dynamic expansion chamber

[rmihalek]

It seems from reading about expansion chambers, that they are designed to be optimal at a specific rpm. I think that a couple websites talking about theory mentioned that the dwell zone (the straight section between the cones) is where the rpm tuning comes in.

I was wondering if anyone has ever designed a pipe that had a variable dwell section, perhaps controlled by some type of servo mechanism, so that the powerband could be stretched out across a much wider rpm range. My old dirt bike (Kawi 200) had a power valve that seemed to function only to increase the volume in the header area. Al Smith mentioned in the Expansion Chamber thread that a buddy of his raced karts with 101 Mac engines and had some way of tuning the pipe with 1/4" sections, but he didn't say which sections were variable.

 

[ehp]

how long your header is has more to do with what rpm's your motor will turn , yes people have built pipes that the dwell can do bigger or smaller

 

[TimberMcPherson]

I dont know about dynamic expansion chambers but in the mid 80's motocross and enduro 2 stroke bikes started coming out with "power valves".
These were valves on the exhaust ports that opened at full revs and as revs dropped,they closed, reducing the port size and thus upping back pressure and therefore torque.
They allowed for the somewhat "peaky" characteristics of the 2stroke bikes to be smoothed out, evening out the power throughout the rev range.
At first they were mechanically operated off the motor, later they became computer controlled. Some had terrible reliability problems, valves would break off and end up being eaten by the engine the the worst cases.

 

[Al Smith]

Louies pipe

Well,first off,Louie had about a dozen or more 101 Macs.He had several porting combinations for these engines.For example,a short track needed,a gutsy low and midrange.A 1/2 miler needed a high rpm.If I recall,he changed the middle section of the pipe.The ultimate in karts was to get max rpms,just prior to braking for the turn.His long track engines,would turn at around 14,000,believe it or not.The life was only about 4 or 5 races,before a rebuild.Each differant engine porting scheme,had a different pipe.How he kept it straight,is still a mystery to me.I have an old gear drive,Mac 650,that has been ported a bit.I get a like affect as a pipe by using a long exhaust,no screen,just the arrester cover.When it hits about 7,000,it instantly goes to around 9.200 to 10,000.I came accross it by accident.It acts like kicking in a turbo.I know very little about the proper design,but niether did Louie.Just blind luck,and iron will.

 

[weimedog]

The trick with saws is to apply tried and true exaust designs in a way where the end result is still a practical saw! Expansion chambers are large. RPM pipes fat! Wonder if a snail type under the saw might be a usable approach. Maybe exaust exiting to the right.

Power valves add weight and complexity. They work really well but add a dimension of maintenace that the average pro-saw user doesn't want to deal with.

The combination of case/reed valves, power valves raising and lowering the exaust port height (variable timing), and expansion chambers did wonders for the racing motorcycle world. Of course water cooling made it possible by allowing all that additional heat to be removed fast enough for an intense motor setup to survive. (The old mid 1980's era Honda's were the ones that varied the volume on the header pipe with a power valve, just put a 1988 CR125 together with its camber/exaust valve deal) But does anyone want to lift all that crap? Espesially when a little larger displacement gives the power without all the complexity, related service and reliability issues and WEIGHT!!

My old 82 cc Homelites are a revelation of how simple but well designed solutions give very practical results.