amounts of HP and RPM that are almost unbelievable. When I was tuning 2 strokes years ago we worked on every single aspect of tuning a motor so that the whole package was enhance by the next component. As compared to chain saws we realized gains of at least 10 times of what chain saws produce. Consequently cranks that far exceeded 20,000 RPM needed special attention with balancing and all. I am guilty of small exhaust modifications to some of my chain saws, but am careful not to make huge noise makers. Thanks

 

[trappermike]

Thanks for your input. Expansion chambers are only for competition saws,but like any 2-stroke give big HP gains even on chainsaws,considering that any hotsaw will have aggressive port timing,carb size etc. I agree they are definitely not for stock or worksaws! That's why this article is in this section Ted.

 

[trappermike]

Click to enlarge

So here is a basic pipe diagram. How it works;
So when the exhaust port opens the exhaust gases in the cylinder rush out under great pressure,travel down the lead pipe into the divergent cone where it funnels out at a specific angle causing negative pressure(vacuum) in the cylinder to help draw out the ex. gases and helps pull fresh fuel up the transfer ports into the cylinder. Then the ex. gases travel down the pipe at sonic speed,until they hit the convergent cone,where a sonic pressure wave is reflected back at the ex. port. If your tuned length is right this pressure wave will arrive right when fresh fuel is normally escaping out the ex. port and are rammed back into the cylinder creating more power.
The diameter and length of the lead pipe and stinger can be adjusted,and angles of the cones adjusted to create what kind of powerband you want,a wide more torquey powerband or a narrower more intense one.
The tuned length of the pipe will dictate what rpm max. power happens at.

 

[trappermike]

Of course a s Ted stated these are not usefull on stock or modded work saws,they are hot,bulky and vibrate like hell,and won't hang on a saw for 1 workday,unless you go to extremes to spring and rubber mount it,and you will probably be deaf after one day. But put one on a race saw and the power and sound is awesome.

 

[mixxer]

Thanks for your input. Expansion chambers are only for competition saws,but like any 2-stroke give big HP gains even on chainsaws,considering that any hotsaw will have aggressive port timing,carb size etc. I agree they are definitely not for stock or worksaws! That's why this article is in this section Ted.

Ted seemed to have not gotten up on the good side of the bed that morning.... Rain clouds for all...!!

A 40% power gain would be expected instantly with the addition of an expansion chamber style pipe correctly matched to the 2 stroke engines application... A better increase in 2 stroke potential simply cannot be had...

It's too bad they aren't practical on work saws.... Years ago some were called "snail pipes" when they were made in a Nautilus snail shape to fit on dirt bikes... That could look cool on a saw... Still race only though..

 

[Little Al]

One of the best guys I know for extracting power from 2Smoke motors always starts with a what do you want it to do at what revs how many times & for how long is the throttle WO with that decided he builds an exhaust & then mods the other parts his usual quote on power is as far as I know is never more than 1/4 HP different from end result ISTR his best was an Aprilla 125cc Moto Gp, disc valve inlet, liquid cooled ,single cylinder, it developed 62.6 Hp at the rearwheel@19,000 RPM it won the World championship the last year before the class going 4 stroke /moto3

 

[trappermike]

Rotary valves are the ultimate 2-stroke induction,you can open and close the intake port exactly when you want at the most ideal times,unlike the piston port intake which opens too late and closes far too late.
For example the rotary valve on one large Rotax motor opens 150 degrees BTDC and closes at 75 degrees ATDC,but that's quite an extreme example. Although too bulky and mechanically complex for modern saws I did once take apart an old saw that had one,an ancient Pioneer or McCulloch.

To be most effective expansion chambers should be spring mounted to the exhaust flange and rubber mounted where the pipe is supported,this isolates their vibrations from engine vibrations,and also helps the pipe remain intact so it doesn't vibrate itself apart or everything else.

 

[Ted Jenkins]

I still believe there is great potential power gains from some modification to the exhaust systems. It looks like some creative person could design exhaust systems that did not interfere with normal operations. But why when all any one has to do to get more power is add cubic displacement. Thanks

 

[Chainsaw Jim]

I still believe there is great potential power gains from some modification to the exhaust systems. It looks like some creative person could design exhaust systems that did not interfere with normal operations. But why when all any one has to do to get more power is add cubic displacement. Thanks

In the saw racing world you don't have that luxury, unless competing in the unlimited class. All classes below that level are sorted by displacement which means when you want more power it has to be gained within the class size range.

 

[Ted Jenkins]

Back in the day I did build a few tuned exhaust systems for saws. The common question has always been how bad do you want to win? Older road bike cylinders and MX cylinders are far more productive for outrageous power. My guess would be that most hot rod saws do not exceed 20 BHP which is not much for a 60 cc motor. Thanks