More Power.

Summer 2008: So after the last test it seemed that we were only getting half the thrust I would have hoped for. I searched again on the net for an engine with the same phyiscal dimensions but with more oomph!!!!! A chap was selling a ‘Stage 3’ engine on ebay for £130 and confirmed to me that the power output was around 6hp. If that was true then 40kg thrust was in easy reach. So without delay I brought a unit off him.

Thinking we were near then end of our journey, I asked Sam if he could help build a steel frame and cage to take the increased power and provide an airborne test bed. After several designs, we chose the simplest we could and Sam and his contacts set about the construction.

The rev counter showed peak revs at 8330rpm. This equated to 1660 prop rpm and 28kg thrust. Sam was very pleased with the amount of thrust being developed but I was unhappy, I wanted more.

A tuned exhaust had come as part of the package with the uprated engine and because we had to modify it to fit the frame I suspected we might have affected its power potential. We tried some flexible exhaust hose to lengthen the header section again but no advantage was gained. We welded the exhaust back to how we had originally modded it.

So, what’s next I thought?????

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One Response to “More Power.”

  1. I realize this post is coming up on 10yrs old, but figured I’d chime in, in case anyone is thinking of making a similar attempt.

    The only way make more HP from a 2 stroke engine by modding the exhaust is to add an expansion chamber. An 2 stroke expansion chamber is shaped roughly like an American Football and comes straight off the exhaust header.

    You gain power with an exc because the exhaust exiting the engine resonates inside the exc. When the chamber is shaped just right the resonation will create a low pressure wave right as the exhaust port is opening. This sucks the exhaust out of chamber and also sucks the fresh fuel and air into the cylinder as both ports are open together momentarily.

    I’ve seen dyno runs where a well tuned exc will an honest 2hp to 2 stroke motor. You can get a little more by modding the head of the piston with shallow channels channels allowing for slightly more time with the intake and exhaust ports to remain opened.

    For maximum effect you’ll probably need to run computer simulations as any mods you make will require adjustments to the exc to ensure you’re low pressure wave is timed just right.

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