Intake tuning
Thought I might give an update of where I am at. The extended-extended electrode NGK plug works. The result was a slight increase in torque. I put the 18” bar on and had it buried in some dry Aussie hardwood. I could lean on the saw using more pressure in the cut with the modified plug. I’m going to continue to use the modified plug, although I am monitoring the outer electrode as it does appear to be getting rather hot.
The next phase of testing was to try and incorporate the intake resonance tuning using a ‘boost bottle. Perhaps I should give some background on this idea.
Many vehicles are now produced with their own intake (and exhaust) tuning. It is quite common to see airboxes that have been tuned, sometimes with multiple inlets of different lengths that open at different RPM.. In the early ‘80s it was a common mod to install a ‘boost bottle’ on the intakes. This was usually done to remove a flat spot in the power curve. The concept of boost bottles and tuned airboxes is related to the physics of a Helmholtz resonator. When done right they can give a 10-15% increase in power.
The Helmholtz resonator will resonate at a lower frequency than a straight pipe. This allows a smaller device to be installed when you are trying to create a torque increase at lower RPM. The final tuning of the bottle is by extending or shortening the hose to the bottle. There are different nodes and anodes in the pipe. The primary wave is the strongest, but you tune for whatever works.
I tried various designs of boost bottles on an enduro bike I had. The computations involve calculus and differential equations (or maybe partial differential equations). I decided to try a straight closed pipe resonator. Instead of the hose going to a boost bottle, I just used a very long hose – it ended up over a meter in length. The computations for a closed tube resonator are a lot simpler than the Helmholtz resonator. In fact, I made it real simple. I ran the tube down the side of my bike and just kept moving a pair of needle nose vice-grips to different positions. When I got the effect I wanted, I cut the hose, plugged it, and then repositioned the hose along the frame inside the rear fender. – Intake tuning for dummies.
Now we come to the saw engine. We don’t want low-end torque, we want the extra torque to come in somewhere around 9000-12000 RPM. The closed pipe can thus be made much shorter to work with the higher RPM – short enough that it probably could be slipped up inside the handle for the saw.
After doing the plug testing, I decided to pull the saw apart to attempt the intake tuning. I had filled a small area on the jug next to the intake with liquid weld material. The area was a bit too small for the size of the hose I wanted to use, but I was willing to give it a try and see if I could tune the intake.
There were several problems with the mod, none the least the fact that the saw was a strato. The extra intake opening would create its own wave. Further, I discovered that the strato port had a longer duration than the intake. I would be trying to pack mixture back into a crankcase that was open to the atmosphere via the strato port.
It was after the torque testing with the modified plug that my big ‘ah-ha’ moment occurred. On previous testing the strato port had remained dry, after all, it only feeds clean air to the engine, however this time the strato port was wet with fuel. After spending a good deal of time really lugging the engine down testing the torque I could see I was getting blowback out the longer duration of the strato port.
Well then, this simplifies everything. I will just tune the strato port and make it pack that mixture back into the engine. With the dry air of the strato port I won’t even have to worry about pooling of mixture in the hose. The intake port will be closed when the wave packs the strato port. This looks do-able.
Except I can’t. Although the intake port had an area that I could modify and install a small hose (admittedly it was a bit iffy), there is no way I can figure a way to hook up a hose to the strato function.
I am writing this up in case someone else has a saw they want to give it a go on. A conventional two-stroke would be a candidate, or a strato where there is room for the modification on the longer duration strato port. If someone gives a go, they don’t need to calculate the hose length, just keep clamping it in different spots to see if you find a ‘sweet spot’ that bumps up the torque.
