a longer conrod wont make much difference, sure, itll make teh compression ratio higher, but it will still stoke the same distance since your using the same crank and its the crank which dictates stroke. So, you wont be pulling the piston down as far and not silling the cylinders as much.
With throttle bodies, as in the S1600s, they are large bore and only work at high rpm as airspeed is far too low to effectively suspend fuel and fill cylinders effectively. So, they run at arounf 7000rpm all the time, thats why they sound like motorbike engines.
The single biggest factor that determines an engines ultimate power potential is the total inlet valve area.
So the problem of power potential becomes one of processing as much air as possible per minute. All the power comes from the fuel burned - the more fuel the engine can burn per minute the more power it can produce. But to burn fuel, or anything else, we need oxygen and that comes, at least in the case of most engines, straight from the air. For best power about 12.6 times as much air and for cruise conditions and good economy about 15 times as much. A gallon of petrol (UK gallon) weighs about 7.5 pounds - so every gallon of fuel needs about 100 pounds of air. Air weighs just over 2 pounds per cubic yard so that translates into about 50 cubic yards of air per gallon of fuel.
The fuel is not a problem - we cab squirt in as much or as little of that as we need just by calibrating the carb or fuel injection system. So what limits the amount of air the engine can process? Well the ultimate limit comes from the flow capability of teh cylinder head. But if the cylinder head cant flow that air then the cylinders cant fill. The flow capability of the cylinder head is in turn dictated by the size and number of the valves because these are what create the biggest restriction to flow. The inlet valves are the critical ones here because all they have going for them to allow air through them is the force of atmospheric pressure. The exhaust valves have the piston acting as a direct pump so they create less of a problem.
if you give me the inlet valve size in sq mm, i can work out the potential power output of the 172 engine, assuming that all induction, exhaust, throttle bodies, cylinder head etc, can be modified to no longer impose any restiction to power potential.
Can anybody give me the inlet velve size?
