Proper flow can be achieved either through back pressure, as in a factory small muffler, or through tuned resonance, like the big bulging resonator pipe on a 2 stroke racing kart or snowmobile or dirt bike. The tuned pipe is far more efficient and capable of producing more power because it is less restrictive, but its effectiveness comes from being designed to support a resonant pressure wave across a wide rpm range, which is difficult do achieve completely from scratch by the average DIYer.
If you ever talk to someone who does 2 stroke racing, be it go kart, dirt bike, or whatever, they'll tell you how swapping out the tuned pipe can change the power band of the engine. It's all about the resonant frequency.
In lieu of a tuned pipe, a restrictive muffler will achieve similar results, at a significant cost, noise, and space savings, but at the expense of additional performance. Perfect example of this is chainsaws. They come with a small, restrictive muffler to keep them compact and relatively quiet, but one of the first mods a competitive Sawyer will do is swap that out for an aftermarket pipe that is tuned to their specific motor.
All that said, there may just be a tuned pipe available for these engines.
THAT!
resonating pipes are surely the way to go if you want performance,
they do produce back pressure when needed and
-what is the best part of a tuning pipe-
create an
underpressure to suck in the fresh gas quicker
so there's that tiny little bit more of it to combust.
Bret (Desertduler) makes his own pipes and he even posted a step by step once..
very interesting, very intimidating for the average diyler
But in case you want to try, he might be able to run your specs through his calculator software
to tell you what dimension pipe you would need to fab up.
Anyways, looking back down the time line...
basically all american racing kart engines of the early days have been two strokes..
essentially none ran a tuned pipe, all are about as open of an exhaust as it could be
(sometimes nothing but a flow direction cap to keep the hot gases from burning your arm)
some minor flow restriction is all they could provide if any at all,
and yet the engines still reved up nicely and provided some serious performance.
So maybe a shorter open header with just a nasty bend or other obstacle can help you out for now.
IDK the Duraforce engines at all, IDK what they are ment to run at,
and what they're capable of when 'adjusted for our needs
'
But 4k seems indeed like there should be some room for improvement.
'sid
