By filing down the piston or raising the exhaust roof you increase exhaust duration. This in effect reduces the amount of time the expanding gasses push down on the pistion during the power stroke, hence less bottom end. And also a greater time from exhaust duration start and transfer start - ie blowdown time.
The problem is that if you run a fixed gear setup (ie no jack shaft kit) you dont want to go for "more HP" or more "power". All these mod tend to do is increase top end (obviously as expected). This is only good if you have gears because as you do these "HP" mods you move your bottom/mid range power and push it more up top. That is why you need gears for these mods - as you essentially narrow the useable power band you have to change teh gear ratio to keep the engine in the power band to use this newly aquired power.
If you dont have gears and you've done these mods your bike will suck up hill and will go great on the flats with a 36T sprocket. Like my first bike - 50km/h over slight hills in the country was good fun and no prob for the engine, as long as you never got to a real hill.
On the other hand, widen your intake and exhaust ports (no more than 33mm for the exhaust across the chord). This allows more flow with out affecting opening duration.
The intake mod is a bit like the exhaust mod (increase open duration) - tunes the enigne for top end at the loss of bottom end. However, my experiance tells me that I may lose like 5-10% bottom end and gain anything up to 20% gain in mid/top end. Which in my books is worth it where as an exhaust mod will more turn out to be a 15% loss in bottom end for a lousy 10% (at most!) gain in top end.
Just remember, your engine once dynoed produces a graph of the power output. The area under this graph is a set amount (some mods will vary this area) BUT what your exhaust duration mod does is shift this graph and skews it towards top end. Meaning you lose big time on bottom end. This is generally the case with many mods.
This is a huge deal because in a fixed gear setup you want to go for highest average torque throught out the RPM range - which allows you to cruse at a good pace on nearly any incline. Mods that do this are increase compression, high flow airfilter, opened (high flow) exhaust, larger carb (16mm bore is prefect for a fixed speed HT 66cc) and last but never yet done on a HT is to either use a larger piston or longer stroke crank.
Lastly, to change the area under the graph you would need to either fit a turbo/super charger or change the engines displacement. In addition, air filter mods and exhaust mods will increase this area slightly in a case where an engine has very restricted in and outputs (such as a HT).
And to who's wondering, the elusive squish band is only a means of increasing compression with out having detonation. Meaning, unless you run like a 7:1 (or more) CORRECTED compression ratio (around 5.75cc combustion chamber volume) , dont worry about it.
Also, a expansion chamber will work like a intake duration change. Slight loss in bottom end but a ok gain in topend (as a top end tuned pipe sacrifices low end for top end). However, many people experiance a HUGE gain in performance. This is not all due to the "expansion chamber" but to the fact that the new pipe has better flow, ie more torque, ie a greater reading on the seat of your pants dyno we all have.
PS - people who tune for "top end" and dont run a balanced crank are foolish as how will you ever get to the "top end" when your engine wouldnt see past 7K RPM?
