Two stroke tune

Two stroke tune google doc

Another mistake commonly made, sometimes even by those who have enjoyed
some success in modifying two-stroke engines, is to believe in a kind of mechanistic
magic. Bigger carburetors, higher compression ratios, altered port timings and expansion
chambers often do bring an improvement in power output, but more and bigger is not
magically, instantly better. All must work in concert with the basic engine, directed
toward the particular application, before they constitute a genuine improvement. You
cannot treat them as a voodoo incantation, hoping that if you mutter the right phrases and
stir the chicken entrails in the prescribed manner, your mild-mannered, all-purpose
chuffer will be transformed into a hyper-horsepower fire-breather. With a lot of luck,
you might get that result; the chances heavily are that you won't.

With all the mysticism filtered out, horsepower at any given displacement is
simply a function of average pressure in the cylinder during the power stroke and the rate
at which power strokes occur, minus work absorbed by friction and scavenging. Raise
pressure and/or the delivery rate of the power strokes, or reduce friction and pumping
losses, and the engine's net output will rise. Unfortunately, there are limitations on all
sides: Pressure must be limited because of thermal considerations (and is further limited
by an engine's restricted ability to recharge its cylinder with a fresh air/fuel mixture
between power strokes). The limit for power strokes per unit of time is established by
what is tolerable in terms of crankshaft rotational speeds, and tolerable here is what the
bearings, rod and piston will survive, in inertia loadings, for what you consider an
acceptable service life; the design engineer has already expressed his opinion in this
matter. Pumping losses can be reduced - relative to the mass flow through an engine -
with a properly designed exhaust system, but otherwise are an inevitable and almost
invariable consequence of pulling air from the atmosphere, moving it through the engine,
and out the exhaust port. Some improvement in output may be obtained with reductions
in friction, but the scope for such improvements is very small compared to what may be
accomplished with cylinder pressure and engine speed.
Obviously, pressure in a cylinder will vary continuously throughout an engine's
entire power stroke. Knowing what those pressures may be in a given engine is useful,
but more useful still is knowing what they should and are likely to be, as such knowledge
can keep you from that futile exercise commonly known as flogging a dead horse - and
from believing a lot of lies about how much power various people are getting from their
engines. Engineers have an overall efficiency rating called “brake mean effective
pressure” (bmep), which they calculate by working their way back through torque
readings observed on the dynamometer, the leverage provided by crankpin offset, and
piston-crown area. Thus, bmep says little about peak cylinder pressures (those
measurements being taken with a pressure transducer and oscilloscope) but it is an
excellent relative indicator of performance and highly useful in projecting power output
from a modified engine.